Two types of cell division occur in humans and many other animals: mitosis and meiosis. Mitosis involves duplication of somatic cells in the body and is represented by the cell cycle (Fig. 6.7). Meiosis is limited to replicating germ cells and takes place only once in a cell line. It results in the formation of gametes or reproductive cells (i.e., ovum and sperm), each of which has only a single set of 23 chromosomes. Meiosis is typically divided into two distinct phases, meiosis I and meiosis II.
Similar to mitosis, cells about to undergo the first meiotic division replicate their DNA during interphase. During metaphase I homologous autosomal chromosomes pair up, forming a synapsis or tetrad (two chromatids per chromosome). They are sometimes called bivalents. They do, however, pair up in several regions. The X and Y chromosomes are not homologs and do not form bivalents. While in metaphase I, an interchange of chromatid segments can occur. This process is called crossing-over (Fig. 6.8). Crossing over allows for new combinations of genes, increasing genetic variability. After telophase I, each of the two daughter cells contains one member of each homologous pair of chromosomes and a sex chromosome (23 double-stranded chromosomes). No DNA synthesis occurs before meiotic division II. During anaphase II, the 23 double-stranded chromosomes (two chromatids) of each of the two daughter cells from meiosis I divide at their centromeres. Each subsequent daughter cell receives 23 single-stranded chromatids. Thus, a meiotic division of one cell forms a total of four daughter cells.
Meiosis, occurring only in the gamete-producing cells found in the testes or ovaries, has a different outcome in males and females. In males, meiosis (spermatogenesis) results in four viable daughter cells called spermatids that differentiate into sperm cells. In females, gamete formation or oogenesis is quite different. After the first meiotic division of a primary oocyte, a secondary oocyte and another structure called a polar body are formed. This small polar body contains little cytoplasm, but it may undergo a second meiotic division, resulting in two polar bodies. The secondary oocyte undergoes its second meiotic division, producing one mature oocyte and another polar body. Four viable sperm cells are produced during spermatogenesis, but only one ovum is produced by oogenesis.