碱基互补配对Base pair

嘌呤总是与嘧啶配对，使双螺旋直径保持一致（约 1.08 nm 的 C1′–C1′ 距离）。A–T 之间形成两个氢键（N6–H···O4、N1···H–N3），G–C 之间形成三个氢键，因此 G–C 含量高的 DNA 更稳定、熔解温度更高。配对的特异性来自氢键供体和受体的几何互补；碱基的稀有互变异构体可能导致错配，是自发突变的一个来源。

A purine always pairs with a pyrimidine, keeping the helix diameter constant (a C1′–C1′ distance of about 1.08 nm). A–T forms two hydrogen bonds (N6–H···O4 and N1···H–N3) and G–C three, so G–C-rich DNA is more stable and melts at a higher temperature. Specificity comes from the geometric match of hydrogen-bond donors and acceptors; rare tautomers of the bases can cause mispairing, one source of spontaneous mutation.

1950 年查加夫发现各种生物 DNA 中 A 与 T、G 与 C 的含量相等（查加夫规则），但不同物种 (A+T)/(G+C) 比例不同。1953 年沃森用硬纸板碱基模型摆出 A–T 与 G–C 的形状相同，与克里克一起将其置于双螺旋内侧，查加夫规则由此得到解释。正确的碱基互变异构形式由多诺霍向沃森指出。

In 1950 Chargaff found that DNA from every organism has equal amounts of A and T and of G and C (Chargaff's rules), although the (A+T)/(G+C) ratio varies between species. In 1953 Watson, playing with cardboard base models, saw that A–T and G–C pairs have the same shape; he and Crick put them inside the double helix, explaining Chargaff's rules. Jerry Donohue pointed Watson to the correct tautomeric forms of the bases.

RNA 中还存在 G–U 摆动配对，使一个 tRNA 能识别多个密码子（克里克 1966 年的摆动假说）。胡斯坦配对可形成三链 DNA 和 G-四链体结构。碱基配对是 PCR、核酸杂交、基因芯片、反义寡核苷酸药物和 DNA 纳米技术的基础；科学家还合成了人工碱基对，扩展了遗传字母表。

RNA also has G–U wobble pairs, which let one tRNA read several codons (Crick's wobble hypothesis of 1966). Hoogsteen pairing gives triple-stranded DNA and G-quadruplexes. Base pairing underlies PCR, hybridisation, microarrays, antisense drugs and DNA nanotechnology, and chemists have made unnatural base pairs that expand the genetic alphabet.