DNA sequencing 1953, James Watson and Francis Crick put forward their double-helix model of DNA, based on crystallized X-ray structures being studied by Rosalind Franklin – and without crediting her. According to the model, DNA is composed of two strands of nucleotides coiled around each other, linked together by hydrogen bonds and running in opposite directions. Each strand is composed of four complementary nucleotides – adenine (A), cytosine (C), guanine (G) and thymine (T) – with an A on one strand always paired with T on the other, and C always paired with G. They proposed such a structure allowed each strand
Non-helical models of DNA structure The SBS structure was hypothetical, and was not based primarily upon physical evidence. It was created to provide a theoretical solution to the angular momentum problem in DNA replication, which Rodley believed to be otherwise unsolvable. The SBS structure is a modification of the original Watson-Crick structure where, instead of one fully right-handed helical twist every 10 base pairs, there is only a "half"-twist of 5 base pairs’ length, followed immediately by a half-twist in the "opposite" direction (that is, a left-handed half-twist, also of 5 base pairs’ length). This alternating pattern of right- and left-handed half-twists is repeated throughout