Three dimension maximum common

It is easy to imagine that the double helix may assume a number of shapes in three dimensions. Three major forms of DNA are recognized A-DNA, ISDN A, and Z-DNA. B-DNA is the most common physiologic form. It is right-handed, with 10 base pairs per turn and a 3.4 A rise per base pair. In the laboratory, it is observed at low salt concentrations and a maximum degree of hydration. When the salt concentration is increased and degree of hydration lowered, DNA may assume the A conformation. It is also right-handed, but there are 10.7-11 base pairs per helical turn with a rise of only 2.3 A per base... 

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