Fidelity and Target Location During Homologous Recombination : A Physicist's View

Homologous recombination plays pivotal roles in DNA repair and in the generation of genetic diversity from 
prokaryotes to man. To locate homologous target sequences at which strand exchange can occur within a 
timescale that a cell's biology demands, a single-stranded DNA-recombinase complex must search among an 
extraordinarily large number of sequences on a genome, by forming synapses with chromosomal segments of DNA. 
A key element in the search is the time it takes for the two sequences of DNA to be compared, i.e. the synapse 
lifetime. Here we visualize for the first time fluorescently-tagged individual synapses formed by RecA, a prokaryotic 
recombinase protein, and measure their lifetime as a function of synapse length and differences in sequence 
between the participating DNAs. Surprisingly, lifetimes can be ~10 seconds long when the DNAs are fully 
heterologous, and much longer for partial homology. Synapse lifetime increases rapidly as the length of a region 
of full homology increases above 30 bases. Few mismatches can reduce dramatically the lifetime of synapses 
formed with nearly-homologous DNAs. These results suggest the need for facilitated homology search mechanisms 
to complete successfully the location of homology within the timescales observed in vivo. 
The talk presupposes no previous knowledge of Biology. All terms will be introduced.