Meiosis

In the eukaryotes (all living organisms except bacteria), there are two forms of cell division—mitosis and meiosis. Mitosis, described in chapter 4, is the basis for division of the single-celled fertilized egg, growth of an embryo, and all postnatal growth and tissue repair. It is essentially the splitting of a cell with a distribution of chromosomes that results in two genetically identical daughter cells. It consists of four stages—prophase, metaphase, anaphase, and telophase.

You may find it beneficial to review figure 4.13 (p. 144) because of the important similarities and differences between mitosis and meiosis. There are three important differences:

1. In mitosis, each double-stranded chromosome divides into two single-stranded ones, but each daughter cell still has 46 chromosomes (23 pairs). Meiosis, by contrast, reduces the chromosome number by half. The parent cell is diploid (2n), meaning it has 46 chromosomes in 23 homologous pairs (see fig. 4.15, p. 146), whereas the daughter cells are haploid (n), with 23 unpaired chromosomes.

2. In mitosis, the chromosomes do not change their genetic makeup. In an early stage of meiosis, however, the chromosomes of each homologous pair join and exchange portions of their DNA. This creates new combinations of genes, so the chromosomes we pass on to our offspring are not the same chromosomes that we inherit from our parents.

3. In mitosis, each parent cell produces only two daughter cells. In meiosis, it produces four. In the male, four sperm therefore develop from each original germ cell. The situation is somewhat different in the female (see chapter 28).

Why use such a relatively complicated process for gameto-genesis? Why not use mitosis, as we do for all other cell replication in the body? The answer is that sexual reproduction is, by definition, biparental. If we are going to combine gametes from two parents to make a child, there must be a mechanism for keeping the chromosome number constant from generation to generation. Mitosis would produce eggs and sperm with 46 chromosomes each. If these gametes combined, the zygote and the next generation would have 92 chromosomes per cell, the generation after that would have 184, and so forth. To prevent the chromosome number from doubling in every generation, the number is reduced by half during gametogenesis. Meiosis is sometimes called reduction division for this reason.30

The stages of meiosis are fundamentally the same in both sexes. Briefly, it consists of two cell divisions in succession and occurs in the following phases: prophase I, metaphase I, anaphase I, telophase I, interkinesis, prophase II, metaphase II, anaphase II, and telophase II. These events are detailed in figure 27.14, but let us note some of its unique and important aspects.

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Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Chapter 27 The Male Reproductive System 1033

Meiosis I (first division)

Meiosis II (second division)

Early prophase I

Chromatin condenses to form visible chromosomes; each chromosome has 2 chromatids joined by a centromere.

Sucsesiv Mejosis

Chromosome Nucleus

Centrioles

Chromosome Nucleus

Centrioles

Mid - to late prophase I

Homologous chromosomes form pairs called tetrads. Chromatids often break and exchange segments (crossing-over). Centrioles produce spindle fibers. Nuclear envelope disintegrates

Metaphase I

Tetrads align on equatorial plane of cell with centromeres attached to spindle fibers.

Anaphase I

Homologous chromosomes separate and migrate to opposite poles of the cell.

Telophase I

New nuclear envelopes form around chromosomes;

cell undergoes cytoplasmic division

(cytokinesis).

Each cell is now haploid

Tetrad

Spindle fibers

Equatorial plane

Metaphase I

Tetrads align on equatorial plane of cell with centromeres attached to spindle fibers.

Spindle Fibers Centromere

Centromere

Cleavage site

Tetrad

Crossing-over

Spindle fibers

Centromere

Equatorial plane

Spermatogenesis Saladin

Metaphase II

Chromosomes align on equatorial plane.

Cleavage site i <7$

Prophase II

Nuclear envelopes disintegrate again; chromosomes still consist of 2 chromatids. New spindle forms.

Metaphase II

Chromosomes align on equatorial plane.

Anaphase II

Centromeres divide; sister chromatids migrate to opposite poles of cell. Each chromatid now constitutes a single-stranded chromosome.

Telophase II

New nuclear envelopes form around chromosomes; chromosomes uncoil and become less visible; cytoplasm divides.

Final product is 4 haploid cells with single-stranded chromosomes.

Figure 27.14 Meiosis. For simplicity, the cell is shown with only two pairs of homologous chromosomes. Human cells begin meiosis with 23 pairs.

Saladin: Anatomy & I 27. The Male Reproductive I Text I © The McGraw-Hill

Physiology: The Unity of System Companies, 2003 Form and Function, Third Edition

1034 Part Five Reproduction and Development

In prophase I, each pair of homologous chromosomes line up side by side and form a tetrad (tetra denoting the four chromatids). One chromosome of each tetrad is from the individual's father (the paternal chromosome) and the other is from the mother (the maternal chromosome). The paternal and maternal chromosomes exchange segments of DNA in a process called crossing-over. This creates new combinations of genes and thus contributes to genetic variety in the offspring.

After crossing-over, the chromosomes line up at the midline of the cell in metaphase I, they separate at anaphase I, and the cell divides in two at telophase I. This looks superficially like mitosis, but there is an important difference: The centromeres do not divide and the chro-matids do not separate from each other at anaphase I; rather, each homologous chromosome parts company with its twin. Therefore, at the conclusion of meiosis I, each chromosome is still double-stranded, but each daughter cell has only 23 chromosomes—it has become haploid.

Meiosis II is more like mitosis—the chromosomes line up on the cell equator at metaphase II, the centromeres divide, and each chromosome separates into two chromatids. These chromatids are drawn to opposite poles of the cell at anaphase II. At the end of meiosis II, there are four haploid cells, each containing 23 single-stranded chromosomes. Fertilization combines 23 chromosomes from the father with 23 chromosomes from the mother and reestablishes the diploid number of 46 in the zygote.

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