Dia 25 de março de 2013 foi o Dia do ADN, por que há exatos 60 anos atrás, saiu o paper da Nature de Watson e Crick (baixe seu original daqui), que desvendou a estrutura exata da dupla hélice da molécula do ADN, base de nosso código genético. Neste mesmo número, saíram outros dois artigos complementares, pois alguns dos experimentos foram utilizados em comum (Wilkins, Stoke & Wilson e Franklin & Gosling).
Foi com as seguintes palavras que a dupla hélice entrou para o rol do conhecimento científico humano:
A structure for nucleic acid has already been proposed by Pauling (4) and Corey1. They kindly made their manuscript available to us in advance of publication. Their model consists of three intertwined chains, with the phosphates near the fibre axis, and the bases on the outside. In our opinion, this structure is unsatisfactory for two reasons:
(1) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidic hydrogen atoms it is not clear what forces would hold the structure together, especially as the negatively charged phosphates near the axis will repel each other.
(2) Some of the van der Waals distances appear to be too small.
Another three-chain structure has also been suggested by Fraser (in the press). In his model the phosphates are on the outside and the bases on the inside, linked together by hydrogen bonds. This structure as described is rather ill-defined, and for this reason we shall not comment on it.
We wish to put forward a radically different structure for the salt of deoxyribose nucleic acid (5). This structure has two helical chains each coiled round the same axis (see diagram). We have made the usual chemical assumptions, namely, that each chain consists of phosphate diester groups joining beta-D-deoxyribofuranose residues with 3′,5′ linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis. Both chains follow right-handed helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions (6) . Read more…