DNA close up Animation

A section from a DNA double helix The backbone of each DNA strand consists of alternating deoxyribose sugars and phosphate groups. The two strands run in opposite directions. One runs from the 5' to 3' direction, the other in the 3' to 5' direction. Think of the deoxyribose units of one strand as being upside down. Hydrogen bonds hold the strands together at their bases. Each base pair consists of a pyrimidine bound to a complementary purine. There are two hydrogen bonds between each adenine-thymine base pair, and three between each guanine-cytosine pair. Adenine always bonds with thymine, and cytosine with guanine; however, the order in which these base pairs appear along the strand can vary. It is this variation that is the foundation for the diversity of life.

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