Question

DNA: How many spacing base pairs between two double-crossover units so that the two double-crossover units are parallel to each other?

Hello! I have no idea what my professor is asking here, any help is greatly appreciated. Thank you so much!

Answer 1

In double crossover, chromatids from two homologous chromosomes come in contact at two points which is refered to as two holliday junctions connected by two double helical arms. There are different types of double crossover molecules which are differentiated by the relative orientations of their helix axes, parallel or antiparallel and by the number of double helical half turns (even or odd) between the two crossovers. Based on this there are five different isomers of double helical structures wherein the helical domains are parallel in three of the five isomers and antiparallel in the other two. The parallel molecules have dyad axis of backbone symmetry parallel to helix axes and in the antiparallel ones it is perpendicular. The double crossovers can also be categorised on the basis of whether the crossover occurs between strands of same polarity or opposite. In parallel it is between same polarity and in antiparallel it is between opposite. As a result of this, there is about a half turn distance between helical domains when parallel and antiparallel species are compared.