The nitrogen bases in a DNA molecule are adenine, guanine, cytosine, and thymine. Adenine and guanine belong to a class of compounds called purines, and cytosine and thymine belong to a class of compounds called pyrimidines. In a DNA molecule, the two strands are connecting by hydrogen bonds between the nitrogen bases of each strand. According to the base-pairing rule, the purine adenine always pairs with the pyrimidine thymine, and the purine guanine always pairs with the pyrimidine cytosine.

The diagram below illustrates the structure of the DNA nitrogen bases.

The diagram below illustrates the base-pairs in DNA.

pur-new-sol

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