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.
