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Figure 2.25 DNA Double-Stranded Helix (a)The sugar-phosphate backbone and the hydrogen bonding between bases.There are two hydrogen bonds between adenine and thymine and three between guanine and cytosine. (b)The spiral staircase of the sugar-phosphate backbone with the bases on the inside.The railings go in opposite directions.

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of thymine molecules and the number of guanines equals the number of cytosines.

Three hydrogen bonds join each G to C, but only two join A to T. Each of the hydrogen bonds is weak, but their large number in a DNA molecule holds the two strands together. In addition to the differences in their sequence of bases, the two complementary strands differ from each other in their orientation. The two strands are arranged in opposite directions. One goes in the 3' to the 5' direction; the other in the 5' to 3' direction. Consequently, the two ends of the helix opposite each other differ; one is a 5' end, the other, a 3' end (see figure 2.25).

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