A sequence-independent, four-stranded, double Watson--Crick DNA helix that could solve the unwinding problem of double helices.

A four-stranded DNA helix is described that consists of two Watson-Crick double helices wound about the same axis, with no restriction on sequence. Its significance is that it permits a simple topological solution to the problem of unwinding a double helix to separate its strands. If a stretch of Watson-Crick double helix is supercoiled on itself as many times as it has helical turns it has no topological winding and its strands can be separated without the ends of the strands rotating about each other. Such a helix is folded on itself to form the four-stranded helix. The dramatic possibility that this is the structure of DNA in chromatin seems to be ruled out by measurements of supercoiling in circular chromosomes, but it could perhaps occur in special situations, for example in chromosome pairing or at irregularities in palindromic DNA sequences. The helix has Watson-Crick base pairing, bases almost perpendicular to the helix axis, eleven nucleotides per strand per turn and CZ-e&o puckering of the deoxyribose ring. The helix axis is an axis of symmetry for the deoxyribose-phosphate backbone.