Module 5 Cell Division

3.5.1 page 2

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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.