Ribbon model of double-stranded B-DNA

To account for the periodicity observed from X-ray data, Watson and Crick postulated that base pairs are stacked one on top of the other, with a distance of 3.4 A between base pairs and with 10 base pairs in one complete turn of the helix. Thus, there is one complete turn of the helix every 34 A. Figure 20.8 shows a ribbon model of double-stranded B-DNA, the predominant form of DNA in dilute aqueous solution and thought to be the most common form in nature. 

Ribbon model of double-stranded B-DNA. Each ribbon shows the pentose-phosphodiester backbone of a single-stranded DNA molecule.The strands are antiparallel, one running to the left from the 5 end to the 3 end, the other running to the right from the 5 end to the 3 end. Hydrogen bonds are shown by three dotted lines between each G-C base pair and two dotted lines between each A-T base pair. 

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See also in sourсe #XX -- [ ]

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