Meiosis
Meiosis is a special form of cell division that only occurs
in the post-pubertal testis and the fetal and adult ovary
(Fig. ). Meiosis differs from mitosis in two main ways;
there are two separate cell divisions and before the first

fig-3-6

Meiosis and gametogenesis. The main chromosomal stages
of meiosis in both males and females. A single homologous pair of
chromosomes is represented in different colours. The final step is the
production of haploid germ cells. Each round of meiosis in the male results
in four sperm cells; in the female, however, only one egg cell is produced,
as the other divisions are sequestered on the periphery of the mature egg
as peripheral polar bodies.

of these there is extensive swapping of genetic material
between homologous chromosomes, a process known
as recombination. The result of recombination is that
each chromosome that a parent passes to his or her offspring
is a mix of the chromosomes that the parent
inherited from his or her own mother and father. The

end products of meiosis are sperm and egg cells
(gametes), which contain only 23 chromosomes: one of
each homologous pair of autosomes and a sex chromosome.
When a sperm cell fertilises the egg, the resulting
zygote will thus return to a diploid chromosome complement
of 46 chromosomes. The sperm determines the
sex of the offspring, since 50% of sperm will carry an X
chromosome and 50% a Y chromosome, while each egg
cell carries an X chromosome.
The individual steps in meiotic cell division are
similar in males and females. However, the timing of the
cell divisions is very different (see Fig. ). In females,
meiosis begins in fetal life but does not complete until
after ovulation. A single meiotic cell division can thus
take more than 40 years to complete. In males, meiotic
division does not begin until puberty and continues
throughout life. In the testes, both meiotic divisions are
completed in a matter of days.