Unit A Lesson 12: Cell Reproduction

 Learning Targets

Inquiry Question: How do various cells reproduce?
Your hair grows longer. You get taller. Your muscles get bigger. You begin to produce gametes for reproduction. These are all possible because of cells reproducing.

At the end of this inquiry, you should be able to answer the following questions:

  • What is mitosis?
  • What is meiosis? 
  • What happens to DNA during the production of sex cells (gametes)?
  • What happens to DNA during the production of body cells (somatic cells)?
Page 46 to 48 in your textbook will help you answer these questions about cell reproduction.


Introduction




How does DNA replicate?

New cells are formed when one old parent cell splits into two new daughter cells. The new cells need all of their organelles to be present and working properly; such as the mitochondria, vacuoles, and nucleus. What must happen for the new cells to receive the correct number of chromosomes?

Cell division is of two types:  mitosis and meiosis.  Both processes have critical processes for the replication of DNA, but each process results in a different number of daughter cells with a different amount of genetic information.

Mitosis is a type of cell division that occurs in somatic cells (body cells).  Mitosis is the cell division responsible for the repair of any injuries and for keeping up with an organism’s growth rate.  It ensures that each somatic cell receives the correct amount of DNA.

Recall the final product of asexual reproduction.  It is an individual that is genetically identical to its parent!  However, if you split an organism in half, will not each cell only have half the genetic material when compared to its parent cell?

Prior to a cell splitting and producing two new cells, its DNA must first be doubled, this will allow the new cells to have the full and correct amount of DNA.  This process of cell division is called mitosis. 

Therefore, through the process of mitosis, two identical daughter cells are produced from one parent cell.  Mitosis is the process in which body cells (otherwise known as somatic cells ) are produced.  Recall that humans have 46 chromosomes.  Mitosis duplicates all 46 chromosomes to make 92, and then the cell splits into two daughter cells, with each containing 46 chromosomes.  Figure 1 is an example of a human body cell undergoing mitosis.

Question to ponder:
Have you ever scraped your knee so badly that a thick scab developed? Eventually, the scab falls off and soft new skin is unveiled. What is happening under the scab?

Answer:
Mitosis is responsible for growth and repair of cells in the body.  When you fall and cut your knee badly, the process of mitosis kicks into high gear.  Your body cells (in this case, skin cells) around the injury start to double their DNA and then divide to produce new skin cells.  Under the scab, new skin cells are knitting the wound closed.

Figure 1 - Mitosis is the splitting of one parent cell into two identical daughter cells.

Watch

What does DNA look like?  How does it function in the body? 
The video "DNA Structure & Testing: How Does DNA Replication Work?" addresses these questions.

 
 
Watch the video "What is DNA and How Does it Work?" to learn more about how genetic information gets passed from generation to generation.


Interactive


DNA and Mitosis

BrainPOP has videos for both DNA and Mitosis. After watching the videos, explore some of the other interactive links on these pages.

Click here to watch the video "DNA".
Click here to watch the video "Mitosis".

 You will need a username and password to access it.
  • Username: 0099
  • Password: students

Watch

Watch "Mitosis" to learn about this fascinating subject.



Meiosis is a special type of cell division that only occurs in very specific parts of the body. The cells produced through the process of meiosis, called gametes, only have half the DNA of a normal cell. But when two of these gametes successfully join during sexual reproduction they form a cell with a full amount of DNA, and a new organism forms.

Geneticists know that regular cells need the full number of chromosomes in their nuclei to perform their function.  However, something unique needs to occur before a sperm and an egg unite. 

We know that each species that exists on earth has an identifying set number of chromosomes. Humans have 46 chromosomes (23 pairs) in their somatic cells.  If the process of mitosis occurs in the reproductive organs, would the sperm and egg cells each have 46 chromosomes? What happens when they unite?

If two normal, somatic body cells came together the offspring would have 92 chromosomes.  Humans have only 46.  Therefore, a different type of cell division occurs in the reproductive organs to ensure that the proper number of chromosomes are passed down to the offspring.  This type of cell division, that produces daughter cells with half the number of chromosomes, is called meiosis. It occurs in the reproductive organs. In human males, the cells produced are the sperm cells in the testes. In females, the eggs cells are produced in the ovaries.


Figure 2 – Meiosis only occurs in the sex cells. There are four daughter cells produced, each with half of the genetic information of the parent cell.

Watch

The process of meiosis produces the male and females gametes (sperm and eggs). 

After meiosis occurs, the sperm and eggs each have half the number of chromosomes.  That way, when they pair together at fertilization, the offspring have the correct number of chromosomes!  In meiosis, the cell with double the amount of DNA will divide twice!


Watch the video “Meiosis”.

 
 
Watch the "Meiosis: the Great Divide".

Summary of Mitosis and Meiosis
       
Although mitosis and meiosis are both types of cell division, they are very different.
     
Where these processes occur in the body, their purpose and the number of daughter cells produced differs. Mitosis produces two daughter cells that are identical to the parent cell whereas meiosis produces four daughter cells each with variation and containing only half of the number of chromosomes as the parent cell.


Connections

Connections: Nature
>> Identical Twins

Identical twins are a mutation that occurs when the fertilized egg splits.  This is an example of a positive mutation in which the two products are genetic clones of one another.  Sometimes, the egg does not split all the way and conjoined (Siamese) twins result with the twins remaining attached to each other in some way.  Fraternal twins are different because two separate eggs and two separate sperms unite.  Fraternal twins can look a lot alike, but they are the same as any other siblings in their genetic makeup.



Watch

Watch "Identical Twins" to learn more about this fascinating mutation.

 
 
Watch "Twins Biology" to learn more about this fascinating mutation.

How does the information on asexual and sexual reproduction, genetic material, and cell division relate to the possible production of purple cows? Or should we say “cattle”?  What do we know about cattle?

  • They are mammals (they belong to the species “Bos primigenius”).
  • There are males and females. 
  • They reproduce sexually. 
  • Their somatic cells (body cells) undergo mitosis, but within the reproductive organs, meiosis would create sex cells with half the number of chromosomes.  

This information would definitely be necessary to create a purple cow!

Try It!

Practice Worksheet: Mitosis and Meiosis

  1. DOWNLOAD this practice worksheet "S9_UA_S3_L12_mitosis_meiosis". If you prefer to use a Google Drive or PDF version of the worksheet, click here. 

  2. Answer the questions and complete the chart. The chart has various species with various numbers of chromosomes in body cells (somatic cells) and sex cells (gametes).

  3. Complete the worksheet.  When you are satisfied with your responses you can check your work by clicking on the "SUGGESTED ANSWERS" button below.

Wait! Don't view the suggested answers first. This practice work is not for marks, it is meant to help you check your understanding. Check the answers AFTER doing the questions! Keep the practice worksheet for study purposes. If you don't understand something, contact your teacher!