Meiosis in a diploid organism
The
diagram shows meiosis in
a
diploid cell with 2n = 2: there are two chromosomes,
each with one chromatid and
a centromere.
The cell is heterozygous at
the
A
locus (Aa). In the
second
stage, as in mitosis, the DNA
replicates
as
part of chromosome duplication: the two chromatids each
remain
attached at a single centromere. In the third stage, a tetrad,
composed of two chromosome pairs (homologs) that
include four chromatids,
lines up at the cell equator. In the fourth stage, each chromosome
is
pulled to one or the other cell pole: note that the
chromatids do not
separate, so that each daughter cell receives
two copies of one or the other
allele (A or a).
In the fifth stage, in each daughter cell, the two
chromatids attached
to each centromere separate to opposite poles (without DNA replication), so
that each of
the new daughter cells receives identical alleles (either a [above] or A [below)]. This
produces a total of
four haploid gametes.
In males, all four haploid cells typically mature as sperm. In females, cell division at the fourth stage typically produces one normal-sized cell and a small cell called a polar body, which does not mature further. The reduction division of the remaining cell also produces two unequal-sized cells, one of which (the second polar body) does not mature. Thus each meiosis in a female produce only a single haploid egg with one or the other allele. However, other meioses produce single eggs with the alternatiive allele, so that overall the number of eggs with each allele is equal.
In males, all four haploid cells typically mature as sperm. In females, cell division at the fourth stage typically produces one normal-sized cell and a small cell called a polar body, which does not mature further. The reduction division of the remaining cell also produces two unequal-sized cells, one of which (the second polar body) does not mature. Thus each meiosis in a female produce only a single haploid egg with one or the other allele. However, other meioses produce single eggs with the alternatiive allele, so that overall the number of eggs with each allele is equal.