dissociates from DNA Lecture 5 Transcription initiation o RNA polymerase big

Dissociates from dna lecture 5 transcription

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dissociates from DNA Lecture 5 - Transcription initiation o RNA polymerase (big) binds to promoter sequence in duplex DNA; start site near 3’ end, stop site further along; this is called the CLOSED COMPLEX because DNA is not yet denatured Eukaryotic RNA polymerases require associated proteins called general transcription factors to find promoters and initiate transcription o Polymerase melts duplex DNA near transcription start site, forming a transcription bubble that is about 14 base pairs, allows rNTPs to begin RNA synthesis; OPEN complex o Polymerase catalyzes phoshosphodiester linkage of 2 initial rNTPs - Transcription elongation o Polymerase advances 3’ 5’ down template strand, melting duplex DNA and adding rNTPs to growing RNA; within bubble there is DNA-RNA hybrid region; nascent RNA, the chain being synthesized, comes off o Multiple polymerase molecules can transcribe the same template DNA strand at the same time o Elongation complex is very stable; polymerase does not fall off until it reaches the stop site
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o Speed of elongation around 1000 nt/min, so small genes are transcribed in a few minutes but big genes (with big introns) can take hours - Transcription termination o At transcription stop site, polymerase releases completed RNA and dissociates from DNA o Primary transcript: completed RNA molecule (mRNA not directly synthesized from DNA) o A specific sequence in the template DNA signals the bound RNA polymerase to terminate transcription - Image 1: E. coli RNA polymerase has 5 subunits; in diagram, DNA bends sharply upon entering the enzyme; inflection site is around the transcription start site, favors local denaturation - Organization of genes is different in prokaryotes and eukaryotes o Prokaryotic genome is very compact; genes with a common function are often arranged linearly in operons & transcribed together on a single mRNA; there are very few non-coding gaps of DNA in prokaryotic genomes (no introns, no RNA processing) o Eukaryotic genome such as in yeast have genes scattered on several chromosomes; that means that coregulation is not achieved simply by physical linkage o In prokaryotes, mRNAs are directly transcribed from DNA; but in eukaryotes, transcripts must go through several processing steps before becoming mRNAs - RNA processing o As the 5’ end of a nascent RNA chain emerges from RNA polymerase, the 5’ cap structure ( 7- methyl-G ) is added to it by several enzymes (5’ to 5’ linkage) o Polyadenylation: addition of 100 to 250 A residues by poly(A) polymerase on 3’ end of mRNA enzymatically o Intron excision, exon ligation (first exon will always include 5’ UTR, last exon will always include 3’ UTR) o mRNAs retain untranslated regions (UTRs) at the 5’ and 3’ ends but they do not encode for proteins; UTRs contain elements that regulate translation of mRNA & recruit ribosome to RNA o open reading frame: part of the RNA that encodes for the protein o the same primary transcript can be alternatively spliced in different tissues; liver cells remove
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  • Fall '06
  • Bureau
  • DNA, RNA, rna polymerase, RNAs

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