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新基因的概念及其起源的普遍性

Course video 35 of 67

核酸演化速率的计算要比蛋白质演化速率计算复杂得多。首先将核酸分为蛋白质编码区和非编码区;还要将碱基的替代分为转换(同类碱基之间的替代)和颠换(不同类碱基之间的替代);同时,有多种模型来定义单个碱基之间替代的速率。以2参数(K-2)模型为例,非编码区每个核苷酸位点的平均替代数计算公式为:K = (1/2)ln(x) + (1/4) ln(y);x = 1/(1 – 2P – Q), y = 1/(1-2Q),P为两序列中发生转换的碱基的比例,Q为两序列中发生颠换的碱基的比例,非编码区核酸的演化速率计算公式为:核酸的演化速率为r = K/2T,编码区的碱基替代又分为同义替代和非同义替代,其演化速率的公式不变,但K = Ks+ Ka;Ks为同义替代位点的平均替代数;Ka为非同义替代位点的平均替代数。Ka和Ks的比值,即Ka /Ks 是等于1、大于1或小于1可以用来判断所研究的基因所受到的选择压力类型。本节课还介绍了基因组学的研究进展,并具体介绍了将基因组研究用于几个物种演化研究的实例。

Peking University
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