base pair

A base pair is a single rung of the DNA ladder: two bases on opposite strands joined across the middle of the double helix. Where one strand has an A, the other has a T directly facing it, and the two together make one A-T base pair; G and C make the other kind.

Each pair is held by hydrogen bonds — two between adenine and thymine, three between guanine and cytosine. Because a large purine always pairs with a smaller pyrimidine, every rung is about the same width, which keeps the helix smooth and regular along its whole length.

Base pair is also the standard unit for measuring DNA length. A gene might be a few thousand base pairs; a human chromosome, tens of millions; the whole human genome, about three billion. Larger lengths are written as kilobases (thousands) and megabases (millions) of base pairs.

It helps to keep two related terms apart: a base pair is two bonded bases across the helix, while a nucleotide is one base together with its sugar and phosphate on a single strand. In double-stranded DNA, each base pair corresponds to two nucleotides, one on each strand.