The structure of DNA.

THE STRUCTURE proposed for DNA (deoxyribonucleic acid) has been described before (for a rerien : see Jordan, 21) and will only be outlined here. It consists of two polynucleotide chains running in opposite directions and twined round one another. The two chains are held together by hydrogen bonds between the bases, each base being joined to a companion base on the other chain. This pairing of bases is specific, odenine going with thymine, and guanine wit.h cytosine. The structure is not only found in extracted DX4 from a wide variety of sources, but is also present in intact biological material such as sperm heads and bacteriophage (36). The x-ray work up to 1954 has already been briefly summarized, with references (6). Since then, the group at King’s College, London, under Dr. N. H. F. Wilkins have published an interim report (17) on their studies of the B form of DK-4 and on their work on nucleoprotamine. Two reviews (9, 37) have also appeared; the one by Wilkins (37) touching on the very recent work of his group on nucleohistone. Some studies of the B form of DB-4 have also been carried out by Wykoff (38), who has shown that the B structure obtained by stretching the -4 form is slightly different from that produced by swelling it. The King’s College group have obtained the B form in a crystalline state, and in particular, ha\-e shown that the lithium salt gives a good lattice (37). According to Wilkins (37:~. rhe structure is now firmly established. The detailed structure described by Crick and Watson (8) has been shown to have too large a diameter, and a drawing has been given of an improved model which is in fairly good agreement with the s-ray data. No coordinates ha\-? 50 fnr been published.* (It should be noted

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