陈汉春转录生化第7版2008下期.ppt

Transcription Transcription is a process in which one DNA strand is used as template to synthesize a complementary RNA.? The biosynthesis of RNA, called transcription, proceeds in much the same fashion as the replication of DNA and also follows the base pairing principle. Again, a section of DNA double helix is uncoiled and only one of the DNA strands serves as a template for RNA polymerase enzyme to guide the synthesis of RNA. After the synthesis is complete, the RNA separates from the DNA and the DNA recoils into its helix. RNA Transcription Process: The RNA transcription process occurs in three stages: initiation, elongation, termination. 原核生物RNA－pol可直接结合DNA模板； 真核生物RNA-pol需与辅助因子结合后才 结合DNA模板。 依赖Rho (ρ)因子的转录终止 非依赖Rho因子的转录终止 Features of eukaryotic transcription: Eukaryotic transcription has same basic principles as prokaryotic transcription does, but is more complicated: DNA is packaged in chromatin with histones in nucleosomes . 5 and 3 ends of primary transcripts are modified (caps, tails) 真核生物的转录起始较为复杂。目前已知RNA聚合酶Ⅱ至少有六种不同的蛋白因子（TF)参与转录复合体的形成。转录因子包括 TFⅡA，TFⅡB，TFⅡD，TFⅡE，TFⅡF， TFⅡH， TFⅡ-I。 转录生成的RNA是初级转录本 (primary transcripts)，需经加工才具有 活性； 真核生物的转录、翻译分区进行， 转录后修饰较复杂。 帽子结构 加尾过程及特点 基因(DNA)序列中没有poly T， 说明poly A的生成不依赖DNA模板， 为转录后加入； 核酸外切酶切去初级转录本的3?端， 再加入poly A； 加尾与转录终止同时进行，核内完成， 先于剪接； Polyadenylation Poly A尾巴的功能 参与mRNA核－质转运； 维持mRNA的翻译模板活性； 增加mRNA的稳定性，延长mRNA的寿命。 Notes：原核生物mRNA无帽、无尾； 组蛋白mRNA无尾。 内含子结构特征： 5?GU-------AG-3? (splicing junction) 内含子功能：有利于物种进化选择； 调控基因表达。 NOTES: 原核生物基因无内含子； 原核生物DNA复制快， 细胞周期短。 RNA splicing: The process by which the introns are removed from pre-mRNA (the primary transcript) is called RNA splicing. 内含子的发现 The RNA of some genes follow patterns of alternative splicing, when a single gene gives rise to more than one mRNA sequence. Different tissues splice the primary RNA transcript of a single gene by alternative pathways,w