Base Pair

A base pair (bp) in molecular biology usually involves two nucleotides on opposite or complementary DNA or ribonucleic acid (RNA) strands that are connected together via hydrogen bonds. In the traditional Watson-Crick base pairing model in DNA, adenine (A) forms a base pair with thymine (T), and guanine (G) forms a base pair with cytosine (C). In RNA, the thyamine is replaced by uracil (U). Non–Watson-Crick base pairing models have alternate hydrogen bonding patterns with pairs such as Hoogsteen base pairs being common.

Base pairs are often used to measure the size of an individual gene, with the total number of base pairs becoming equal to the number of nucleotides in one of the strands. With extremely complex genomes, the detailing of base pairs can be very complicated. In fact, the human genome is estimated to have 3 billion base pairs, which contain 20,000 to 25,000 distinct genes. For dealing with these large numbers, there are kilo base pairs (kb or kbp), which consist of 1,000 base pairs, mega base pairs (Mb) which have 1 million base pairs and giga base pairs (Gb).