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DNA and RNA Reference library
...why the amounts of G and C, and of A and T, are approximately the same in DNA. The sugar phosphate backbones of the two chains are on the outside. They are twisted into a helical structure and carry the negative charges on the phosphate groups. The bases lie on the inside, perpendicular to the axis of the helix. The double helix structure resembles a spiral staircase, with the base pairs forming the steps. The model requires the two strands to run in opposite directions. Looking along the helix, therefore, one strand lies in the 5′ to 3′ direction, while its...