Methodology in Gene Transcription (II)-复旦《医用分子遗传学》课件.ppt

BMB400, 3_RNA processing_04 Methodology in Gene Transcription (II) Measurement of transcription rate Nuclear run-on transcription Reporter gene transcription Post-transcriptional Processing of RNA Making ends of RNA RNA splicing Primary Transcript Primary Transcript-the initial molecule of RNA produced hnRNA hnRNA-heterogenous nuclear RNA In prokaryotes, DNA →RNA →protein in cytoplasm concurrently In eukaryotes nuclear RNA >> Cp RNA Pre-messenger RNA Processing Types of RNA processing A) Cutting and trimming to generate ends: rRNA, tRNA and mRNA B) Covalent modification: Add a cap and a polyA tail to mRNA Add a methyl group to 2’-OH of ribose in mRNA and rRNA Extensive changes of bases in tRNA C) Splicing pre-rRNA, pre-mRNA, pre-tRNA by different mechanisms. 5’ and 3’ ends of eukaryotic mRNA Capping of pre-mRNAs Cap=modified guanine nucleotide Capping= first mRNA processing event - occurs during transcription CTD recruits capping enzyme as soon as it is phosphorylated Pre-mRNA modified with 7-methyl-guanosine triphosphate (cap) when RNA is only 25-30 bp long Cap structure is recognized by CBC stablize the transcript prevent degradation by exonucleases stimulate splicing and processing The structure of the 5’cap of eukaryotic mRNAs. Polyadenylation and termination Functions of 5’ cap and 3’ polyA Need 5’ cap for efficient translation: Eukaryotic translation initiation factor 4 (eIF4) recognizes and binds to the cap as part of initiation. Both cap and polyA contribute to stability of mRNA: Most mRNAs without a cap or polyA are degraded rapidly. Shortening of the polyA tail and decapping are part of one pathway for RNA degradation in yeast. mRNA Half-life ?t? seconds if seldom needed ?t? several cell generations (i.e. ~48-72 h) for houskeeping gene ? ≈avg 3 h in eukaryotes ? ≈avg 1.5 min in bacteria Split gene and mRNA splicing(断裂基因与mRNA剪接） Exon and Intron Exon is any segment of an interrupted gene that is represented in the mature RNA product. I