Details
In prophase of meiosis I, duplicated homologous chromosomes pair and align end-to-end. Crossover can occur between aligned chromatids, leading to exchange of homologous segments by homologous recombination.
Chromosome segregation through meiotic divisions leads to novel genotypes, first in gametes, then in offspring.
In the original Holliday model for homologous recombination, single-strand breaks occur at the same point on one strand of each parental DNA. Free ends of each broken strand then migrate across to the other DNA helix, where the invading strands are joined to the free ends they encounter. The resulting crossover junction is called a Holliday junction.
As each crossover strand reanneals to its original partner strand it displaces the original complementary strand ahead of it, causing the Holliday junction to migrate. This creates heteroduplex DNA segments.
Cleavage and rejoining to re-establish two separate DNAs occurs in two ways. This is most easily visualized by first rearranging the Holliday structure. DNA molecules with this apparent structure have been observed.
This structure can be resolved by cleavage in the horizontal plane, leading to two molecules that do not show crossover of markers in genes A and B.
If, instead, the same structure is cleaved in the vertical plane, both of the resulting recombinant molecules show crossover of markers in genes A and B. All products, regardless of cleavage plane, are heteroduplexes in the region of Holliday junction migration.
Read more about this topic: Holliday Junction
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