Genetic recombination: Formal implications of a crossed strand-exchange between two homologous DNA molecules

Formal genetic consequences of generation and break-up of a structure with cross connection have been studied on the basis of a simple quantitative model with a symmetric exchange of two strands between parental DNA molecules. In recombination analysis, this intermediate structure gives rise to a critical frequency R(ξ) (see Fig. 3), which corresponds to recombination between two markers separated by a distance equal to the mean genetic length ξ of the hybrid region at the time of scission of the cross connection. As concerns the routine mapping practice, a fundamental inference emerges: since genetic exchange has no invariable equivalent on molecular level it fails basically to serve as statistically independent elementary event in fine scale mapping. Instead of the number of exchanges between markers (or proportional metrics) the use of the number of the sites of initiation of hybrid regions (or proportional metrics) as a measure of map distance is suggested. Usage of the mean length of hybrid region ξ as a natural unit of map distance is shown to be of particular interest.In the case of bacteriophage T4 under the conditions of standard crosses, the structure with cross connection is characterized by the value R(ξ) equal to 3·0.10^?2, at the time of scission of the cross connection this structure acquiring either of the two alternative isomeric configurations with equal probability. About four percent of the total number of mismatches emerging as a result of formation of hybrid regions were repaired.