BSC 314 Lecture Notes - Lecture 35: Genetic Recombination, S Phase, Telophase
27 Jun 2018
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Sexual Reproduction: Meiosis
A second type of cell division called meiosis takes place in multicellular eukaryotes.
This is a reduction division in which the daughter cells receive exactly half the number
of chromosomes of the mother cells.
Meiosis occurs in the production of gametes—the sperm of the males and the eggs of the
females. When a sperm fertilizes an egg, a zygote is produced with the appropriate number
of chromosomes for the species—in humans (and potatoes) the zygote and the somatic
(body) cells produced from it have 46 chromosomes. This is the diploid (2n) number of
chromosomes, half of which have come from the sperm nucleus, half from the egg. The
sperm and egg are haploid ( n); they carry half the number of chromosomes of the body
cells (in humans, 23 in each sperm and egg). Meiosis thus makes it possible to maintain a
constant number of chromosomes in a species that reproduces sexually by halving the
number of chromosomes in the reproductive cells. Meiosis uses many of the same
mechanisms as mitosis and is assumed to have been derived from mitosis after the latter
procedures were in place in some early organisms millenia ago.
Figure 1 shows the stages of mitosis, and Figure 2 shows the stages of meiosis. Note that
the names for the stages are the same as those of mitosis, with the addition of a numeral to
designate either the first or the second divisional stage. Both divisions are part of meiosis; not
until the final four daughter cells are produced is the process complete.
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