2. Meiosis and Gametogenesis

Interphase

Meiosis is preceded by a stage called interphase. In this stage, the DNA of the chromosomes is replicated, so that each cell contains two copies of each chromatid. This means that in a human cell, both copies of chromosome 1 are copied to produce 4 chromatids, both copies of chromosome 2 are copied to produce 4 chromatids, and so on. The cell also grows, and produces enough of the enzymes and structures required for meiosis during this interphase period.

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Meiosis I

Early in meiosis I, the chromosomes can be seen clearly microscopically. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. The homologous chromosomes now become arranged in the center of the cell, with the ends of each pair of homologous chromosomes facing opposite poles. The orientation of each pair of homologous chromosomes at the center of the cell is random.

This randomness, called independent assortment, is the physical basis for the generation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a human are originally inherited as two separate sets, one from each parent. One set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. These pairs line up at the midway point between the two poles of the cell, and their arrangement in regard to the two poles is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each pair is independent of the orientation of the other 22 pairs.

In each cell that undergoes meiosis, the arrangement of the chromosomes is different. There are two possibilities for orientation (for each pair); thus, the possible number of alignments equals 2ⁿ where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (2²³) possibilities. Other mechanisms not discussed in this class can increase the variation in each cell produced as well. Given both independent assortment and these other mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition.

Next, protein fibers in the cell pull the linked chromosomes apart. The fibers pull the chromosome to the opposite poles of the cell. At each pole, there is just one member of each pair of the homologous chromosomes, so only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though there are duplicate copies of the set because each homolog still consists of two “sister: chromatids that are still attached to each other. Cytokinesis, where the cell membrane pinches off in the center of the cell to separate it into two, now occurs, splitting each cell into two cells containing one full set of chromosomes.

Meiosis II

In meiosis II, the connected sister chromatids remaining in the haploid cells from meiosis I will be split to form four haploid cells. The two cells produced in meiosis I go through the events of meiosis II in synchrony. Overall, meiosis II resembles the mitotic division of a haploid cell.

First, the nuclear envelope breaks down and the chromosomes are clearly visible under a microscope. The sister chromatids then line up at the center of the cell. Protein fibers pull one of each pair of sisters toward the poles of the cell.The chromosomes arrive at opposite poles.. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four genetically unique haploid cells. At this point, the nuclei in the newly produced cells are both haploid and have only one copy of the single set of chromosomes. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of other sources of variation not discussed here.

Comparing Meiosis and Mitosis

Mitosis and meiosis, which are both forms of division of the nucleus in eukaryotic cells, share some similarities, but also exhibit distinct differences that lead to their very different outcomes. Mitosis is a single nuclear division that results in two nuclei, usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original. They have the same number of sets of chromosomes: one in the case of haploid cells, and two in the case of diploid cells. On the other hand, meiosis is two nuclear divisions that result in four nuclei, usually partitioned into four new cells. The nuclei resulting from meiosis are never genetically identical, and they contain one chromosome set only—this is half the number of the original cell, which was diploid (Figure 2).

The differences in the outcomes of meiosis and mitosis occur because of differences in the behavior of the chromosomes during each process. Most of these differences in the processes occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are bound together, experience chiasmata and crossover between sister chromatids, and line up along the metaphase plate in tetrads with spindle fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I, never in mitosis.

Homologous chromosomes move to opposite poles during meiosis I so the number of sets of chromosomes in each nucleus-to-be is reduced from two to one. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level in mitosis.

Meiosis II is much more analogous to a mitotic division. In this case, duplicated chromosomes (only one set of them) line up at the center of the cell with divided kinetochores attached to spindle fibers from opposite poles. As in mitotis, one sister chromatid is pulled to one pole and the other sister chromatid is pulled to the other pole during meiosis II. Meiosis II is not a reduction division because, although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.

Cells produced by mitosis will function in different parts of the body as a part of growth or replacing dead or damaged cells, but cells produced by meiosis will only participate in sexual reproduction.