X-ray diffraction from the side-by-side model of DNA.

An intriguing topological problem posed by the double-helical Watson-Crick model of DNA is that of unwinding the intertwined strands during replication. Several workers have recently proposed novel side-by-side (SBS) structures for DNA. In all these models the two strands are joined by complementary Watson-Crick base pairs and the antiparallel polynucleotide strands alternate between short segments of right- and left-handed helix, thus both reducing the amount of intertwining and alleviating the unwinding problem. We show here that there are unacceptable discrepancies between the observed diffraction pattern of B-DNA and that calculated for the original SBS structure. We also describe a simple modification of this model which resolves some of the more serious discrepancies. However, the agreement is still markedly inferior to that obtained for a Watson-Crick model of DNA.