之前的文章中,小编简介过各种进阶版的PCR(详情点这里 ),也详说了进阶PCR的一种,colony PCR ,本期我们就来说说,另外一种进阶版PCR,巢式PCR (Nested PCR)。
为什么
PCR非常简单,最完美的PCR就是用设计的上下游引物完整地复制出目标片段。我们一般希望,在30-40个循环之后,产物几乎都是目标片段,然而现实不一定都是这样。如果上下游引物结合互补链的特异性不强,那么PCR产物有各种长度的片段,电泳结果就是弥散条带而不是我们期望的高亮而专一的条带。不纯的PCR产物会严重影响后续的克隆、筛选、表达等等实验结果。
怎么做
一张原理图
简单来说,第一级引物用于扩增包含目的片段的较长片段,这个较长片段就是所谓的巢片段,第二级引物用于扩增目标片段。从统计角度看,两对引物框定的片段有更小的可能性不是目标片段。小编一般的做法是,先加入一级引物进行正常的PCR反应,等到6-10个循环之后,再加入第二级引物,等到完成所有循环(30-35 cycles),即可回收产物了。建议使用第一级扩增的巢片段要比目标片段稍微长一些(比如长500 bp 或更长),这样产物回收的时候,就可以清晰分辨目标片段和巢片段了,毕竟我们想要的只有目标片段而已。
Extra tips
- BLAST搜索,弄清母板、巢片段、目标各是什么,这样在设计引物的时候就知道到底会不会存在非特异性结合;
- 更长的引物,长引物有更小的可能性会出现非特异性结合,同时也会提高退火温度;
- 引物的G+C含量,一般控制在40-60%,GC太低会导致引物结合能力不强,而出现各种飞特异性结合;
- 使用添加剂,稳定DNA模板和阻止可能存在的二级结构,各种添加剂功能和使用,请戳这里:PCR反应中各种添加剂的作用(上) ,PCR反应中各种添加剂的作用(下) 。
有什么关于巢式PCR的想法,欢迎下方留言讨论哦~
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