Module 5 Cell Division

Unit C - Cell Division, Genetics and Molecular Biology

Unit C Introduction

From one generation to the next, from single cell bacteria to people, we are all part of a competition to create more of our own species. How do we ensure our best is passed on to the next generation? Now that science has discovered most of the mechanics of the process of inheritance, a new question is arising; should we now manipulate inheritance to suit our whims? Should we have the power to decide exactly what the next generation will look like? This unit is all about the cycle of life. From the secrets of cellular reproduction, to the story of why you may have your grandmother’s nose, this unit will explore how our biology is caught in an increasingly complex drive to survive.

In Unit A you examined how the human body used the nervous or endocrine system to maintain homeostasis. In Unit 2 you examined closely how the human reproductive system ensured the continuation of our species.

There are three modules in Unit C: modules 5, 6 and 7.

In Module 5, you will again examine reproduction, but this time at the cellular level. Individual cells, like humans and all other multi-cellular organisms, need to ensure the next generation has all that it needs to survive and excel. In human reproduction, you saw how this creates a cycle as one is born, grows, mates, and gives birth to the next generation. In a similar way, cells follow a life cycle of growth, preparation, and division. In this unit, you will learn how the cell life cycle naturally progresses. You will examine how cells copy their instructions for the next generation. You will also discover how cells divide to form new, complete cells, or divide to form incomplete cells that must find another cell with which it will join in order to become a new, whole organism. You will compare these methods of reproduction, examining their advantages and limitations. As you work through this unit, you will begin realize just how important the regulation of the cell cycle is as you consider cancerous or “wild” cell growth.

In Module 6, you will come to understand that the instructions copied by cells during the cell cycle code for traits observed in organisms. You will look more closely at how an organism passes on his/her traits to the next generation. As you consider these patterns of inheritance, you will learn about the works of Gregor Mendel and Thomas Morgan and how they contributed to our understanding of genetics today. You will practice using predictive tools that will allow you to understand and explain the movement of a disease or condition through a family pedigree.

In Module 7, you will examine the molecular basis for these traits in the cell, and you will gain an understanding of how cells express these traits through protein synthesis. As you examine the molecules present in our cells, you will reflect on how mutation can change the intended expression of our genetically inherited traits. Genetic change can result in disease. It can also result in enhanced abilities, and can be the basis of evolution as explored in Unit D.

The major concepts you will explore and the skills you will develop in this unit are:

• Explain the rules and steps involved in mitosis and meiosis that regulate the transmission of genetic information from one generation to the next.
• Describe the similarities and differences that exist in mitosis and meiosis that allow for growth, healing, and reproduction of organisms.
• Hypothesize how the understanding of the molecular nature of genes and DNA can help explain the transmission of traits, and how mutation at the molecular level results in changed proteins.
• Analyze how the knowledge of the molecular nature of genes and DNA has led to new biotechnologies and treatment of genetic disorders.

Think about the following questions as you complete this unit:

• What are the cellular processes that an organism uses for growth, healing, and reproduction to ensure the survival of his/her species?
• What regulates the transmission of genetic information from one generation to the next?
• How is DNA responsible for the production of proteins?
• How has the molecular knowledge of genes and DNA led to new biotechnologies and the treatment of genetic disorders?

Concept Organizer

The concept organizer below shows the relationship between the cell cycle, patterns of inheritance, and molecular genetics. A properly functioning cell cycle is needed for DNA to be properly duplicated and divided into new cells for the next generation. This generation will express traits based on logical patterns of inheritance that can be understood by careful observation and recording. These traits are a result of the proper creation of proteins from the genes found on DNA.

The chart below organizes the main ideas of each module. You may wish to create your own chart by using the following chart as a starting point and adding much more detail as you go through this unit.

Module Descriptions

Module 5: Cell Division—The Processes of Mitosis and Meiosis

Scratch your arm or cut your finger and you will be glad to see your skin heal over the wound. What would happen if that re-growth did not stop? How do our cells know when to live, or die, or replace themselves? This module will explore the cell life cycle, the production of somatic cells, the formation of gametes, and the reproductive strategies of various species. This module will also look at how scientists use their understanding of the cell cycle to combat cancer and aging.

The following questions will be explored:

• Is there a life cycle clock common to all organisms?
• What are the different ways cells reproduce?
• What are the purposes of reproduction?
• What are the sources of variation or consistency in cell division?

Module 6: Mendelian Genetics—the transmission of traits to the next generation

Should you look like your parents? This is one of the questions that will be discussed in this section. Genetics is a complex topic, and this section will explain the fundamentals of inheritance. Here, you will examine how simple traits are passed from one generation to the next. You will explore the patterns of inheritance that can be traced and predicted.

The following questions will be explored:

• How are traits passed-on from generation to generation?
• What controls an organism’s physical appearance?
• Are all genes the same?
• Does the inheritance of traits always happen the same way?
• Can technology change the inheritance of traits?

Module 7: Molecular Genetics—DNA, RNA and protein synthesis

Unraveling the code of life was one of the greatest discoveries of the past century. This section will explore how traits are coded for by genes that are carried in our DNA, and how these genes are expressed as proteins. This section will also look at how scientists can manipulate these genes or transfer them from one organism to another, resulting in variations never before possible.

The following questions will be explored:

• How does DNA carry the genetic code?
• How does DNA direct the synthesis of proteins?
• What changes in the genetic code result in mutations and variations?
• How much of my genetic code is uniquely mine?  Do I share any part of my DNA code with my relatives?

Module 5 -Cell Division

Introduction

In living organisms, the production of new cells is essential. New cells replace damaged cells, they allow for growth, and they are the basis of organism reproduction. Cells divide to form more new cells by either mitosis or meiosis. You should be familiar with some of the basics of mitosis from previous science courses. In this module, you will build on that knowledge as you examine the cell cycle of division and compare the processes of mitosis and meiosis. You will examine the opportunities for variation that exist during cell division, and you will become familiar with technologies that allow you to observe that variation. You will also discover that, although organisms don’t all use the same reproductive strategies, the basic principals are universal. Just like any biological process, cell division has its complications.  Cancer is an example of mitotic cell division gone wrong.  Cancerous cells divide at an uncontrolled rate causing abnormal mass cells.   In this module you will further develop your understanding of cell division.

Big Picture

This module is about cycles. Our body cells all follow a cycle. No doubt somewhere in your childhood you skinned your knee or cut your finger. Once the tears were done and the bandage was applied, you were probably told it will just take some time to heal. Over time, our cells are constantly growing and dividing to replace what is old or damaged. Skin cells can replace themselves every three days. Cells of the digestive tract and respiratory tract replace themselves rapidly because of the damage they experience from every day use. Muscle cells are much slower to replace, and bones can take months to heal.

Our life is part of a cycle too. In the previous unit you studied in detail how human life begins. In this module, you will learn how other organisms may follow similar or very different patterns in reproduction. As you consider your own family, every one is at a different stage in their life cycle, and so are the cells within their bodies.

What would happen if we could step outside of one of these cycles? What would it be like to get our cell lines to stop growing older? To simply grow and reproduce at the same rate as when we’re twenty? Aside from the social implications there may be very serious biological implications to consider. Cancer cells are in essence our own body cells that no longer respect the cell cycle. They reproduce as quickly as they can and never stop. Clearly cancerous cells are not the target of the anti-aging industry! As you continue through this module think about the internal clock that governs your cell reproduction and aging. How important is it for you to try to beat aging?

As you explore healthy and unhealthy or uncontrolled cell cycles, you will focus on the big question of how cellular processes allow for growth, healing and reproduction in the support of survival of living organisms.

This module will explore the following focusing questions:

• What structures pass genetic information on to the next generation and in what ways can cells ensure they are passed on successfully?
• What are the stages and phases of the cell cycle and do they change with age?
• How do meiosis and mitosis compare in the creation of new cells?
• When is consistency desired over variation, and which processes ensure the proper outcome?
• What differences exist between fraternal and identical twins?
• How do chromosome disorders occur and why does their occurrence increase with age?
• What are the advantages or disadvantages of different reproductive strategies?

This module relies on prior knowledge of the cell and how it works. If you feel you need to review the concepts of the cell before you begin this module, read pages 546 and 547 in your textbook. If you are comfortable with your knowledge of the cell, continue on with the module.

In this Module

Lesson 1: Cell Division and Chromosomes

In this lesson you will identify the types of cellular division and understand their function and purpose. You will be able to recognize the structures within the cell that carry genetic information. You will learn about the significance of chromosome number in cells, and learn how to read a picture of human chromosomes.

In this lesson the following focusing questions will be examined:

• What kinds of cell division exist and when are they used?
• What are the structures that pass genetic information to the next generation, and how are they observed?

Lesson 2: Cell Cycle and Cancer

In this lesson you will learn to identify the phases of the cell cycle. You will learn how a normal cell regulates this cycle, and how some cells can exit the cycle or may even ignore these clues.

In this lesson the following focusing questions will be examined:

• What are the stages and phases of the cell cycle?
• Do all cells have the same ability to reproduce and does this change with age?

Lesson 3: Mitosis

In this lesson you will learn to describe the stages of mitosis. You will learn why this type of growth is important and how new daughter cells compare to their parents.

In this lesson the following focusing questions will be examined:

• How are the phases of mitosis identified and described?
• How does mitosis maintain consistency in plants and animals?

Lesson 4: Meiosis

In this lesson you will learn to describe the stages of meiosis, you will come to understand when meiosis is necessary and how it differs from mitosis. You will learn the major sources of genetic diversity, and why this is important to a species.

In this lesson the following focusing questions will be examined:

• How does meiosis contribute to genetic variation?
• What differences exist between fraternal and identical twins?

Lesson 5: Cell Cycle Disorders and Genetic Testing

Cell reproduction does not always proceed as planned. In this lesson you will learn of common disorders resulting from improper cell division and you will be asked to consider the ethical issues involved in prenatal tests and working with embryonic cells.

In this lesson the following focusing questions will be examined:

• How do chromosome disorders occur and why does their occurence increase with age?
• How can embryonic cells be used, and what tests may be done on an unborn fetus?

Lesson 6: Variation and Reproductive Strategies

In this lesson you will learn about the wide variety of reproductive strategies found in different organisms. You will also gain an appreciation for the variety of ways species balance their life cycles.

In this lesson the following focusing questions will be examined:

• What are the advantages or disadvantages of different reproductive strategies?
• Why do some organisms vary their chosen reproductive strategy?

Lesson 3.5.1

Lesson 1—Cell Division and Chromosomes

Get Focused

Life is about cycles. At the cellular level, the cell cycle involves reproducing identical cells through mitosis. At the organism level, mitosis can once again produce identical cells—this time their offspring. Organisms with variations are produced through meiosis, another type of cell cycle. The similarities and differences in the offspring can be explained by examining chromosomes.

As far as the human race is concerned, genetic material from both the male and female are necessary to create a new human being. How the child ends up with the characteristics of either parent continues the cycle of cellular division and reproduction. Our reproductive cycle is a strictly sexual affair.

Sometimes in nature, reproduction can be rather diversified and unique. Aphids, for example, all hatch out of their eggs female. Perhaps more alarming is the fact that they already have live nymph aphids developing inside them. These too will be born female and pregnant. In little time, aphids can dominate a crop and cause serious economic damage. This sounds like a winning reproductive strategy. Why have males anyway? However, the aphid’s tale is not over. In the fall, males will suddenly be born. They will mate with females and produce eggs that will hatch over winter. Why the change? Why would aphids go through all the trouble of changing strategies just when conditions are getting tougher?

In this lesson you will identify the types of cellular division and reproduction, and understand their functions and purpose. You will be able to recognize the structures within the cell that carry genetic information. You will learn about the significance of chromosome number in cells, and learn how to read a picture of human chromosomes.

In this lesson the following focusing questions will be examined

• What kinds of cell division exist and when are they used?
• What are the structures that pass genetic information to the next generation and how are they observed?

This lesson will take approximately 60 minutes to complete.

Module 5: Lesson 1 Assignment

 Once you have complete all learning activities for this lesson, you can complete the online assignment.

Bio30 3.5.1 online assignment

 The other questions and activities in this lesson are not marked by the teacher; however, you should still attempt all of the work offered here. They are designed to help you review important information and build key concepts that may be applied in future lessons.

Here is a tutorial video for this lesson that you can watch if it suits your learning style.  Bio30 tut#3.5.1 Cell Division

3.5.1 page 2

Explore

Review

Review the diagram on page 547 of the text book on your own or with a friend as a way to review the different parts of a cell. If you are comfortable with your knowledge of the parts of the cell continue on with the lesson.

Read

As you may recall from earlier science courses, life does not spontaneously occur. Instead, life comes from existing life and is organized around small units called cells. The first part of the cell theory was proposed by two German biologists, Mathias Schleiden and Theodore Schwann. Based on their observations they concluded that all plants and animals were made of cells. This conclusion has been extended to include all living things and, since their discoveries, no exceptions have been found.

Types of Cell Division

The continuity of life from one cell to another is based on the reproduction of cells via cell division and occurs as part of the cell cycle. There are two major types of cell division: mitosis and meiosis.

Mitosis

Mitosis is simple cell replication. Begin with one parent cell and end with two complete daughter cells, nearly identical to the original. If the cell being considered is a unicellular organism, then this kind of division is also reproduction and known as binary fission. Mitotic division in multi-cellular organisms is responsible for growth, development, and repair. To review why organisms must grow through cell division and not by increasing cell size, read page 550 of the textbook and consider “Figure 16.1” illustrating surface area to volume ratio.

Some multi-cellular organisms can also reproduce through mitosis under special conditions. This is called asexual reproduction since it involves only one parent. Taking a cutting from a plant, or cutting a flat worm in half are good examples. Some insects can also use this method to rapidly populate an area, like aphids that are born pregnant with more female aphids that will also be born pregnant!

The advantage of mitosis is in speed and energycosts. Only one cell is needed to start and from that many thousands can result. This is why some bacterial infections can spread so rapidly. The disadvantage of this kind of replication is that daughter cells are genetically identical to the parent cell. Therefore little or no variation exists in the population, and it may be susceptible to changes in the environment, such as the use of drugs to treat bacterial infection. However, some bacteria mutate very rapidly and develop a resistance to drugs. These are termed the super bugs and they are a high concern in the medical community. In Unit D, you will study the factors that determine the success of populations surviving or why they might not be successful in surviving and their population number size decreases.

Meiosis

Meiosis is a more complex process. After a meiotic division, the resulting cells will have half the needed genetic information. These cells are called gametes. To complete replication, a gamete from one parent will need to unite with a gamete from another parent to restore the complete amount of genetic information; a process known as fertilization, which occurs during sexual reproduction.

While meiosis takes longer, involves more than one parent, and costs more energy to carry out, it does result in variation. By encouraging genetic variation in a population by using meiosis and fertilization, a species will be better suited to overcome change in the environment. Variation increases the chances for survival and reaching reproductive age so that adaptive traits can be maintained in the population.

Genetic Material

The essence of a cell, all of what it is and can do, is determined by the genetic code found on DNA (deoxyribonuclieic acid). DNA is organized inside our body cells into segments called chromosomes. For most of a cell’s life these segments of DNA are diffuse and cannot be observed under a microscope. However, when the cell undergoes division, the chromosomes condense and become visible. For more detail on the organization of chromosomes readpages 551 and 552 in the textbook.

Every organism has a specific number of chromosomes in its cells. For example human cells have 46 chromosomes while dog cells have seventy-eight chromosomes. This number must be maintained from generation to generation for normal function to occur. Chromosomes can be further organized into homologous chromosomes. Homologous chromosomes are roughly the same size and shape and contain the same type of genetic information.

In a human body, or somatic cell, there are 22 pairs of homologous chromosomes, known as autosomes and one pair of sex chromosomes. The sex chromosomes determine the gender of the organism. A human with two X chromosones is femaile and one with one X and one smaller Y chromosome is a male. Read pages 552 and 553 in the text book to learn more about chromosomes and how they are arranged in cells.

Karyotype

As you’ve just read, the number and arrangement of chromosomes is very important to regular function. Scientists can make a picture of chromosomes in a cell by staining cells that are about to undergo cellular division. At this point the chromosomes are most dense and can be sorted into homologous pairs by their length, position of the centromere (a region that appears pinched in), and banding pattern.

When sorted, scientists can determine the gender of an organism and whether or not an abnormal number of chromosomes are present. Certain major syndromes are a result of too many or too few chromosomes because chromosomes did not separate equally during cell division. This is termed nondisjunction. In humans, Down syndrome is a result of an extra chromosome 21 and Klinefelter’s syndrome is a result of an extra sex chromosme (XXY). An individual’s chromosome set is known as their karyotype.

3.5.1 page 3

Watch and Listen

In the previous unit you considered the creation of sperm and eggs and how they united to start a new human life. You also talked about how the zygote underwent continuous cell division and differentiation to eventually become a new person. In this lesson we have identified those processes as mitosis for growth and differentiation, and meiosis for sperm and egg production. Consider the following video comparing mitosis and meiosis cell division. Without worrying about the specific steps in either process, which will come in a later lesson, use this video to gain an understanding of how these processes differ.

Consider these questions:

• How does the cell prepare for either division?
• What structures are involved during the processes?
• How many daughter cells usually result from meiosis or mitosis?
• How do the daughter cells of mitosis compare with those from meiosis?
• In what ways is mitosis similar to meiosis? How is it different?

Module 5: Lesson 1 Assignment—Lab

Complete the following:  Use the exploration guide to learn how to use the Gizmo to complete the assignment.

Exploration Guide

Human Karyotype Gizmo

 (Log in to the top login box with your Moodle username and password.)

Discuss

Cell division as introduced in this lesson can be dramatic and amazing. From a tiny sperm and egg comes a complete new person. However, we have come to accept less dramatic growth or repair once we have reached maturity. Sure you are still thankful that your bone mended after that break, or your skin closed after that cut, but if you lost a finger or an ear, you would accept there was nothing that could be done.

That may not be the case now. Science is constantly looking for ways to extend natural re-growth or repair. Re-growth of finger tips has been reported with the addition of ground-up pig bladder (a source of stem cells). Perhaps more shocking is our experimentation with growing human tissue or organs inside of animals for transplant. Use the search Internet search terms “mouse human ear BBC” h and follow the link to the BBC website on “Artificial Liver ‘could be grown’ or “Girl may be the first to grow artificial ear.” In both of these reports there is a disturbing picture of a human ear growing on the back of a mouse for transplant.

As promising as these developments are, they bring up many ethical questions. Should scientists be experimenting with tissue or organ re-growth using animals? Discuss this question with your classmates and your teacher in the discussion area.

3.5.1 page 4

Reflect and Connect

Self-Check

Complete the following review questions on cell division.

SC 1. In what three general ways is cell division important to your body?

SC 2. In general terms, compare binary fission to mitosis.

SC 3. Give the main advantage of meiosis and why this is important to populations.

SC 4. Differentiate between chromosome, chromatid, and chromatin.

SC 5. What characteristics are common between homologous chromosomes?

SC 6. What can scientists learn from creating a karyotype of a developing fetus?

3.5.1 page 5

Lesson Summary

During this lesson you explored the following focusing questions:

• What kinds of cell division exist and when are they used?
• What are the structures that pass genetic information to the next generation and how are they observed?

Mitosis and meiosis are the main types of cellular division. Most of what we can see or observe around us is an example of mitotic growth or repair. Perhaps that is because it is fast and accurate. However, in later lessons, as you consider the survival of a species and the changing environmental pressures all life must face, you will soon see the value of meiotic reproduction.

In either type, DNA organized into chromosomes is at the heart of proper cell replication. In humans there are 22 pairs of autosomal chromosomes and a sex pair that determines gender. Together all 46 chromosomes must be duplicated and then passed on to each new daughter cell for those cells to carry on their roles. Chromosomes can be stained and arranged in a karyotpe so scientists can determine the health and gender of a young developing fetus.

Lesson 3.5.2

Lesson 2—The Cell Cycle and Cancer

Get Focused

Ponce de Leon, a Spanish explorer, spent years looking for the fountain of youth in a part of North America now called Florida. Of course he never found it, but imagine if there was one, and you could drink from it! Life would be yours for hundreds of years. No wrinkles until you were at least 500 years old! Alas, what is it that makes human cells last only a little more than one century at best? Why do cells age? Is there a secret to keeping body cells vigorous and young which has not yet been discovered?

In this lesson you will learn to identify the phases of the cell cycle. You will learn how a normal cell regulates this cycle, and how some cells can exit the cycle or may even ignore these clues.

In this lesson the following focusing questions will be examined:

• What are the phases of the cell cycle?
• Do all cells have the same ability to reproduce and does this change with age?

This lesson will take approximately 80 minutes to complete.

Module 5: Lesson 2 Assignment

Complete the online assignment  Bio30 3.5.2 online assignment after you have worked through the lesson materials and watched the tutorial video.

The other questions and activities in this lesson are not marked by the teacher; however, you should still attempt all of the work offered here. They are designed to help you review important information and build key concepts.

Here is a tutorial video for this lesson that you can watch if it suits your learning style.  Bio30 tut# 3.5.2 DNA intro

3.5.2 page 2

Explore

Before we begin, it is worth mentioning that the process you are about to study is exactly that - a process. Cell division should be viewed as having a definite beginning and end. In between divisions, that is, when a cell is not undergoing cell division, a cell performs normal vital activities such as respiration, photosynthesis, protein synthesis, absorption, secretion, and excretion. These processes take up most of a cell's life. Perhaps the most important process is the replication of genetic material in the nucleus. This must be completed before a cell divides so that each daughter cell receives the same genetic complement as the parent cell. Once the replication is complete, the cell is ready to divide.

Read

Cell Cycle

Our bodies are made up of an amazing array of specialized cells that keep us healthy and able to maintain an internal balance in an environment that seems bent on changing. During our daily combat with the environment, cells must be replaced as they wear out. Exactly how fast this occurs depends mostly on the role those cells perform. Skin, which is constantly being scratched, rubbed or cut, replaces itself very quickly. However, muscle or nerve cells may remain healthy for most of our life and therefore may not need to divide to replace themselves. Some cells, such as red blood cells, which lack a nucleus and genetic material, or gametes, which only have one of each chromosome, will never reproduce.

The typical life cycle of a cell can be broken up into two main phases; Interphase and M phase. Interphase is the much longer phase, and is the phase where the cell will carry out its intended function in the body. M phase, or Mitosis phase, is shorter and moves the cell through a complex sequence of steps that are carried out in order to divide the cell’s genetic material equally. This stage ends with cytokinesis, which is the physical division of the cell into two daughter cells. Read pp. 553 – 555 in your textbook to preview the stages of the cell cycle discussed below.

Interphase is further broken down into three phases; G1 phase, S phase, and G2 phase.

During G1, or Growth 1, the cell is growing rapidly, producing proteins and carrying out its intended function. In the case of muscle or nerve cells, they may remain healthy and functioning at this stage for so long that they may be referred to as being “stuck” in G1 or, as it has more recently been refered to, in G0 phase. If, however, this is a regular cell, it will reach a point where it moves on to the S phase.

The G1 stage is critical if the cell is to divide properly later on. In S phase, or synthesis phase, the cell will duplicate its DNA exactly. Each single chromosome makes a copy of itself and holds on to the copy. These doubled chromosomes do not contain any new genetic material. Rather, they are identical copies of each other. While together, they are known as sister chromatids, and are joined together by a regional structure called the centromere.

After the DNA has been successfully doubled, the cell will enter G2 or Growth 2. Here the cell continues to carry out its role in the body. Nearing the end of Interphase, the cell will get ready for M phase by storing energy and building proteins and other structures needed for cell division.

During M phase or Mitosis phase, the cell will divide each doubled chromosome into two separate single chromosomes. We will consider this process in detail in the next lesson. At this point it should be clear to you that the cell goes through an orderly set of steps necessary to ensure that its genetic material is correctly divided into two complete and equal sets. Following this division, the cell will go through cytokinesis in order to physically divide the cell into two.

Watch and Listen

The animation above introduced the checkpoints within the cell that regulate division. These are of great interest when considering how cells ensure that they are growing and are in fact capable of division.

There are three major checkpoints of the cell cycle. One is at the end of G1. Here, the cell evaluates if it is large enough and strong enough to continue with the division process. The second checkpoint is at the end of G2. This is a very important checkpoint for the cell. At this time, the cell must evaluate if it has properly duplicated all of its chromosomes. If it has not, it may attempt to carry on with the division, or it may simply self-destruct. The last major checkpoint occurs during M phase. At this checkpoint the cell evaluates whether the spindle apparatus has properly attached itself to each of the chromosomes, and whether the rest of the cell is ready for cytokenisis, or physical cell division. If something is wrong at this stage, the cell will often simply die. 

Read

Reasons and Limits for Cell Growth

There are many factors that may lead cells to reproduce. The most common stimulation is the need for replacement due to damage or age. Cells work very hard, and many simply become too brittle or build up toxins too fast to continue to function. These cells are broken down by the body and their raw materials are re-used.

Once the call for cell reproduction is given, how do cells know when to turn off? Let’s consider skin cells as a working example. Your skin is one of the most active areas on your body for cell reproduction. Skin cells use two common clues to determine when to start or stop reproduction. The first is density-dependent inhibition.

For density-dependent inhibition to work, cells must pay attention to their neighbors. First off, regular skin cells will not divide if alone. Next, if a gap is created in a layer of skin cells,then those that remain will automatically start to divide until the gap is covered. Also, once skin cells bump up against their neighbors, they stop dividing.

Another clue that cues cell growth is anchorage dependence. Using skin cells again as the example, skin will grow naturally if anchored to a substratum of tissue. Conversely, they will not grow if simply free floating in a nutrient bath.

Cancer

Cancer is a broad group of diseases characterized by a rapid, uncontrolled division of cells. Cancer cells appear to ignore all the regulations in place for cell growth. They do not wait or stop at any of the check points. They do not appear to be density-dependent, nor are they anchorage dependent. Cancer cells grow and reproduce constantly.

Module 5: Lesson 2 Assignment

Knowing that certain chemicals interfere with the process of cell division, researchers set out to find drugs that would help in curing cancer. This has led to a cancer treatment method called chemotherapy, or treatment by chemical drugs. Since cancer cells divide rapidly and continually, any chemical which blocks cell division or kills cells while they are dividing will have a much greater effect on cancerous cells than on normal cells. However, these drugs will also destroy other fast-growing cells in the body, such as hair follicles. This explains the loss of hair by cancer patients on chemotherapy.

There are now more than two dozen different anticancer drugs which can be used to treat cancer. One drug used in chemotherapy is methotrexate, which attaches to certain enzymes involved in chromosome (DNA) replication and prevents these enzyme from doing their job. Without these enzymes, new molecules of DNA cannot be synthesized. If cell division does not take place among these drug-damaged cells, none of the newly formed cells will survive. Methotrexate is generally quite successful at first, but like other similar drugs, it loses its effectiveness over time. Studies show that the cancer cells become resistant to these anticancer drugs. Researchers believe that resistance to methotrexate occurs because the drug-treated cancer cells produce multiple copies of the specific gene that is affected by the drug. Methotrexate alters the DNA molecules in cancer cells so that some genes begin to multiply uncontrollably. One of these genes directs the synthesis of the DNA-replicating enzyme, the exact enzyme that the drug inhibits. Multiple copies of this gene cause a pronounced increase in the production of the DNA-replicating enzyme, which in turn causes a dramatic increase in the rate of DNA replication within the cancer cells. This leads to an increase in the rate of cell division. Daughter cells from these altered cells also show multiple genes and a more rapid rate of cell division. Ironically, the very drug that stops cancer cells from dividing also has the effect of making these cells more resistant. Eventually, the chemical's inhibition of cell division in cancerous cells becomes ineffective and essentially useless.

Two other drugs used in the treatment of certain cancers are vinblastine and vincristine. These two drugs were discovered in the Madagascar periwinkle plant, Catharanthus roseus. Vincristine is very effective in the treatment of leukemia, and vinblastine in the treatment of Hodgkin's disease. Vinblastine doubles the chance of surviving Hodgkin's disease. Currently the only practical source of the two drugs is from this plant. However, to produce 5.0 g of vincristine, an expensive and laborious process requiring 1000 kg of periwinkle stems is used. Chemists have successfully synthesized the substances, but this is even more expensive. At present, new methods of culturing the plants are currently being developed to speed up the production of these drugs. The medical potential of Madagascar periwinkle is a good example of why conserving plant diversity is so important.

3.5.2 page 3

Reflect on the Big Picture

Finding the Fountain of Youth?

In addition to ignoring clues or checkpoints that essentially control cellular growth, cancer cells also never stop dividing. Probably the best example of this comes from cancer cells removed from Henrietta Lacks in 1951. Those cells are still in existence today. They are alive in a variety of laboratories around the world, and were even included in the Discover 17 satellite and shot out into space. Though Henrietta died eight months after the cells were removed, she has in some way reached immortality.

Healthy cells have a built in countdown timer. At best, a cell can divide approximately 50 times. After that, the cell will cause its own death, or self-destruct. Many cosmetic companies and late night TV commercials would like us to believe that we can cheat this number. That we can instead, keep our cells healthy and dividing as usual for longer and longer periods. Is this a reasonable claim? Can regular cells cycle continually?

Earlier in the lesson, we learned about the S phase of the cell cycle. During this phase the DNA is duplicated for the next generation of cells. This is crucial, since it is our DNA that contains all of the instructions necessary for cellular function. The problem is that our DNA accumulates errors over time. Whether it be from the duplication process itself or from exposure to environmental mutagenic agents, with time, our somatic cell DNA get’s pretty beat up. These errors found in DNA can cause serious problems or diseases like cancer. That’s why every cell contains specific instructions to self destruct after about 50 divisions. It seems our cellular clock exists for a good reason.

Discuss

Now that you have gained further knowledge on the cell cycle and its regulations or limits on cell growth, do you believe that research should continue to be focussed on finding a fountain of youth? Is it time to accept our limits and focus on the quality of our lives now rather than focus on the potential length of our lives? Discuss your opinion with your classmates and your instructor.

Lesson Summary

During this lesson, you were to explore the following focusing questions:

• What are the phases of the cell cycle?
• Do all cells have the same ability to reproduce and does this change with age?

Healthy cells will move through Interphase and M phase of the cell cycle when conditions are correct. From growth and production in G1, to the synthesis of DNA in S phase and all the way through and including the preparation of G2, the cell is constantly checking it’s own performance and readiness for the next step in the cycle. Finally, during M phase and cytokinesis, the cell will physically divide or split itself in two to form two new young cells, each of which contains one copy of the parent cell’s genetic instructions.

Regular cells have a limit to how many times they can divide, and they respect their neighbors. Cancerous cells do not respect either of these criteria, and this can have terrible results. If the fountain of youth is to be found by gaining control of our cellular division, it will not be by following cancer’s example!

Lesson 3.5.3

Lesson 3—Mitosis

Get Focused

You are not the same person you were a year ago. In fact, you are not the same person you were a few seconds ago. Even as you are reading this, your cells are growing, dying, and dividing, again and again. New cells replace the old cells, leading to an eventual complete turnover of many body tissues over a matter of a few years. This continuous cell replacement maintains your body and keeps you looking like you. Yet your appearance does change over time. Why? The replicate cells that replace worn out and damaged cells should be exact copies of their predecessors, but are they?

In this lesson you will learn more about the M phase, or mitosis phase of the cell cycle. You will also learn to describe and explain what is occurring in the different phases of mitosis. And finally, you will learn why this type of growth is important, and how new cells that result from mitosis relate to their parent cell.

In this lesson, the following focusing questions will be examined:

• How are the different phases of mitosis identified and described?

• How does mitosis maintain consistency in plants and animals?

This lesson will take approximately 80 minutes to complete.

Module 5: Lesson 3 Assignment

You will complete a lab on cell division for assessment.  You will use information collected in your cell division lab to complete the online assignment.

Bio30 3.5.3 online assignment.

The other questions and activities in this lesson are not marked by the teacher; however, you should still attempt all of the work offered here. They are designed to help you review important information and build key concepts that may be applied in future lessons.

Here is a tutorial video for this lesson that you can watch if it suits your learning style.  Bio30 tut#3.5.3 Meiosis

3.5.3 page 2

Read

In previous lessons you have learned how our cells follow a cycle. From origin through G1, S, and G2 of Interphase, the cell grows, divides its DNA, then gets ready for cell division. If you need to refresh your understanding of this process, take a moment to review the following animation.

Mitosis is an orderly process that carefully divides a cell’s chromosomes. These chromosomes were copied precisely in S Phase so that each one has the identical genetic content. When Mitosis is complete, cytokinesis divides the cell physically into two identical daughter cells, which are exact replicas of the parent cell. Consider the summary on p. 557 in your text. You may wish to summarize this information for your course folder. Study both the computer graphic and the actual slides of each phase.

Mitosis is a process, and is actually more of a continuum than a set of snap shots. However, for study purposes, it can be divided into four distinct phases. Prophase, Metaphase, Anaphase, and Telophase. These phases are very important, and if you can come up with an easy mnemonic for PMAT it would be a great study and memorization tool! Your can read more about these phases on pages 557 and 558 of your textbook.

Watch and Listen

You should now have functional knowledge of each of the phases of mitosis. Review how they work together by watching the following video. 

Going Beyond

You could be creative too. Why not come up with your own poem or song about mitosis and share it with your classmates? Be sure to let your instructor know that you would like to complete this activity, and once you have finished, save your work to your course folder.

Read

Consider the two sets of slide images below. How are they similar? How are they different?

If you have good observation skills you should notice two clear differences between plant and animal mitosis. Firstly, plant cell walls are rigid and cannot go through cleavage. Instead, a new cell wall is formed between the daughter cells called a cell plate. Secondly, plant cells do not have centrioles. They do form a spindle apparatus to move chromosomes around, but must anchor this apparatus to the cell wall instead of to the centrioles as animal cells do.

Labs

 Use the Exploration Guide to learn how to use the Cell Division Exploration activity needed to complete the assignment.

Complete the online Cell Division Exploration.

(Put your username and password into the top login box.)

Make sure you allow the simulation to run for around 100 simulation hours or more to obtain the best results.

Self-Check

Here is a chance to check your understanding. Complete the following questions on Mitosis and cellular division, and then check your answers. If any of these questions give you trouble, ask your instructor for clarification about that concept.

1. Label the following terms on the flow chart below: mother cell, daughter cells, chromosomes, S phase, metaphase, separation of chromatids, prophase.
   
   
2. A skin cell taken from a chimpanzee contains forty-eight chromosomes.
   
   
   1. How many chromosomes would there be in the nerve or bone cells of this animal?
   2. If a skin cell of the chimpanzee underwent cell division, how many chromosomes would there be in each daughter cell?
3. What role do centrioles play in cell division of animal cells?
4. Match the event described in each statement with the correct stage of mitosis labeled on the diagram.
   
   

   A. Interphase	B. Prophase	C. Metaphase	D. Anaphase	E. Telephase
   Diagram	Event
   ________ a.	Normal growth and functioning of the cell occurs here.
   ________ b.	Chromosomes replicate to produce two sets of chromosomes in preparation for cell division.
   ________ c.	Chromosomes with their duplicates still attached shorten by coiling, thus becoming visible under the microscope.
   ________ d.	Centrioles migrate to opposite sides of the cell and the nuclear membrane dissolves.
   ________ e.	Spindle fibres grow from each centriole and attach to the centromere of each chromatid pair.
   ________ f.	Chromatid pairs still joined at the centromere line up along the middle of the cell, called the metaphase plate.
   ________ g.	Chromatids are pulled apart by shortening of the spindle fibres. One complete set of chromosomes is pulled to each pole.
   ________ h.	Chromosomes uncoil, spindle fibres dissolve, and cytoplasm divides. Two daughter cells are formed.

5. Name the process of cytoplasmic division and describe how it is different in plant and animal cells.

3.5.3 page 3

Reflect and Connect

By creating copies of our DNA in S phase, and then carefully separating those identical sets of DNA into new cells during Mitosis, our cells ensure that the next generation has all it needs to continue life. Each new cell has the same number of chromosomes as its parent cell. Consistency seems assured.

However, as we considered in the introduction, our bodies do change over time. For example, our skin is not as elastic when we’re old as it is when we’re young. What could be causing this? The source of aging seems to be two-fold. One factor is built-in to the process of copying of our DNA, while the other is linked to environmental stress.

Our environment is harsh. There are all kinds of chemicals and radiation that can break up our DNA or cause changes known as mutations. One common class of these nasty chemicals is known as oxidants. These are highly reactive substances containing oxygen that are present always, but increase with infection, as well as with consumption of alcohol, cigarettes and highly processed foods. Another concern is high levels of glucose, which can bind to our DNA and cause it to stop functioning. Mutation, oxidants and high glucose can all cause our cells to die. Even if they escape death, the cell line may be forever reduced in function or may end up with cancerous growth.

In addition to environmental stress, our DNA faces challenges from within. Each time our DNA is copied in S phase, it is not perfectly copied. Instead, the ends of our chromosomes, known as telomeres, are shortened just a bit. Telomeres protect the chromosome in much the same fashion a plastic tip protects a shoe lace. When these ends are too short, the chromosomes can no longer be copied, nor do they function properly, and as a result the cell and its line die. Cells can at best divide 50 times, no more.

Discuss

A lot of attention has recently been given to telomeres and their role in the aging process. Cancer cells seem to pay attention to them too, as most cancers turn on a gene to create telomerase, an enzyme that will prevent the shortening of telomeres. However, they are not the single factor in aging, as you’ve read. Instead, excess sugar, alcohol, smoke and processed foods full of chemicals seem to cause far greater damage to our DNA and our life span. Given that research and funding is limited, should more focus be given to finding gene treatments to lengthen our telomeres, or should focus and funding go to programs that encourage us to reduce behaviours that put our health and longevity at risk? Take a position on this statement and engage your classmates and your instructor in discussion.

Self-Check

Try reviewing this material by answering the following questions:

1. What three functions does mitosis serve in your body?
2. In which phase of mitosis does each of the following events occur?
   
   
   1. Migration of sister chromatids to opposite poles
   2. Condensation of chromatin into compact chromosomes
   3. Formation of a nuclear membrane
   4. Alignment of chromosomes along the cell equator
3. Sketch the four phases of mitosis. Include labels to explain what is happening in each phase.
4. What role does the spindle apparatus play in cell division?
5. Briefly explain the link between cell cycle regulation and cancer.
6. The scientists in a lab have isolated a substance that prevents cells from synthesizing microtubules. What impact would this substance have on cell division? Explain.
7. A scientist studying a group of somatic cells notices that upon the completion of the cell cycle, half of the daughter cells have no chromosomes and the other half have 92 chromosomes. In what phase of mitosis did an error likely occur? Explain your reasoning.

Discuss your answers with your instructor and save them to your course folder for review. These questions are also found on page 561 of your text.

Going Beyond

Pop a pill or change your life style.

In this lesson you were introduced to two forces that cause aging. One is built in to our cells, but the other was environmental. Environmental factors have been linked to at risk behaviour like overeating, alcohol consumption or smoking. However, a lot of research goes in to creating medications or treatments that combat the effects of these behaviours. Using the Internet, your local library, or other community resources you might have access to, conduct your own research to find out about new treatments or medications that are being introduced to the market to combat the negative effects of today’s lifestyle choices. Organize your findings into a presentation style of your choice and share it with your classmates.

Lesson Summary

During this lesson you explored the following focusing questions:

• Identify and describe the phases of mitosis.

• How does mitosis maintain consistency in plants and animals?

Mitosis is the orderly separation of doubled chromosomes into new daughter cells. Through prophase, metaphase, anaphase, and telophase the cell carefully ensures each new cell has a complete set of chromosomes. By this division,  plant or animal tissue lines are continued faithfully until time and exposure to the environment cause a breakdown with age.

Lesson 3.5.4

Lesson 4—Meiosis

Get Focused

There is an urban legend that everyone on the planet has a double somewhere. Someone you could be mistaken for. Could you really have an identical double out there running around and you don’t know about it? Well, genetically it is impossible, unless you were an identical twin at birth and were separated. The reason for this variation in individuals is the focus of this lesson: cell division by meiosis. Unlike mitosis where daughter cells are nearly identical, cell division or chromosome separation by meiosis creates unique daughter cells called gametes. How unique? Well an average human male will create about 525 billion sperm (male gamete cells) over a life time and not one will be the same genetically. Recall from the previous unit that a sperm must fertilize an egg (female gamete cell) to create a new person, and you will begin to understand the impossibility of there randomly being exactly two of you! This part of our life cycle, the creation of gamete cells, is all about variation. This variation ensures the survival of a species, as you will come to appreciate in Unit D.

In this lesson you will learn to describe the stages of meiosis, you will come to understand when meiosis is necessary, as well as how it differs from mitosis. You will learn the major sources of genetic diversity, and why this is important to a species.

In this lesson you will examine the following focusing questions

• How does meiosis contribute to genetic variation?
• What differences exist between fraternal and identical twins?

This lesson will take approximately 120 minutes to complete.

Module 5: Lesson 4 Assignment

You will complete Investigation 16.C: Modeling to Compare Meiosis and Mitosis, and complete the online assignment cloning for assessment. 

Bio30 3.5.4 online assignment

The other questions and activities in this lesson are not marked by the teacher; however, you should still attempt all of the work offered here. They are designed to help you review important information and build key concepts that may be applied in future lessons.

3.5.4 page 2

Explore

Here is a video to watch: 

Crash Course - Meiosis

Read

In the previous lesson, you learned how cells can replicate by mitosis. At the end of mitosis, you have two identical daughter cells with the same number of chromosomes as the parent cell. This is great for growth and repair, but it creates very little variation in a species if used for reproduction. As you may recall from Biology 20, variation is what drives natural selection and allows a species to survive.

Meiosis

In order to create variety within the species, many organisms turn to meiosis and fertilization, also called sexual reproduction, to create offspring. The main goals of meiosis are to create cells with half the normal chromosome number and to vary the combinations of genes present on those chromosomes. 

Reduction Division

In Lesson 1, you learned that humans have 46 chromosomes organized into 22 homologous pairs and one sex pair. Homologous pairs have the same type of genes but may not have the exact same forms of genes as each other. For example, they may each carry the gene for blood type, but one chromosome codes for Type A and the other codes for type B. Cells that have homologous pairs have two complete sets of genetic information. These cells are known as diploid or 2n.

The cells that result from meiosis have only one complete set of genes and are known as haploid, or n. The number of genetic sets in a cell is referred to as its ploidy count. In humans, all of our body cells, called our somatic cells, are diploid (2n). Only our gametes, our sperm or eggs, are haploid (n).

To prepare for meiosis, the cell duplicates it’s chromosomes in S phase. Then it goes through two division cycles; meiosis 1 and meiosis 2. The goal of meiosis 1 is to separate homologous pairs of chromosomes. This will reduce the number of chromosomes by half. The goal of meiosis two is to pull apart the sister chromatids, similar to mitosis.

To learn each specific stage in meiosis and to see how each one functions, read pp. 564-565 of your textbook. Pay close attention to Prophase 1. A lot of very important work occurs in Prophase 1 of meiosis. Here, homologous chromosomes come together and find their pair in a process called synapsis. Since each chromosome is made up of two sister chromatids there will be four chromatids together in a pair. The temporary bundle they form is called a tetrad.

When meiosis 1 is complete, the chromosomes number has been reduced, but they are still made up of two chromatids, or doubled. Meiosis 2 follows a pattern exactly like mitosis and separates the two chromatids into new cells.

The final result of meiosis is four haploid cells that have originated from one diploid cell. In humans, that means the starting cell has 46 chromosomes, and the resulting cells, known as gametes have 23 chromosomes.

Try This

To emphasize the difference in chromosome movement in meiosis, complete this simple interactive flash quiz. In this quiz you will choose either mitosis or meiosis. Then you will need to separate the chromosomes or chromatids accordingly.

3.5.4 page 3

The diagram above uses colour and size to clarify differences in chromosomes. Homologous chromosomes are the same size and have the same kinds of genes. They are different in colour to illustrate that they contain different forms of genes. These different forms are known as alleles.

Since there can be many alleles on each chromosome it is common to represent alleles with letters instead of colour. So an Upper case “B” and a lower case “b” shown on a chromosome would be different alleles of the same gene (since they are the same letter). Homologous chromosomes are ones that have the same letters, but the case may vary. The significance of this practice of using upper and lower case letters will be reviewed in a later lesson when we study the results of potential crosses.

Read

Sources of Genetic Variation—Crossing Over

Meiosis goes further than simply shuffling the genetic deck. It actually makes new cards! Recall that during Prophase 1 of meiosis the homologues pair up and form close groups called tetrads. Those groups are so close and so tight that sometimes small pieces from two neighboring chromatids will break off and fuse with the other chromatid. This is known as crossing over, and it results in a chromosome with a gene combination never before seen. Read the description of crossing over on p. 566 and consider the illustration in figure 16.14. As you can see without crossover, there would be no chromosomes with an Upper case A and a lower case b possible.

Watch and Listen

Meiosis has three key components as highlighted above; reductional division, crossing over during synapsis, and the independent assortment of homologous chromosomes. The following animation is a good summary of those features. While you are watching, pay close attention to how the resulting gametes are different from their parent cell. You may wish to make summary notes, a flow chart, or a labeled diagram to summarize this information. Store this information in your course folder.

Unique features of Meiosis

Try This

Although the end results of mitosis and meiosis are very unique, they do follow similar steps when it comes to sorting chromosomes. It is very important that you have a clear understanding of the differences of these types of cellular division. The following animation shows a step-by-step comparison of mitosis to meiosis when starting with the same cell.

Self-Check

Work through the following meiosis tutorial and quiz. Each problem has hints and explains why a given answer is correct. If any question gives you trouble, ask your instructor for clarification.

Read

© Stephen Mcsweeny/shutterstock

You’ve just read about how meiosis creates unique gamete cells. This important part of the human life cycle ensures variation within our species. Given the incredible amount of variation that can result, how is it that identical twins exist? Is it some kind of faulty meiosis?

Identical twins start from a single egg and a single sperm. Those gametes unite to form a single diploid zygote. As the zygote begins to divide through mitosis, a disruption may occur and cause the creation of two cell masses instead of one cell mass. At this stage, cells have not begun to specialize. Any cell is capable of becoming anything in the human body. They are known as totipotent. Since they have not specialized, and conditions are perfect for growth and development, each new mass grows to become a full person. Since twins started from the same sperm and egg they will be genetically identical.

Fraternal twins are a different story. They are the result of two different eggs being released at roughly the same time. During fertilization each egg receives its own sperm nuclei, and the result is two separate zygotes. Each of these zygotes is genetically unique, and no more related than any other brother and sister.

3.5.4 page 4

Reflect and Connect

Meiosis is the part of our life cycle that gives rise to variety. Through the recombination of chromosomes, random assortment during separation, and finally random fertilization, it is clear that no two people will look alike. That is true as long as the production of that organism is a result of meiosis and fertilization. However, in the case of identical twins, the two separate organisms are actually the result of an anomaly in mitosis, not meiosis. The fertilization event has already occurred before the single mass separates into two new cell groups. From this point on, each cellular division occurs by mitosis. Mitosis is all about keeping the genetics the same, so identical twins result.

Self Check

Complete the questions 1-4 and 7-9 on p 572 in your textbook. You may wish to discuss your work with your instructor.

Try This

Discuss

In 1996 a sheep named Dolly made history as the first animal to be cloned from an existing adult. In many ways it was like making an identical twin, only much later in life. This was an amazing scientific development, and all kinds of groups began discussing how we could use this technology. Some advocated cloning people to help infertile couples. Others wanted to clone all kinds of livestock to make vast herds of perfect producers. However, some problems with this procedure became evident about six years after Dolly was born. She developed arthritis and lung cancer and was put down. Dolly’s breed of sheep normally lives 12 to 15 years. The cell she was created from was about six years old. Some speculate her chromosomes were already 6 years old and therefore really died of old age.

Retrieve your copy of Module 5: Lesson 4 Assignment that you saved to your computer earlier in this lesson. Complete Part 2. Save your completed assignment in your course folder. You will receive instructions later in this lesson on when to submit your assignment to your teacher.

Lesson Summary

During this lesson you were to concentrate on the following focusing questions:

• How does meiosis contribute to genetic variation?
• What differences exist between fraternal and identical twins?

Meiosis is the orderly separation of homologous chromosomes into haploid gametes. Through crossover events in Prophase 1, and through independent assortment in Metaphase 1, meiosis ensures the creation of unique gametes. When fertilization later occurs, the new offspring will have a genetic combination never before seen.

Fraternal twins result from the fertilization of two individual and different egg gametes by two individual sperm cells. Since meiosis ensures that each gamete is unique, they will each be genetically different from each other. Identical twins start out from the same sperm and egg. Early on in development, the cell mass splits into two, and from there onward, each mass grows by mitosis into a full person. Since mitosis does not create variation, these twins are genetically the same, or identical.

Lesson 3.5.5

Lesson 5—Cell Cycle Disorders and Genetic Testing

Get Focused

Chromosome replication and separation does not always go according to the patterns you’ve learned. Occasionally, the process falters, and the results can be dramatic. In this lesson, you will learn of common disorders resulting from improper cell division and consider the ethical issues involved in prenatal tests and working with embryonic cells.

In this lesson, the following focusing questions will be examined:

• How do chromosome disorders occur, and does their frequency increase with the age of the parent individual?
• How can embryonic cells be used, and what technologies exist to test the genetic condition of an unborn fetus, thereby allowing parents to know about the condition of their child prior to birth?

This lesson will take approximately 60 minutes to complete, but you will need an additional 60 minutes for work on the assessment projects.

Module 5: Lesson 5 Assignment

 Once you have completed all of the learning activities for this lesson, you can complete the online assignment.

Bio30 3.5.5 online assignment

The other questions and activities in this lesson are not marked by the teacher; however, you should still attempt all of the work offered here. They are designed to help you review important information and build key concepts that may be applied in future lessons.

3.5.5 page 2

Explore

Read

In order to consider common chromosome disorders in people, we will first look at the formation of human gametes.

Spermatogenesis

Recall from your studies in unit B that the male gamete is created in the testicles. Here, in the seminiferous tubules, diploid germ cells known as spermatogonia can either divide by mitosis for growth, repair, and replacement, or they divide by meiosis to create four haploid sperm cells (each with only 23 chromosomes). The process is outlined well in your text on page 569. As you read, you should make summary notes, a flow chart, a diagram, or any other form you choose to summarize this information for your course folder.

The process of creating sperm is known as spermatogenesis, and it occurs in males at around the rate of tens, or hundreds of millions of sperm produced per day once puberty is reached.

Read

Oogenesis

Gamete creation in females is similar to that in males in regards to its location, the female gonad, which is the ovary, and in the reduction of the chromosomal number. However, it also differs in several key ways. First, the two meiotic divisions have unequal cytokinesis, resulting in one larger cell, and three small cells that deteriorate and are reabsorbed. The small cells are called polar bodies. Since meiosis involves two chromosome separation events, oogenesis results in one large haploid (23 chromosomes) egg cell and three small polar bodies, which are also haploid.

Another key difference is timing. In males, spermatogenesis starts at puberty, and the process is very rapid. In females, oogensis starts before birth, but freezes or stops at prophase 1. Oogenesis remains at this stage until puberty. At the onset of puberty, one egg or primary oocyte will continue through meiosis 1, and then is released into the fallopian tubes for fertilization. This is the egg cell or secondary oocyte that is matured each month as part of the menstrual cycle you studied in unit B.

If the egg is fertilized, then it will complete meiosis 2, and the final reduced nucleus will fuse with the sperm nucleus to start a new life. The whole process is summarized in figure 16.16 on page 569 in your text. You should summarize this information in a chart, a flow chart, a labeled diagram or into summary notes for future reference when you are studying. Store your work in your course folder.

Read

Now that we’ve reviewed meiosis and gamete creation, you should better understand how errors could occur in the process. Sometimes homologous chromosomes or sister chromatids fail to separate when forming haploid gametes. This is known as nondisjunction. Nondisjunction leads to gametes with either too many or two few chromosomes. Examine figure 16.15 in your text on p. 567. It compares nondisjunction in meiosis I to nondisjunction in meiosis II. Which one leads to more gametes with an abnormal number of chromosomes?

Try This

Recall from Lesson 1 that chromosome counts are done by creating a karyotype of cells undergoing mitosis. From this picture of a cells chromosomes you can determine how many of each type of chromosome there are, and the gender of the person being tested. Some common disorders that result from non-disjunction are Down syndrome (trisomy 21) or Edward syndrome (trisomy 18). Trisomy means three, so in each of these cases, there is an extra chromosome present. Syndromes are conditions with a multitude of specific characteristics that can vary widely in severity.

For practice in analyzing karyotypes and in diagnosing the syndrome present, complete Lab 16.A. on page 554 of your text. Ask your instructor for a sample karyotype. After you have completed the analysis questions and conclusion, compare your results with a fellow student or discuss your work with your instructor.

3.5.5 page 3

Read

There is a dramatic increase in the incidences of nondisjunction in female gametes as a woman ages. For example, the chances of having a Down syndrome child at 25 is 1:1500, by age thirty the risk is 1:910, and by the age of 45 it is 1:30 a specific cause for this is unknown.

Given that the oocytes have been suspended in prophase I for so many years, it is likely that the apparatus to complete meiosis properly has deteriorated. New research also indicates that a woman’s womb can becomes less selective for genetically compromised embryos making it more likely to carry an embryo with a genetic disorder to term.

Genetic Testing

You may recall from your studies in unit B the various technologies used to test a fetus for genetic problems. Three common procedures are amniocentesis, cordiocentesis, and chorionic villi sampling. Each differs on when it can be done, which material it tests, and how quickly the answers come back. Each test is the same in terms of what they are looking for: an abnormal number of chromosomes.

Try This

Research the following genetic testing technologies by completing the following table, and share your work with your fellow students and your instructor.

Ethical Concerns

Science is very good at seeking knowledge. However, it is not the role of science or even scientists to decide what should be done with that knowledge. Instead, this role falls on the shoulders of society. What are some ethical concerns that can arise from prenatal genetic testing? What is the tension or debate that focuses on prenatal testing?

Module 5: Lesson 5 Assignment—Stem Cell Research

While some groups within society may object to prenatal tests because of the possible risks to the fetus, or because of the future actions parents may take in their pregnancy, the debate is focused on a possible action. Stem cell research may have greater ethical concerns because of the certain death it causes to the blastocyst from which these cells are taken.

3.5.5 page 4

Reflect and Connect

Cell division and chromosome separation does not always follow the normal pattern. Nondisjunction events can lead to too many or too few chromosomes in gamete cells, which leads to individuals with serious conditions. Currently, science can conduct tests to analyze the chromosome counts of an unborn fetus, but there are no genetic treatments to reverse the problem of nondisjunction. After the process of fertilization involving a gamete with trisomy or monosomy, the resulting zygote will have too many or too few chromosomes, and each daughter cell from that zygote will also carry that error since mitosis will faithfully repeat the chromosome count to all of the body’s somatic cells.

Discuss

Stem cell research holds much promise for finding new treatments to many serious diseases. By starting with young, undifferentiated cells, scientists may be able to generate any number of missing or malfunctioning cells in adults. However, the collection of these cells destroys the blastocyst from which they are taken. From your reading of the text on p. 527, and from your review of the video on Canada’s guidelines for stem cell research, take a position on whether or not Canada’s guidelines give enough freedom to scientists while still respecting ethical concerns. Discuss your position with your classmates.

Lesson Summary

During this lesson you were to focus on the following questions:

• How do chromosome disorders occur and does their frequency increase with age?
• How can embryonic cells be used, and what tests may be done on an unborn fetus?

Chromosomes may not separate properly in meiosis I or meiosis II. This is known as nondisjunction. When this occurs in a gamete that is later fertilized and becomes a new individual, the result is a person with a serious condition, like Down syndrome for instance.

Tests like amniocentesis, cordiocentesis and CVS can diagnose the presence of abnormal chromosome numbers in an unborn fetus. However, no treatments can be done to change chromosome numbers since the error is found in every cell of that new life.

Early embryonic cells are totipotent. As a result, they can become any type of cell present in the human body. It is this characteristic that causes them to be of particular interest to scientists who are trying to find cures for diseases caused by adult cells shutting down or stopping regular function. Society will need to decide upon the guidelines which will be govern how scientists are allowed use these amazing cells.

Lesson 3.5.6

Lesson 6—Variation in Reproductive Strategies

Get Focused

You probably don’t remember much about the time of your life when, for a short while, you lived as a sperm and an egg. It seems somewhat silly to talk about our life as a gamete, but for many life forms it is a major part of their life cycle. For example, moss gametes grow and divide to become free living organisms before they create a single cell that will fertilize another to become a new generation of moss. The drive to reproduce and survive has lead to some very interesting strategies.

In this lesson you will learn about the diversity among reproductive strategies for a range of organisms, as will you gain an appreciation for the variety of ways species balance their life cycles.

In this lesson the following focusing questions will be examined:

• What are the advantages or disadvantages of different reproductive strategies?
• Why do organisms vary their reproductive strategies?

This lesson will take approximately 80 minutes to complete.

Module 5: Lesson 6 Assignment

Here is a tutorial video for this lesson that you can watch if it suits your learning style.  Bio30 tut# 3.5.6 Alternation of Generations

3.5.6 page 2

Read

Recall from lesson one that there are two basic methods of cell reproduction. Mitosis, which creates two new daughter cells from a parent cell quickly but with little variation, and meiosis, which creates daughter cells with a reduced number of chromosomes that must fertilize or be fertilized by other cells to create varied offspring. If mitotic division leads to a new organism, it is generally referred to as asexual reproduction. If meiosis, followed by fertilization, leads to new offspring, it is known as sexual reproduction.

Asexual Reproduction

Sometimes reproduction can be very simple. Consider bacteria. They do not have chromosomes, and hold their entire DNA in a single loop. If they wish to divide, they duplicate their DNA loop, attach the ends of each to their cell wall and grow a new wall between the loops. This process is called binary fission and can be completed in as little as 20 minutes. This leads to large populations of bacteria, or prokaryotes, quickly. As a result, each organism is identical, and the whole population could be susceptible to a toxin or a change in the environment. This toxin, or environmental change could lead to the extinction of the population because no individual would have the genetic variation for resistance.

Another quick method common to small aquatic organisms is budding. In budding, the parent organism begins growing a new organism from its body through mitosis. The new organism, or the ‘bud’, then separates and becomes a new individual. In a similar way, many plants grow long horizontal stems that reach new areas and then grow new full plants on the ends of those stems. An excellent example you may have in your garden would be strawberry plants (we sometimes call them runners when they spread).

An artificial way to reproduce much like budding is to take a cutting of a plant and encourage it to root. This kind of reproduction is called fragmentation. In this way we are actually making a clone of the parent plant since the new cells will grow through mitosis. To review these and other asexual methods of reproduction read pp. 573-575 of your text.

While asexual reproduction is rare in animals, some unique cases are worth mentioning. You may be familiar with sea stars and their ability to reproduce through fragmentation. Flat worms can do it too. Even more strange, if a flat worm is cut only partway down the middle it will re-grow each half of the head that is missing and end up sharing a tail!,

Other animals can produce unfertilized eggs that will grow into whole organisms. This is called parthenogenesis. A queen bee does this to create more sterile male drones. Female Komodo dragons can produce offspring this way when there are no males present! If there are males present they can reproduce in the regular sexual way.

Try This

One of the advantages of asexual reproduction is the speed at which you are creating more offspring. To get an idea of how different the resulting numbers of offspring can be between sexual and asexual reproduction, try out this generation calculator. When you load the calculator, start with a simulation of 10 generation and only two offspring per generation.

1. Why is there such a big difference between asexual and sexual?
2. What happens if you increase the number of offspring per generation?
3. Is there a point where sexual reproduction approaches the high numbers attained by asexual reproduction?

Sexual Reproduction and Alternation of Generations

With sexual reproduction there are always two important processes: meiosis and fertilization. These processes act as gate keepers between life as a diploid organism and life as a haploid organism. Recall that diploid cells have two sets of chromosomes present while haploid cells only have one. Haploid cells do not contain homologous chromosomes.

Animals do alternate between diploid and haploid. However their life cycle is nearly completely dominated by the diploid generation. The haploid side does not grow and produce large cell masses through mitosis.

Plants alternate between haploid and diploid generations with growth on both sides. This variance between generations is known as alternation of generations. The diploid generation is referred to as the sporophyte generation, as it produces haploid spores to start the next generation. Each haploid spore grows through mitosis into a multi cellular structure referred to as the gametophyte. It is the gametophyte that actually produces male and female gamete cells that will fertilize to form a new sporophyte generation.

Ferns are a good example of an organism that spends much time in both parts of the life cycle. Consider the graphic here, or in your text on p. 576. Can you tell which generation is haploid and which is diploid? Remember, haploid means only one set of chromosomes, so no homologues , while diploid means to have two sets or (2n). What process changes an organism from haploid to diploid? If you guessed fertilization, you were right.Which process reduces the organism back to haploid? Meiosis is correct.

Some life forms will reproduce either sexually or asexually. Bacteria can grow extensions into neighboring bacteria and exchange genetic information. This is called conjugation, and it results in a new, genetically varied daughter cell. Yeast can also reproduce many times asexually, and if conditions are harsh, they will fuse with another yeast cell and form a spore through meiosis. This spore can resist drying out and may lay dormant for a long time. When conditions are right, the spore will germinate into haploid yeast cells that will likely colonize through budding (mitosis).

Try This

What are the potential advantages and disadvantages of each type of reproduction? Read this section on pp. 578–579 of your textbook. Create a summary table for each type of reproduction and save it in your course folder for review.

Watch and Listen

Watch the following video on asexual reproduction and the alternation of generations. It will review mitosis and meiosis and go through the reproductive strategies we have introduced above. Compare their lists of advantages and disadvantages with the list you just created in the Try This.

Alterations of Generations: 

3.5.6 page 3

Self-Check

Here is a chance to check your understanding. Complete the following questions on alternation of generations and then check your answers. If any give you trouble, ask your instructor for clarification about that concept.

Carefully study this illustration and then answer the question that follows.

1. What kind of cell division is involved in the budding process? Explain.

2. When a yeast cell undergoes meiosis how many ascospores are produced? Is this consistent with what you would expect for this kind of division? Explain.

3. The life cycle of yeast can be divided into two phases. Which one is the sexual phase and which one is the asexual phase?

4. What seems to trigger sexual reproduction in yeast cells?

5. What kind of cell division is involved in the production of gametes in mosses? Explain why.

6. Identify the units produced by the reproductive process in mosses and the structure where meiotic divisions occur.

7. Why do mosses need moist conditions to reproduce?

8. With regards to dispersal, what is the advantage of producing spores?

9. Is the dominant stage in a moss’s life cycle haploid or diploid?

10. Use the fern life cycle to answer the following questions:

a. Is the dominant stage of a fern a gametophyte or a sporophyte?

b. Are the cells of a fern diploid or haploid?

c. Do ferns produce gametes or spores for reproduction?

d. Are spores produced by meiosis or mitosis?

e. Are spores haploid or diploid?

f. Is a small prothallus a gametophyte or sporophyte?

g. Does the heart-shaped prothallus produce gametes or spores?

h. Does the zygote grow into a sporophyte or a gametophyte?

3.5.6 page 4

Lesson Summary

In this lesson the following focusing questions were examined

• What are the advantages or disadvantages of different reproductive strategies?
• Why do organisms vary their reproductive strategies?

Organisms can reproduce quickly and effectively with mitosis and asexual reproduction. The resulting offspring may takeover a habitat and the whole species will benefit by a sheer growth in numbers. However, the entire population will be genetically identical. They may thus all be susceptible to a virus or disease and experience rapid death.

To balance the need for high numbers of offspring and to reproduce quickly, many organisms include a part of their life cycle that requires meiosis and sexual reproduction. This increases genetic variability and allows for offspring that are different from their parents. Some of these organisms may not be susceptible to a disease or a change in the environment that hurts the general population. These organisms will live to reproduce again.

Some organisms can change their strategies depending on conditions. This allows for the best of both reproductive worlds. When conditions are good, mitotic or asexual reproduction can quickly help colonize an area. However, when the conditions become unfavorable, meiosis or sexual reproduction will allow for genetic variation that will help the species survive.

Diploma Connection

Answer the following questions from a previous Biology 30 Diploma Exam.

Use the following information to answer the next question.

- from Campbell, 1993

1.   Which structures in the life cycle of the Ulva are haploid (monoploid)?

A.  Zoospores and the zygote

B.  The sporophyte and the zygote

C.  Zoospores and the gametophytes

D.  The sporophyte and the gametophytes

Use the following information to answer the next two questions.

2.   In humans, what process must have occurred to obtain the cells at U?

A.  Mitosis

B.  Meiosis

C.  Fertilization

D.  Differentiation

3.   In humans, what process must occur before cell V forms cells W and X?

A.  Mitosis

B.  Meiosis

C.  Recombination

D.  Nondisjunction

Use the following information to answer the next question.

Numerical Response

4.   Identify the stages in the conifer life cycle, as numbered above, that correspond with the letters that represent these stages on the diagram.

Stages:                       _____             _____             _____             _____

Diagram:                    A                     B                     C                     D

Use the following information to answer the next question.

5.   The row below that identifies the chromosome number at the first stage and the chromosome number at the second stage is

Module Summary

Module Summary

Throughout this module you will learn about cell growth, repair and reproduction. By understanding the life cycle of your cells, you will gain insight into how your own body renews itself over the course of your lifetime. As you get older you will understand the differences between combating the signs of aging, and actually keeping your cells in a healthy cell cycle of growth and repair. You will better understand how cancer can lead to serious health concerns, and how various treatments show promise in combating this disease. Finally, by examining emerging technologies involved in cell research, you will be better prepared to help society guide scientists in setting limits or guidelines for new research aimed at combating serious illness and disease. 

Module Assessment

 The assessment for this module includes the following assignments.

Bio30 3.5.1 online assignment

Bio30 3.5.2 online assignment

Bio30 3.5.3 online assignment

Bio30 3.5.4 online assignment

Bio30 3.5.5 online assignment

You should also submit a tutorial summary for each of the tutorial videos for this module .