Transcribes specific genes o Sigma interacts with tetrameric core to recognize promoter in order to start transcription at the beginning of the gene Funtions of the subunits: o α: assembly of the tetrameric core o β: ribonucleotides triphosphate binding site o β’: DNA template binding region o σ: initiation of transcription specifically at a promoter Initiation of Transcritption Initiates about 5-9 base pairs down from the end of the -10 sequence The 5’ end for the RNA is usually a purine +1 position: First purine (T or C) after the -10 sequence o Will start copying from T or C and A or G will be the first base in the new strand Structure of promoter: 2 block sequence information – 10 sequence and -35 sequence Sequences are known as consensus sequences- found most often in these positions o -10 sequence: Very A/T rich localized unwinding occurs here 5’ TATAAT 3’ “Pribnow” Box o -35 sequence: σ factor binding site is here 5’ TTGACA 3’ 4 different ways to orient the duplex, you will need to go in from 5’ ends (might not be given, have to find the -35 sequence to know which is the 5-3 strand)
MBG*2040 Exam Notes December 2015 Elongation Basic mechanism Synthesis proceeds 5’-3’ Grows one base at a time The Transcription Bubble: Area where transcription elongation takes place RNA polymerase has helix unwinding and rewinding activities Rho- Independent or Intrinsic Termination- Prokaryotes only Rho- D: Protein that catches up to RNA polymerase when it pauses and stops it Eukaryotes only o Has to tell it where to stop, or it would go on forever o Sequences in DNA which get made into RNA which tell RNA polymerase to stop Common mechanism in bacteria Weak H-bonding at U:A residues allows mRNA release from DNA when polymerase pauses at termination Polymerase recognizes that hairpin is formed and pauses it from transcription. When polymerase pauses they come apart and transcription stops Green match up and can pair with each other, they are self- complementary, can form hairpin structure Hairpin structure causes polymerase to pause. Weak U-A bonds result in its release
MBG*2040 Exam Notes December 2015 Transcription and Translation are Tightly Coupled in E. coli Fibres come off somewhat perpendicular, chains of blobs represent mRNA that are being made by RNA polymerases and are transcribing genes, blobs represent ribosomes Can tell which way the polymerase has gone depending on the length of the mRNA chains o The shorter the chain, the closer it is to the 3’ end of the DNA o The longer the chain, the further it is from the 3’ end of DNA Transcription and RNA processing in Eukaryotes Polytene: many double stranded DNA strands that have copied but stay together Puffs in chromosomes: sites of localized unwinding due to gene transcription o Can open where transcription is occurring and they can close Regions of chromosomes where tightly compacted chromatin have made loops
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- Fall '13