7.3 Meiosis

7.3 Meiosis

Meiosis is the nuclear division of a sexual life cycle. It consists of two divisions, meiosis I and meiosis II, and the cell does not copy its DNA between them. Starting from one diploid germ cell, meiosis produces four haploid nuclei. Those nuclei are not genetically identical to the parent cell or to one another. Meiosis I is the reduction division: homologous chromosomes separate, and the chromosome number is cut in half. Meiosis II separates sister chromatids. Using a centromere count, each nucleus is already haploid at the end of meiosis I, even though each chromosome is still duplicated. Meiosis II does not halve that count again.

In animals, meiosis produces haploid cells. How those cells are packaged as sperm or eggs belongs to reproduction, not to the chromosome movements in this section. Plants use meiosis to produce haploid spores rather than gametes; the spore then grows by mitosis.

Prophase I

Prophase I is the long stage that makes meiosis different from mitosis. Homologous chromosomes find each other and synapse, pairing along their length. The synaptonemal complex is the protein structure that holds the paired homologs in register. Because each homolog is already duplicated, the paired unit contains four chromatids. That unit is a tetrad, also called a bivalent.

Synapsis, tetrads, and chiasmata

While the homologs are synapsed, non-sister chromatids can break and rejoin. That exchange is crossing over. The exchange sites are visible as chiasmata (singular: chiasma). Crossing over occurs in prophase I. It does not occur in prophase II. Prophase II has no synapsis of homologs, because the homologs were already separated in meiosis I. After a crossover, the sister chromatids of a chromosome may no longer be identical, because one of them can carry a new combination of alleles.

Other events of prophase I resemble mitotic prophase. The chromosomes condense, a spindle forms, and the nuclear envelope breaks down. The features that mark this stage as meiosis are homolog pairing and crossing over.

Metaphase I and independent assortment

In metaphase I, tetrads line up on the metaphase plate. In each pair, the two homologs attach to microtubules from opposite poles. Sister chromatids of one homolog face the same pole and will move together. The orientation of one tetrad does not fix the orientation of another tetrad. That independent orientation is independent assortment.

Independent assortment shuffles whole chromosomes. For an organism with 3 chromosome pairs, the maternal and paternal members can be assorted into 8 different haploid combinations, which is 2 raised to the third power, before crossing over is considered. Crossing over adds more variety, because it creates new allele combinations on a single chromosome before the homologs are pulled apart. Alleles of genes on different chromosomes assort independently. Alleles of genes on the same chromosome do not always do so, because they can travel together unless crossing over separates them.

Anaphase I through the end of meiosis I

In anaphase I, homologous chromosomes separate. The maternal homolog of a pair moves toward one pole, and the paternal homolog moves toward the other. Sister chromatids do not separate in anaphase I. They stay joined at the centromere and travel together. Each pole receives a haploid set of duplicated chromosomes: one member of each pair, still made of two chromatids.

Telophase I and cytokinesis may follow. In many organisms the chromosomes decondense somewhat, nuclear envelopes reform briefly, and the cytoplasm divides into two haploid cells. In others the chromosomes move into meiosis II with little pause. Either pattern still uses the same chromosome movements. What must not happen is another round of DNA replication. There is no S phase between meiosis I and meiosis II.

Meiosis II

Meiosis II resembles mitosis, but it starts with a haploid set of duplicated chromosomes. In prophase II a spindle is in place and there is no new pairing of homologs. In metaphase II, sister chromatids line up on a metaphase plate, with kinetochores attached toward opposite poles. In anaphase II, sister chromatids separate and move apart. Each is then its own unduplicated chromosome. Telophase II reforms nuclear envelopes, and cytokinesis divides the cytoplasm.

From one diploid germ cell the usual result is four haploid cells. A direct check against mitosis: meiosis of a diploid cell yields four haploid cells, not two diploid cells.

A numeric check

Suppose a diploid germ cell has 6 chromosomes, which is 3 homologous pairs. DNA replication before meiosis produces 6 duplicated chromosomes, or 12 chromatids. The chromosome count is still 6 because the centromere count is still 6. Meiosis I sends 3 duplicated chromosomes to each pole. Meiosis II then separates the chromatids of those 3 chromosomes. Each of the four products has 3 unduplicated chromosomes. The haploid number is 3, half of 6. Crossing over and independent assortment make the four products genetically different from one another and from the starting cell.

Spores and gametes

The chromosome movements are the same idea in animals and in plants. An animal uses meiosis to produce haploid cells on the way to gametes. A plant inserts mitosis after meiosis: the haploid spore divides by mitosis and builds a haploid body, and gametes form later from haploid cells without another meiosis. Do not describe plant meiosis as the direct manufacture of sperm and eggs.

Two common wrong results are easy to reject once the divisions are separate in your mind. Two diploid nuclei are the result of mitosis in a diploid cell, not the result of meiosis. Crossing over placed in prophase II is also wrong, because the tetrad and the chiasma belong to prophase I, and sister chromatids are what meiosis II pulls apart.