the helical structure assumed by two strands of deoxyribonucleic acid (nucleotides), which entwine like vines and are held together throughout their length by hydrogen bonds between bases on opposite strands like rungs on a ladder (see figure below). The geometry of the helix can assume one of three slightly different forms. The ‘B’ form, described by Watson and Crick, is considered to be the main one in cells. It has a width of 2 nanometers, and extends 3.4 nanometers for each 10 base pair (bp) of sequence, which is also roughly the length of sequence in which the helix makes one complete turn about its axis. The ‘B’ form helix twists 360° for every 10 bp of sequence. The other forms, ‘A’ and ‘Z’, have slightly different geometries and dimensions compared to ‘B’: ‘A’ can appear in dehydrated samples of DNA and in hybrid pairings of DNA and RNA strands, while ‘Z’ occurs in cells that have been methylated, and in which the strands adopt a mirror image of ‘B’ in twisting around the helical axis. Referred to as Watson-Crick base pairing, the history of how the DNA double helix was discovered is one of the most intriguing stories in science. Credit must be given to Linus Pauling (1901-1994) who found in 1951 that many proteins had a three-dimensional spiral structure (i.e., a helix). This led Pauling and others, including John D. Watson and Francis H.C. Crick (1916-2004), to envisage that DNA might be a helix as well. The first person, however, to postulate this possibility was Alec Stokes (1919-2003). It was the Watson-Crick model, consisting of two helices, paired bases, and the sugar structure discovered by Rosalind E. Franklin (1920-1958), that gained final acceptance, and for which the pair were awarded the Noble Prize for Medicine or Physiology in 1962, together with Maurice H.F. Wilkins (1916-2004). They announced their model on February 21, 1953, one week after concluding that it was feasible, and which was followed by the publication of their famous Nature paper ‘A structure for deoxyribose nucleic acid’ on April 25.
