Cambridge International A Level Biology Revision Guide

Chapter 16: Inherited change
Gametogenesis in humans
In animals such as humans, meiosis occurs as gametes
are formed inside the testes and ovaries. The formation of
male gametes is known as spermatogenesis (Figure 16.10)
and the formation of female gametes as oogenesis
(Figure 16.11).
Sperm production takes place inside tubules in the testes.
Here, diploid cells divide by mitosis to produce numerous
diploid spermatogonia, which grow to form diploid primary
spermatocytes. The first division of meiosis then takes place,
forming two haploid secondary spermatocytes. The second
division of meiosis then produces haploid spermatids, which
mature into spermatozoa.
Oogenesis follows a similar pattern, but many fewer
gametes are made than during spermatogenesis, and the
process takes much longer, with long ʻwaiting stagesʼ. It
takes place inside the ovaries, where diploid cells divide by
mitosis to produce many oogonia. These begin to divide by
meiosis, but stop when they reach prophase I. At this stage,
they are called primary oocytes, and they are, of course,
still diploid. All of this happens before a baby girl is born,
and at birth she has around 400 000 primary oocytes in
her ovaries.
When she reaches puberty, some of the primary
oocytes get a little further with their division by meiosis.
They proceed from prophase I to the end of the first
meiotic division, forming two haploid cells. However, the
division is uneven; one cell gets most of the cytoplasm, and
becomes a secondary oocyte, while the other is little more
than a nucleus, and is called a polar body. The polar body
can be thought of as simply a way of getting rid of half of
the chromosomes, and has no further role to play
in reproduction.
mitosis
diploid
mitosis
diploid
spermatogonia
oogonia
(produced in embryo)
371
growth
growth
meiosis
meiosis
primary spermatocyte
diploid
primary oocyte
(produced in embryo)
diploid
stop
secondary spermatocyte
haploid
secondary oocyte
(after puberty)
haploid
stop
polar body
spermatid
secondary oocyte
(one a month)
maturation of sperm
maturation of oocyte
(after ovulation)
spermatozoan
haploid
ovum
polar body
haploid
Figure 16.10 Spermatogenesis in humans.
Figure 16.11 Oogenesis in humans.