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UNIT SIX
First Meiotic Division
Prophase I.
Individual chromosomes appear as thin
threads in the nucleus, then shorten and thicken. Nucleoli
disappear, the nuclear membrane temporarily disassembles,
and microtubules begin to build the spindle that will sepa-
rate the chromosomes. The DNA of the chromosomes has
already been replicated.
As prophase I continues, homologous chromosomes pair
up side by side and tightly intertwine. During this pairing,
called
synapsis,
the chromatids of the homologous chro-
mosomes contact one another at various points along their
lengths. Often, the chromatids break in one or more places
and exchange parts, forming chromatids with new combina-
tions of genetic information
(f g. 22.2)
. One chromosome of
a homologous pair is from a person’s mother and the other
is from the father, so an exchange, or
cross over,
between
homologous chromosomes produces chromatids that contain
genetic information from both parents.
Metaphase I.
During the first metaphase, chromosome
pairs line up about midway between the poles of the devel-
oping spindle, and they are held under great tension, like two
groups of people playing tug-of-war. Each chromosome pair
consists of two chromosomes, which equals four chroma-
tids. Each chromosome attaches to spindle F bers from one
pole. The chromosome alignment is random with respect to
maternal and paternal origin of the chromosomes. Each of
the 23 chromosomes contributed from the mother may be
on the left or the right, and the same is true for the paternal
chromosomes—it is similar to the number of ways that 23
pairs of children could line up, while maintaining the pairs.
Chromosomes can line up with respect to each other in many
combinations.
FIGURE 22.2
Crossing over mixes up genetic traits. (
a
) Homologous
chromosome pair, (
b
) chromatids cross over, (
c
) crossing over recombines
genes. The dif
erent colors indicate that one oF the homologous
chromosomes comes From the individual’s Father and one From the mother.
FIGURE 22.1
Sex cells are Formed by a special type oF cell division, meiosis. This illustration Follows two representative pairs oF homologous
chromosomes.
(23 chromosomes, each
with 2 chromatids)
Paired homologous
chromosomes
(46 chromosomes,
each with 2 chromatids)
(23 chromosomes, each
with 2 chromatids)
(23 chromosomes,
each chromatid now an
independent chromosome)
First meiotic
division
Second meiotic
division
(a)
(b)
(c)
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