Models of Recombination

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Models of Recombination
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Models of recombination

• Initiation by nicking of DNA
• Exchange of single nucleotide strands between
  chromatids (DNA duplexes), which creates
  heteroduplexed areas.
• Mismatch repair of heteroduplexes or not.
• Resolution of the intermediate (reciprocal
  recomnination for flanking makers, or not).

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The Holliday model
(a) pair of chromatids
(b) a single strand cut is made in each chromatid
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(e) Branch migration occurs, giving regions of
heteroduplex DNA
(f) Resolution of the Holliday junction gives two
DNA molecules with heteroduplex DNA. Depending
upon how the Holliday junction is resolved, we
either observe no exchange of flanking markers
(left) or an exchange of flanking markers (right)

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If the heteroduplex is repaired, the result is
either a chromatid conversion or a normal
chromatid, depending on which allele is removed.
If the heteroduplex is not repaired, then when
the resulting DNA replicates, one daughter DNA
molecular is , while the other DNA molecular is
m. The result is a half-chromatid conversion
wherein only half the chromatid is converted.
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Summary of the Holliday model

• Single-strand DNA nick on both chromatids.
• Strand exchange generates the Holliday junction.

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A Modification of the Holliday ModelThe
Meselson-Radding model

• A single DNA strand is nicked.
• Strand displacement (invasions) and subsequent
  DNA synthesis generates the Holliday junction

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The Meselson-Radding model
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The Meselson-Radding model
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Double-Strand Break-Repair model

• A single double-strand break is generated in one
  chromatid (DNA molecule)
• Strand displacement (invasion) and subsequent DNA
  synthesis generates the Holliday junction.

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Double-Strand Break-Repair model
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Double-Strand Break-Repair model
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Proc. Natl. Acad. Sci. USA Vol. 94, pp.
5213-5218, May 1997 Genetics Clustering of
meiotic double-strand breaks on yeast chromosome
III Frédéric Baudat and Alain Nicolas
Institut Curie, Section de Recherche, Centre
National de la Recherche Scientifique, Unité
Mixte deRecherche 144, Compartimentation et
Dynamique Cellulaires, 26 rue d'Ulm, 75248 Paris
Cedex 05, France
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ABSTRACT In the yeast Saccharomyces
cerevisiae, meiotic recombination is initiated by
transient DNA double-strand breaks (DSBs) that
are repaired by interaction of the broken
chromosome with its homologue. To identify a
large number of DSB sites and gain insight into
the control of DSB formation at both the local
and the whole chromosomal levels, we have
determined at high resolution the distribution of
meiotic DSBs along the 340 kb of chromosome III.
We have found 76 DSB regions, mostly located in
intergenic promoter-containing intervals. The
frequency of DSBs varies at least 50-fold from
one region to another. The global distribution of
DSB regions along chromosome III is nonrandom,
defining large (39-105 kb) chromosomal domains,
both hot and cold. The distribution of these
localized DSBs indicates that they are likely to
initiate most crossovers along chromosome III,
but some discrepancies remain to be explained.
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Figure 2. Location and amount of meiotic DSBs on
chromosome III.