An explanation of fine structure map expansion in terms of excision repair

Summary

Fine structure map expansion is a marker effect which can be explained on the basis of the repair of mismatched bases in hybrid DNA. The chance of a mismatched base pair being corrected independently of a closely linked mismatched pair will sharply increase as the distance between the two sites becomes greater than the length of the DNA segment involved in the correction process. The consequences of this explanation are worked out and it is shown that, if it is true, the mapping curve should show three phases: an initial additive phase when the recombining sites are closely linked, a phase of increased slope—map expansion-and a final additive phase of reduced slope beyond the expansion region. Comparison of the initial and final slopes should yield information on the relation between gene conversion and crossing-over. Many sets of experimental data show a clear transition from the initial additive region to that of map expansion, but evidence for the predicted final phase is scanty, possibly because fine structure maps cover too small a region of the chromosome. Using data from genes with known products, estimates can be made of the minimum length of the DNA segments involved in correction. These are calculated as about 40 nucleotides in fission yeast and at least 130 nucleotides in Neurospora.