Module 5 Cell Division

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.

 

 

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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.

 

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© 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.