In the model of DNA developed by James Watson and Francis Crick, the DNA molecule consists of two connected chains of nucleotides forming a ladderlike structure. The sides of the "ladder" are composed of alternating phosphate and deoxyribose molecules.

Each rung of the ladder consists of a pair of nitrogenous bases bonded together by hydrogen bonds. The two chains of the DNA molecule are twisted to form a spiral, or double helix.

The four nitrogenous bases of DNA nucleotides bond together in only one way: adenine (A) pairs with thymine (T), by a double hydrogen bond; and cytosine (C) pairs with guanine (G), by triple hydrogen bond. Because the bases pair together in only one way, the two strands of a DNA molecule are complementary.

Where there is an adenine nucleotide on one strand, there is always a thymine nucleotide on the other; where there is a cytosine on one strand, there is a guanine on the other. If you know the order of bases in one strand, you also know the order in the second.

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