Meiosis is a special type of cell division resulting in four genetically nonequivalent daughter cells, each containing half the number of chromosomes of the parent cell. Germ cells (egg and sperm) are produced by meiosis.
Figure 8-C-1. Illustration of meiosis.
The meiosis involves two consecutive cell divisions. Each division is composed of four stages as in mitosis. However, the first division differs in two important aspects:
After the first division, the chromosome number in each daughter cell becomes 2n. The process of the second division is the same as in mitosis except that there is no DNA replication during the interphase. As a result, the produced cell is a haploid (1n).
During synapsis, DNA recombination may occur between homologous pairs of sister chromatids. Details are given in Section D.
In the metaphase I of meiosis, two pairs of homologous sister chromatids align side by side. One pair (the maternal homolog) comes from the egg and another pair (the paternal homolog) comes from the sperm. Subsequently, the two homologous pairs are separated into two different daughter cells. Since they are randomly aligned, which homolog will enter which daughter cell is random. As a result, each daughter cell will contain some maternal homologs and some paternal homologs. For human cells, there are a total of 8.4 million (= 223) possible combinations.