Essential Cell Biology 5th edition

658 CHAPTER 19 Sexual Reproduction and Genetics
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FOUR CHROMATIDS IN A BIVALENT
maternal chromatids
paternal chromatids
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one of the maternal chromatids
one of the paternal chromatids
DOUBLE-STRAND
BREAK PRODUCED
BY RECOMBINATION
PROTEINS
NUCLEASE
DIGESTS 5′ ENDS
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STRAND EXCHANGE
DNA SYNTHESIS
CAPTURE OF
SECOND STRAND
ADDITIONAL DNA
SYNTHESIS FOLLOWED
BY DNA LIGATION
DNA STRANDS CUT
AT ARROWS, FOLLOWED
BY DNA LIGATION
DNA
double
helices
Figure 19−9 During meiosis I, non-sister chromatids in each
bivalent swap segments of DNA. The process begins when protein
complexes that carry out homologous recombination (not shown)
produce a double-strand break in the DNA of one of the chromatids.
(Here, the maternal chromatid has been broken, but the paternal
chromatid is equally vulnerable.) These proteins then promote the
formation of a cross-strand exchange with the undamaged chromatid.
When this exchange is resolved, each chromatid contains a segment
of DNA from the other. Many of the steps that produce chromosome
crossovers during meiosis resemble those that guide the repair of
DNA double-strand breaks in somatic cells (see Figure 6−31).
homologous recombination, a process in which two identical or very
similar nucleotide sequences exchange genetic information. In Chapter
6, we discussed how homologous recombination is used to mend damaged
chromosomes from which genetic information has been lost. This
type of repair uses information from an intact DNA double helix to restore
the correct nucleotide sequence to a damaged, newly duplicated sister
chromatid (see Figure 6−31).
A similar process takes place when homologous chromosomes pair during
the long prophase of meiosis I. In this case, the recombination occurs
between the non-sister chromatids in each bivalent, rather than between
the identical sister chromatids within each duplicated chromosome. In
the process, the maternal and paternal homologs can physically swap
homologous chromosomal segments, an event called crossing-over
(Figure 19−9).
Crossing-over is a complex, multistep process that is facilitated by the
formation of a synaptonemal complex. As the duplicated homologs pair,
this elaborate protein complex helps to hold the bivalent together and
align the homologs so that strand exchange can readily occur between
the non-sister chromatids (Figure 19–10). Each of the chromatids in a
duplicated homolog (that is, each of these very long DNA double helices)
can form a crossover with either (or both) of the chromatids from the
other chromosome in the bivalent.
axial cores
cohesin
sister chromatids of duplicated
maternal homolog
transverse
filaments of
synaptonemal
complex
100 nm
sister chromatids of duplicated
paternal homolog
BIVALENT WITH CROSSOVER BETWEEN
TWO NON-SISTER CHROMATIDS
Figure 19–10 The synaptonemal complex helps to align the duplicated homolog
pairs. The sister chromatids in the maternal (red ) and paternal (blue) homologs
are held together by a protein complex called the axial core (gray), which interacts
with the cohesins (green) that link the sisters together (see Figure 18−18). When the
duplicated homologs pair, the axial cores associated with each are pulled closely
together in a zipperlike fashion by a set of rod-shaped transverse filaments (yellow),
forming the synaptonemal ECB5 complex. m17.55/19.10
ECB5 e19.10/19.09