HUMAN BIOLOGY

CeLL reprOduCtIOn 363

What does meiosis do?

• Meiosis is the first step in the formation of gametes—sperm
  and eggs—for sexual reproduction.


• Meiosis reduces the parental chromosome number by half,
  to the haploid number.


• In males, meiosis and gamete formation are called
  spermatogenesis. In females, these two processes are
  called oogenesis.

taKe-hoMe Message

The early stages of oogenesis unfold in a developing
female embryo. Recall from Chapter 16, however, that until
a girl reaches puberty, her primary oocytes are arrested in
prophase I. Then, each month, meiosis resumes in (usually)
one oocyte that is ovulated. This cell, the secondary oocyte,
receives nearly all the cytoplasm; the other, much smaller
cell is a polar body. Both cells enter meiosis II, but the
process is arrested again at metaphase II. If the secon dary
oocyte is fertilized, meiosis II continues. It results in one
large cell and one to three small polar bodies. The polar
bodies are “dumping grounds” for three sets of chromo-
somes so that the egg will end up with only the necessary
haploid number. The large cell develops into the mature
egg (ovum). Its cytoplasm contains components that will
help guide the development of an embryo.

sperm

male germ cell

A male germ cell develops into a primary spermatocyte
as it replicates its DNA. Both types of cell are diploid.
Meiosis I in the primary spermatocyte results in two
secondary spermatocytes, which are haploid.
Four haploid spermatids form when the secondary
spermatocytes undergo meiosis II.
Spermatids mature as sperm (haploid male gametes).

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Figure 18.9 Animated! Spermatogenesis is the process

that forms sperm. (© Cengage Learning)

A female germ cell (an oogonium) develops into a primary oocyte as it
replicates its DNA. Both types of cell are diploid.
Meiosis I in the primary oocyte results in a secondary oocyte and a first
polar body. The polar body is much smaller than the oocyte. Both cells are
haploid. Polar bodies typically degenerate.
Meiosis II in the secondary oocyte results in a polar body and an ovum,
or egg. Both cells are haploid.

egg

female germ cell

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Figure 18.10 Animated! Oogenesis is the process

that forms eggs. For clarity, only one polar body is

shown. (© Cengage Learning)

Oogenesis and spermatogenesis make gametes that are

available for fertilization. As you may recall from Section

16.6, fertilization restores the diploid number of chromo-

somes in the resulting zygote.

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