Module 5 Cell Division

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Crash Course - Meiosis

 

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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.