Biology 12

Appendix 4 • MHR 557

phase, but here meiosis involves two division
steps. The first step, meiosis I, reduces the
number of chromosomes without separating
the chromatids. During the second division,
meiosis II, the chromatids are separated to
form one or more haploid gamete cells.

Spermatogenesis

Spermatogonia are diploid germ cells that
produce haploid sperm cells. In mammals and
many other organisms, spermatogonia divide by
mitosis to produce a dormant cell (that replaces
the parent cell) and some cells that will actively
divide by meiosis. This process is shown in
Figure A4.3.
During meiotic prophase, homologous
chromosomes come together and overlap in
synapsis. At this time, enzyme complexes
(recombination nodules) promote crossing over,
the exchange of genetic material between
homologous chromosomes. These synaptic
pairs remain together until they are separated
in anaphase I. Synapsis reduces the chances of
an unequal distribution of chromosomes, which
is also called nondisjunction.
At metaphase I, spindle fibres line the paired
chromosomes at the equatorial plane and
separate the chromosomes to opposite poles.
The daughter cells then proceed through the
steps of mitosis to form four haploid gametes,
which then mature to form sperm cells.

Oogenesis

The egg cell, or oocyte, has a different function
from the sperm cell. The egg cell provides
the energy and materials necessary to support
the growth and division of the embryo until
additional materials are available. As a result, egg
cells are relatively large and meiosis is modified
to help this accumulation of cytoplasm by
producing one haploid egg and two or three
small cells called polar bodies. This process is
illustrated in Figure A4.4 on the next page.
In most mammals, all the oogonia begin
oogenesis before the organism is mature — even
before birth. There are no reserve germ cells,
and the oocytes are arrested during prophase
in meiosis I. In human females, the hormones
LH and FSH (discussed in Chapter 6) trigger
the completion of the process by way of the
ovarian cycle.

Figure A4.3A brief overview of meiosis. Synapsis in

prophase I allows the exchange of genetic information

between homologous chromosomes.

During this arrested period, which may

last up to 50 years for humans, some DNA

sequences are transcribed to produce mRNA

and proteins that support several mitotic

divisions of the early embryo. Other proteins

Meiosis

homologues align

independently

synapsis and

crossing-over

occur

homologues separate

daughter

cells form

sister

chromatids

separate

daughter nuclei are not genetically

identical to parent cell