rna的生物合成2学习.pptx

转录后加工（post-transcriptional processing）过程：;（一） rRNA前体加工;Processing of preribosomal RNA transcripts in bacteria. (a) Prior to cleavage of the 30S RNA precursor, it is methylated at specific bases. (b) Cleavage produces 17S and 25S intermediates, and (c) the final 16S and 23S rRNA products are made by the action of specific nucleases. The 5S rRNA arises from the 3 end of the 30S precursor. From the midsection (and sometimes the 3 end), one or more tRNAs are formed. ;2. 真核生物rRNA前体的加工：;Processing of preribosomal RNA transcripts in eukaryotes. (a) The 45S precursor is methylated at more than 100 of its 14,000 nucleotides, mostly on the 2-OH groups of ribose units retained in the final products. (b) A series of enzymatic cleavages produces the 18S, 5.8S, and 28S rRNAs. The 5S rRNA arises separately.;（二） tRNA前体加工;tRNA前体;tRNA前体的切断和剪接加工;tRNA3-末端-CCA序列的添加;tRNA的碱基修饰;（三）mRNA前体的一般加工; - 5?端加帽（m7G 5?ppp5? NmpNp- ） - 3?加poly A尾巴 - 拼接(去除内含子并连接外显子) - 核苷酸甲基化;Processing of hnRNA in eukaryotes.;The capping of eukaryotic pre-mRNAs.;7-甲基鸟嘌呤;The 5 cap of 7-methylguanosine is found on almost all eukaryotic mRNAs. ;Addition of the poly(A) tail to the primary RNA transcript of eukaryotes. The RNA is cleaved 11 to 30 nucleotides 3 to AAUAAA, and polyadenylate polymerase synthesizes a poly(A) tail of 20 to 250 nucleotides, beginning at the cleavage site.;1. 识别mRNA的3ˊ端的AAUAA保守序列，在AAUAA下游大约11－30个核苷酸的部位断裂。 2. 多聚腺苷酸聚合酶（PAP）结合于转录产物的游离的3ˊ端，加上大约20－250个腺苷酸的3’端的poly（A）尾巴。;（四） RNA的拼接和编辑;四种方式有两点相似之处： （1）磷酸二酯键的断裂和再连接， （2）RNA本身或者以cis方式（第I类和第II 类内含子自我剪接），或以trans方式（通过称之剪接体的核糖核蛋白复合体进行的hnRNA剪接）作为催化剂。;Splicing mechanism of group I introns. ;在第一次转酯基反应中，游离的鸟苷的3ˊ羟基起着一个亲核体的作用，结果与内含子中的5ˊ核苷酸形成一个新的3ˊ,5ˊ-磷酸二酯键，释放出上游的外显子。;RNase P的RNA（M1RNA）具有催化活性。;The structure and self-splicing pathway of group II introns.;真核生物hnRNA拼接 (拼接体的拼接);The splice sites that mark the intron-exon boundaries of many eukaryotic mRNAs have some conserved sequences. ;Splicing mechanism in mRNA primary transcripts.;U1 、U4被释放 U6结合在5?拼接点;Ove