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Meiosis

 

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 (eggs and sperm) are produced by meiosis.

Figure 9-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:

  1. In the metaphase I, each pair of sister chromatids aligns with its homologous pair side by side, forming two lines of sister chromatids at the equatorial plane (there is only one line in mitosis).  Such alignment is called synapsis.
  2. In the subsequent anaphase I, each pair of sister chromatids moves away from its homologous pair, but the sister chromatids are not separated.

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.  

Independent assortment

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 maternal 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.

Figure 8-C-2.  Illustration of independent assortment.  Sperm cells (1n) come from the meiotic division of primary spermatocytes (2n).  In their 23 chromosomes, some of them (pink) are inherited from the maternal homolog and others (blue) are from the paternal homolog.  There are a total of 8.4 million (= 223) possible combinations, but only 4 of them are shown here.