Meiosis

Meiosis is a special form of nuclear division which occurs only in the ovaries and testes. This form of division results in daughter cells with only half of the normal number of chromosomes; they are haploid. These daughter cells are ova (sing. - ovum) (eggs) and sperm. With the union of sperm and ovum at fertilization, the full number of chromosomes (diploid) is reestablished in the new individual. Without meiosis, union of sperm and ovum would produce a new individual with twice as many chromosomes as the previous generation. Meiosis is divided into eight phases.

Prophase I. Like prophase of mitosis (see chapter 4) the nuclear membrane disappears, and the chromosomes appear. Unlike mitosis, however, chromosomes of the same type pair up together to form twenty three pairs of homologous chromosomes. Since each chromosome has two chromatids (strands), each pair has four chromatids and is called a tetrad. During this phase parts of one chromatid may exchange with the identical part from another chromatid. This process is called crossing over and results in a unique mixture of genes.

Metaphase I. In metaphase I the chromosome pairs line up in the middle of the cell. This is different from the metaphase of mitosis where the chromosomes do not line up in pairs.

Anaphase I. In anaphase I the chromosomes in each pair are separated. The chromatids of each chromosome remain attached by the centromere. In contrast the chromatids of mitosis separate.

Telophase I. Like telophase in mitosis the nuclear membrane reforms. Cytokinesis (cell division) always follows telophase I of meiosis.

Telophase I may be followed by a brief interphase. Usually the daughter nuclei move directly into prophase II. The stages of prophase II, metaphase II, anaphase II, and telophase II are basically the same as in mitosis. The chromosomes line up in the middle of the cell, the chromatids are pulled to each pole (and then each is called a chromosome), and then the nuclear membrane reforms. Cytokinesis always occurs during telophase II of meiosis.

The result of meiosis is four daughter nuclei, each different from the parents and from each other because of the crossing over which occurred in prophase I. The four nuclei are contained in four cells produced as a result of the two cytokineses.

In males each of the four cells forms into a sperm. In females the cell divisions are unequal. The first cytokinesis produces one large cell (the secondary oocyte) and one small cell termed a polar body. The secondary oocyte divides again to form the ovum and another polar body. The original polar body divides to form two polar bodies. The three polar bodies degenerate. In human females the second meiotic division occurs years after the first meiotic division.

Exercise 35.1

Study figure 35.1- Label the stages of meiosis. Crossing over is not shown in order to simplify the illustration. Crossing over is shown in your textbook.

Figure 35.1 Meiosis

Anaphase I Anaphase II Centriole Chromatid Interphase Metaphase I Metaphase II Prophase I Prophase II Spindle Telophase I Telophase II Tetrad

1. _ How many stages are in meiosis?

2. _ What two events make prophase I of meiosis different from prophase of mitosis?

4. _ In humans the diploid number is 46, what is the haploid number?

5. _ How many sperm result from one meiosis and cell divisions?

6. _ (6) chromosomes are chromosomes which are alike.

7. _ (7) assures that the chromosomes of children are different from each other and from the parents.

8. How is anaphase different in meiosis I and mitosis?

9. _ Cytokineses in conjunction with meiosis in females results in the production of one ovum and three (9) .

10. _ Does Figure 35.1 best illustrate meiosis in a male or a female?

THE REPRODUCTIVE

SYSTEM

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