Module 5

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

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In Lesson 3 you discovered that mitotic division results in two identical daughter cells with the same number of chromosomes as the parent cell. Mitosis is efficient for growth and repair, but it creates very little variation in a species if used for reproduction. In Biology 20, you learned that variation drives natural selection and allows a species to survive.

 

Variation is the result of a species that reproduces sexually. Sexual reproduction involves meiosis and fertilization. Meiosis creates cells with half the normal chromosome number and varies 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 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 the body cells, called somatic cells, are diploid (2n). Only the gametes, sperm or eggs, are haploid (n).

 

 

To prepare for meiosis, the cell duplicates its chromosomes in S phase. Then it goes through two division cycles: meiosis I and meiosis II. The goal of meiosis I is to separate homologous pairs of chromosomes. This will reduce the number of chromosomes by half. The goal of meiosis II is to pull apart the sister chromatids. This same event was a goal in mitosis when sister chromatids were separated.

 

To learn about each specific stage and function in meiosis, read pages 563 to 565, up to “Sources of Genetic Recombination,” in your textbook. Pay close attention to prophase I. A lot of very important work occurs in prophase I 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, when homologous chromosomes come together in a pair, there are four chromatids. The temporary bundle they form is called a tetrad.

 

 

 

 

When meiosis I is complete, the chromosome number has been reduced, but they are still made up of two chromatids, or doubled. Meiosis II follows the pattern of steps you studied in mitosis and separates the two chromatids into new cells.

 

The final result of meiosis is four haploid (n) 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.

 

 

Watch and Listen

 

To ensure your understanding of meiosis, watch the “Stages of Meiosis” animation. Note the movement of chromosomes and compare the resulting haploid gametes to the original diploid parent.

 

Try This

 

TR 1. To find an excellent animation illustrating the concepts of mitosis and meiosis, do an Internet search using terms such as “Life’s Greatest Miracles” + “How Cells Divide” + “mitosis versus meiosis.”

 

TR 2. To emphasize the different ways chromosomes move in meiosis, do another Internet search using the term “biology activities and exhibits online + cell division exercise.” When you get to the Biology in Motion website, click on “cell division excercise” and complete the activity on mitosis and meiosis.

 

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