title: Recombination in the AM gene of Neurospora crassa--a new model for conversion polarity and an explanation for a marker effect.
abstract: It is argued that polarised intragenic recombination is not necessarily due to hybrid DNA extending into the gene for variable distances from one side; it can as well be explained by hybrid DNA usually covering the whole gene, the two complementary DNA strands of the gene having unequal chances of undergoing the interchromatid transfer involved in hybrid DNA formation, and excision from base-pair mismatches proceeding predominantly in one chemical direction (perhaps 5' to 3'). This alternative model for polarity is thought to be in better accord with the behaviour of flanking markers in the am data, which appears to show that conversion-associated crossing-over can occur with almost equal likelihood on either side of the gene (a feature previously held to support negative interference--Fincham, 1974). It also provides an explanation for an unusual marker effect involving two am mutational sites separated by only three base pairs. The nature of the sites (defined by amino acid replacement analysis) is such that one forms a purine-purine mismatch on the same chromatid as the other forms a pyrimidine-pyrimidine mismatch, and vice versa. They show an approximately two-fold difference in recombination frequencies in crosses to other am mutants mapping to the left, and a difference of similar magnitude, but opposite sign, in crosses to mutants mapping to the right.
mesh_terms: D003434:Crossing Over, Genetic; D004247:DNA; D005787:Gene Frequency; D005796:Genes; D008954:Models, Biological; D009491:Neurospora / Q000201:enzymology*; D009492:Neurospora crassa / Q000201:enzymology*; D009693:Nucleic Acid Hybridization; D011995:Recombination, Genetic
chemical_list: D004247:DNA