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Lectures 9-11 2B03 2017.pptx - Lectures 9-11 Transcription...

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Lectures 9-11 Transcription in Prokaryotes and Eukaryotes Based on Chapter 29: Transcription and the Regulation of Gene Expression 1
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mRNA, rRNA, and tRNA All participate in protein synthesis All RNAs are synthesized by DNA-dependent RNA polymerases via transcription Only mRNAs direct the synthesis of proteins Transcription is tightly regulated in all cells A small percentage of genes in a typical prokaryotic or eukaryotic cell are undergoing transcription at any given moment The metabolic conditions and growth status of the cell dictate which gene products are needed at any moment 2
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Transcription in Prokaryotes In prokaryotes, all RNA molecules are synthesized by a single species of DNA-dependent RNA polymerase RNA polymerases link NTPs (ATP, GTP, CTP, and UTP) in the order specified by base pairing with a DNA template The polymerase moves along the DNA strand in the 3'-5' direction Thus, the RNA chain grows 5'-3' during transcription Subsequent hydrolysis of PPi to inorganic phosphate by pyrophosphatases makes the polymerase reaction thermodynamically favorable (energy required for process to oc 3
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Template and Non-template Strands Certain conventions are used in describing information transfer from DNA to RNA: The strand of duplex DNA that is read by RNA polymerase is termed the template stand The strand not read is the nontemplate strand The template is read by the RNA polymerase moving 3'-5' along the template, so the transcript grows in the 5'-3' direction By convention, when the order of nucleotides in DNA is shown as a single strand, it is the 5'-3' sequence of nucleotides in the nontemplate strand that is shown 4 5’ 5’ 3’ 3’ TEMPLATE (BLUE): ATGGCTCGAGT 3’ 5’ TGAGCTCGGTA BY CONVENTION
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Template and Nontemplate Strands 5
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Prokaryotic RNA Polymerase RNA Template DNA β chain Dimeric chains β' chain 6 Prokaryotic RNA polymerase holoenzyme is made of α 2 ββ’σ The core polymerase is α 2 ββ’ ββ’ form enzymatic core for RNA synthesis Dimeric chains serve to recruit β and β’ Non-template DNA DNA sits in a channel between β and β’ NTPs enter from the back channel
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Recognition of Transcription Start Sites by Sigma in Prokaryotes Binding of the σ ( sigma ) subunit allows the polymerase to recognize different DNA sequences that act as promoters Promoters are nucleotide sequences that identify the location of transcription start sites , where transcription begins Without σ bound , the core polymerase cannot initiate transcription 7
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Transcription Has Four Stages Transcription can be divided into four stages: 1) Loading of RNA polymerase onto a promoter 2) Initiation of polymerization (of mRNA) 3) Chain elongation (mRNA elongation) 4) Chain termination 8
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Prokaryotic Initiation and Elongation 9
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Prokaryotic Initiation and Elongation 10
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Open Complex – Transcription Bubble -10 region -35 region RNAP contacts promoter around –35 and –10 regions -12 to +2 region is melt ” (14 base pairs) (this is the region that opens up) Transcription bubble (AT-rich) 11
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