History of chemistry quantum mechanics were able to predict qualitatively some chemical features for biologically relevant molecules, they were, till the end of the 20th century, more a collection of rules, observations, and recipes than rigorous ab initio quantitative methods. This heuristic approach triumphed in 1953 when James Watson and Francis Crick deduced the double helical structure of DNA by constructing models constrained by and informed by the knowledge of the chemistry of the constituent parts and the X-ray diffraction patterns obtained by Rosalind Franklin. This discovery lead to an explosion of research into the biochemistry of life. In the same year, the
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