只要涉及进化，转换(transitions)和颠换(transversions)是常遇到的两个词。下面引用百度百科资料简单介绍一下：
点突变，也称作单碱基替换（single base substitution），指由单个碱基改变发生的突变。
可以分为转换（transitions）和颠换（transversions）两类。
转换：嘌呤和嘌呤之间的替换，或嘧啶和嘧啶之间的替换。
颠换：嘌呤和嘧啶之间的替换。
方便理解下面再附上一张示意图，如下：
Transition versus Transversion
图片引自：http://www.mun.ca/biology/scarr/Transitions_vs_Transversions.html
相关英文介绍：
DNA substitution mutations are of two types. Transitions are interchanges of two-ring purines (A 
G) or of one-ring pyrimidines (C T): they therefore involve bases of similar shape. Transversions are interchanges of purine for pyrimidine bases, which therefore involve exchange of one-ring and two-ring structures.
Although there are twice as many possible transversions, because of the molecular mechanisms by which they are generated, transition mutations are generated at higher frequency than transversions. As well, transitions are less likely to result in amino acid substitutions (due to "wobble"), and are therefore more likely to persist as "silent substitutions" in populations as single nucleotide polymorphisms (SNPs).
