Sexual Reproduction: Meiosis
A second type of cell division called meiosis takes place in multicellular eukaryotes. This is a reduction division in which the daughter cells receive exactly half the number of chromosomes of the mother cells.
Meiosis occurs in the production of gametes—the sperm of the males and the eggs of the females. When a sperm fertilizes an egg, a zygote is produced with the appropriate number of chromosomes for the species—in humans (and potatoes) the zygote and the somatic (body) cells produced from it have 46 chromosomes. This is the diploid (2n) number of chromosomes, half of which have come from the sperm nucleus, half from the egg. The sperm and egg are haploid ( n); they carry half the number of chromosomes of the body cells (in humans, 23 in each sperm and egg). Meiosis thus makes it possible to maintain a constant number of chromosomes in a species that reproduces sexually by halving the number of chromosomes in the reproductive cells. Meiosis uses many of the same mechanisms as mitosis and is assumed to have been derived from mitosis after the latter procedures were in place in some early organisms millenia ago.
Figure 1 shows the stages of mitosis, and Figure 2 shows the stages of meiosis. Note that the names for the stages are the same as those of mitosis, with the addition of a numeral to designate either the first or the second divisional stage. Both divisions are part of meiosis; not until the final four daughter cells are produced is the process complete.
Meiosis and mitosis have many similarities. There are, however, several fundamental differences. Compare Figure 1 (mitosis) with Figure 2 (meiosis). In meiosis:
In Prophase I, homologous chromosomes come together in synapsis and form pairs called bivalents or tetrads (because there are four chromatids in the pair); each bivalent has two chromosomes and four tetrads.
In Metaphase I, bivalents align randomly on the equatorial plane, which means that each daughter cell has an equal chance of getting either the chromosome from the sperm or one from the egg.
In Anaphase I, the chromosomes separate, each with two chromatids, and move to opposite poles; each of the two daughter cells is now haploid ( n).
There is no S phase, and the chromosomes line up immediately in Metaphase II, their chromatids separate in Anaphase II and in Telophase II new cell walls form around the four haploid cells. (Events of the second division are similar to those of mitosis.)
Synapsis in Prophase I is a decisive interval in determining the inheritance of the daughter cells. At this time, genetic recombination can occur; that is, daughter cells may receive combined traits of their two parents rather than simply the trait from one or the other. This is possible because the phenomenon called crossing over often occurs when the chromatids lie together—segments containing similar alleles break apart and rejoin to the corresponding segment of the opposite chromatid, thus mixing the traits from individual parents.