MEIOSIS

In unit 11 you learned that mitosis was the division of a parent cell into two daughter cells which were identical to each other and the parent cell. In this unit you will learn about meiosis. In meiosis a parent cell will divide into daughter cells with half the number of chromosomes as the parent cell. The only cells which use meiosis are the sex cells, or gametes (sperm and egg). All other cells of your body go through mitosis to divide. You will see that many of the stages of meiosis are similar to mitosis, but, again, the important idea to remember about meiosis is that the chromosome number will be cut in half in the daughter cells.

Sexual Reproduction

To reproduce sexually, organisms need two cells to join. These two cells must have half the number of chromosomes as all other cells of the body so that when they unite the new cell will have a complete set of chromosomes for the new organism. These sexually reproducing organisms have meiosis to provide a way of producing these cells with half the number of chromosomes. Before you learn about the steps of meiosis, you need to learn some of the terminology that is used with sexual reproduction and meiosis.

“Cells”

The reproductive cells that meiosis produces are known as gametes. The gametes are sperm and egg. When sperm and egg unite, a complete set of chromosomes are formed in a process called fertilization. The new cell formed as a result of fertilization is called a zygote. This zygote is the beginning of the new individual. Because the zygote has traits of both parents (from the sperm and egg), the new organism will not be exactly like either parent. Cells that are specialized for producing gametes for sexual reproduction are known as germ cells. All other cells of the body are called somatic cells. Somatic cells have nothing to do with sexual reproduction. If somatic cells need to divide they will divide by way of mitosis.

Chromosome Number

All organisms have a unique number of chromosomes. A dog has 78 chromosomes, a frog has 26, and different species of ferns have over 500. Humans have 46 chromosomes. As you can see with these examples, the chromosome number has nothing to do with the complexity of the organism. However, the number of chromosomes is vital to the organism. Too many or too few chromosomes and the organism may not develop and/or function properly.

In terms of humans, we have 46 chromosomes which are made up of 2 copies of 23 types of chromosomes. One copy came from our father by way of the sperm and the other copy came from our mother by way of the egg. The cells (gametes) that have only one set of each type of chromosome are called haploid. The cells (somatic) that have two sets of each type of chromosome are called diploid. The symbol n is used to represent the number of chromosomes in one set of chromosomes, or how many there are of each type of chromosome. In humans, the haploid number is n = 23, because the human has 23 types of chromosomes. When you consider a somatic cell, it has 2 sets of 23 chromosomes so the diploid number is 2n = 46.

“Chromosomes”

When looking at a diploid cell you will find two of each type of chromosome. These two chromosomes of each type are described as homologous chromosomes. Homologous chromosomes are similar in size and shape, and the genes that they contain, however, the specific code of the gene will be different between the two chromosomes. Even though eye color is controlled by more than one gene we will use this as an example. On one of your mother’s chromosomes is the gene for eye color. On your father’s homologous chromosome (same size, shape, and genes as the mother’s) is also the gene for eye color. However, the gene for eye color on the mother’s chromosome may be for blue and the gene for eye color on the father’s may be for brown. They are homologous chromosomes because they have the same size, shape, and genes, although they have different forms of the gene.

When looking at the 46 chromosomes of the human, there are two types of chromosomes. Autosomes are chromosomes with genes that do not determine the sex or gender of an individual. Sex chromosomes are chromosomes with genes that do determine the sex or gender of an individual. The sex chromosomes are referred to as “X” and “Y” chromosomes. A male has the sex chromosomes “XY”, while a female has the sex chromosomes “XX”. Keep in mind, humans are diploid organisms and have two of each type of chromosome. So one “X” comes from the mother and the other “X” or “Y” comes from the father.

Some scientists can examine the chromosomes by looking at a karyotype. A karyotype is a picture of chromosomes. A human karyotype will show, after it has been prepared and arranged, all homologous chromosome pairs lined up from largest to smallest with the sex chromosomes placed last. Chromosome pairs 1 to 22 are the autosomes and pair 23 are the sex chromosomes.

Stages of Meiosis

We will now look at the stages of meiosis. This will explain how one diploid germ cell begins the process and becomes four haploid gametes with half the number of chromosomes. Meiosis involves two divisions of the nucleus called meiosis I and meiosis II. In meiosis I the homologous chromosomes are separated producing two haploid cells, but the sister chromatids are still attached. In meiosis II, the sister chromatids separate producing four haploid cells. Meiosis II is very similar to mitosis as you will see. Meiosis I involves four stages (prophase I, metaphase I, anaphase I, and telophase I), and meiosis II also involves four steps (prophase II, metaphase II, anaphase II, and telophase II).

Stages of Meiosis I
Prophase I	Metaphase I	Anaphase I	Telophase I
· Homologous chromosomes form tetrads. · Crossing-over occurs.	· Tetrads line up on the equator.	· Tetrads separate.	· Cytokinesis occurs.

Prophase I

In prophase I, the nuclear membrane disappears, centrioles move to opposite poles, spindle forms, and chromatin becomes chromosomes. Before prophase I, the germ cell DNA is copied. Prophase I involves the same events as prophase of mitosis with a couple more to add. Something unique to prophase I is the formation of tetrads (bivalent). Tetrads are formed when two homologous chromosomes pair up side-by-side. When tetrads have been formed crossing over can now occur. Crossing over is when homologous chromosomes exchange portions of genetic material while in a tetrad. Remember, homologous chromosomes are the same in size, shape, and genes, but the form of gene they have may be different. When crossing over occurs the gene forms switch from being associated with one set of genes on one chromosome to another set of genes on another chromosome. The sites where crossing-over occurs on the non-sister chromatids of the tetrad are known as the chiasmata. This event of crossing over provides variation in the genetic material being passed on.

Metaphase I

In metaphase I, the tetrads line up on the equator. It is not so much the centromeres lining up on the equator as it is now the tetrad pair lining up on the equator.

Anaphase I

In anaphase I, the tetrads separate. It is important to remember that the sister chromatids are still attached by the centromere. This phase is just separating the tetrad.

Telophase I

In telophase I, the homologous chromosomes, with their two chromatids still attached, are separated into two new nuclei. The spindle breaks down. A new nuclear envelope reforms around each set of chromosomes. In some cells the chromosomes will uncoil (decondense). Cytokinesis also occurs during this phase to produce two new cells. In the end of meiosis I, two cells are produced with half the number of chromosomes, but the problem is the chromosome is still made of two chromatids. Meiosis II is needed to split the centromere and separate the chromatids.

Stages of Meiosis II
Prophase II	Metaphase II	Anaphase II	Telophase II
· New spindle forms	· Centromeres line up on the equator.	· Centromeres separate.	· Cytokinesis occurs.

Prophase II

In prophase II, the nuclear membrane disappears, centrioles duplicate and move to opposite poles, and the spindle forms. The DNA (chromosomes) are not copied between Meiosis I and Meiosis II, as they were already copied before meiosis I. [Remember there are two cells going through meiosis II that came from the end of meiosis I].

Metaphase II

In metaphase II, the centromeres line up on the equator.

Anaphase II

In anaphase II, the centromeres split separating the chromatids to opposite poles. Once the centromere splits the chromatids are now called chromosomes.

Telophase II

In telophase II, a new nuclear envelope forms around the chromosomes. Cytokinesis also occurs during this phase to produce four new haploid cells (two new cells for each of the two cells produced at the end of meiosis I). In the end of meiosis II, four cells are produced with half the number of chromosomes from one diploid germ cell.

Unit 12 Meiosis Worksheet

UNIT VOCABULARY REVIEW

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Now answer questions 1 through 20.