Stereochemical and dynamic aspects of genetic recombination.
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The conformational features of three key intermediates in the gene conversion pathway are described. We have found that the dimensions of the trans turned structure involved in crossover are incompatible with normal H-bond formation occurring in opposing strands within the confines of 23 A axially separated double helices. However, if the separation is reduced to 18 A, slight rotation around the axis can give rise to crossover. A mechanism is proposed in which the crossover junction for short sequences migrates by torsional oscillations. This process is rapid enough to permit strand exchange of 100 bases in less than a millisecond. It is shown that the rotational diffusion mechanism becomes rate limiting for the crossover processes involving longer sequences.
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