Essential Cell Biology 5th edition

Meiosis and Fertilization
659
duplicated
paternal
homolog
duplicated
maternal
homolog
Figure 19−11 Crossover events create
chiasmata between non-sister chromatids
in each bivalent. (A) Schematic set of
paired homologs in which one crossover
event has occurred, creating a single
chiasma. (B) Micrograph of a grasshopper
bivalent with three chiasmata. (C) Schematic
of the three crossovers between the paired
homologs in (B). Each sister chromatid is
numbered. (B, courtesy of Bernard John.)
(A)
chiasma
(B)
(C)
4
3
1 2
By the time meiotic prophase ends, the synaptonemal complex has disassembled,
allowing the homologs to separate along most of their length.
But each bivalent remains held together by at least one chiasma (plural
chiasmata), a structure named after the Greek letter chi, χ , which is
ECB5 E19.11/19.11
shaped like a cross. Each chiasma corresponds to a crossover between
two non-sister chromatids (Figure 19−11A). Most bivalents contain more
than one chiasma, indicating that multiple crossovers occur between
homologous chromosomes (Figure 19−11B and C). In human oocytes—
the cells that give rise to the egg—an average of two to three crossover
events occur within each bivalent (Figure 19−12).
Crossovers that take place during meiosis are a major source of genetic
variation in sexually reproducing species. By scrambling the genetic constitution
of each of the chromosomes in the gamete, crossing-over helps
to produce individuals with novel assortments of alleles. But crossingover
also has a second important role in meiosis: it helps ensure that the
maternal and paternal homologs will segregate from one another correctly
at the first meiotic division, as we discuss next.
Chromosome Pairing and Crossing-Over Ensure the
Proper Segregation of Homologs
In most organisms, crossing-over during meiosis is required for the correct
segregation of the two duplicated homologs into separate daughter
nuclei. The chiasmata created by crossover events keep the maternal
and paternal homologs bundled together until the spindle separates
them during meiotic anaphase I. Before anaphase I, the two poles of the
spindle pull on the duplicated homologs in opposite directions, and the
10 µm
Figure 19−12 Multiple crossovers
can occur between the duplicated
homologous chromosomes in a bivalent.
Fluorescence micrograph shows a spread
of chromosomes from a human oocyte
(egg-cell precursor) at the stage where
both maternal and paternal homologs are
still tightly associated: each single long
thread (stained red ) is a bivalent, containing
four sister chromatids. Sites of crossingover
between the chromatids within each
bivalent are marked by the presence of
a protein (stained green) that is a key
component of the meiotic recombination
machinery. Blue staining marks the positions
of centromeres (see Figure 19−8). (From
C. Tease et al., Am. J. Hum. Genet. 70:
1469–1479, 2002.)