Synthesis-Dependent Strand Annealing In Meiosis

In organisms that reproduce sexually, sex cells (gametes) are produced by the specialized cell division called meiosis, which halves the number of
chromosomes from two sets (diploid) to one (haploid). During meiosis, homologous DNA molecules exchange genetic material (in a process called
homologous recombination), thereby contributing to genetic diversity. In addition, a subset of recombinants, called crossovers, creates connections
between chromosomes that are required for those chromosomes to be accurately segregated.

Accurate segregation ensures that gametes contain one and
only one copy of each chromosome. Recombination is initiated by chromosome breakage. A regulatory process then selects a subset of breaks to be healed
by a mechanism that forms crossover recombinants. Many of the remaining breaks are healed to form so-called "noncrossover" recombinants (also
referred to as "gene conversions"). Until recently it was thought that both crossovers and noncrossovers were formed by nearly identical pathways;
which form arose was thought to depend on how the last enzyme in the pathway attacked the last DNA intermediate.

However, more recent observations
suggested noncrossover recombinants might arise by a mechanism involving less stable intermediates than those required to make crossovers. In this
week's issue of PLoS Biology Dr. Douglas Bishop, Dr. Melissa McMahill, and Dr. Caroline Sham show how a yeast strain was constructed that allowed
detection of a genetic signature of such unstable recombination intermediates. This strain provided evidence that meiotic crossovers and
non-crossovers do indeed form by quite different mechanisms.



Citation: McMahill MS, Sham CW, Bishop DK (2007) Synthesis-dependent strand annealing in meiosis. PLoS Biol 5(11): e299.
doi:10.1371/journal.pbio.0050299


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