RNA Metabolism Transcription - process by which DNA (genetic info) gets made into RNA (mRNA, rRNA, or tRNA) by an RNA polymerase mRNA - messenger RNA -

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Presentation transcript:

RNA Metabolism Transcription - process by which DNA (genetic info) gets made into RNA (mRNA, rRNA, or tRNA) by an RNA polymerase mRNA - messenger RNA - encodes the amino acid sequence of > 1 protein specified by a gene(s) rRNA - ribosomal RNA - constituents of ribosomes (proteins synthesized here) - catalytic and directly involved in protein synthesis tRNA - transfer RNA - reads the information encoded in the mRNA and transfers the appropriate amino acid to a growing polypeptide chain during proteins synthesis During DNA replication usually the entire genome is copied, but during transcription there is more selectivity (only particular genes are transcribed, some of DNA genome never transcribed)

RNA Metabolism Transcription - uses DNA-dependent RNA polymerase RNA pol requires: 1. DNA template 2. rNTPs (ATP, GTP, UTP, CTP) 3. Mg 2+ NO primer needed 3’-OH acts as a nucleophile, attacking the  -phosphate of the next rNTP RNA pol elongates an RNA in the 5’  3’ direction

RNA Metabolism Transcription - uses DNA-dependent RNA polymerase Polymerization is “asymmetric” - only one strand of DNA used as template - new RNA chain is identical in sequence to the nontemplate strand

RNA Metabolism Transcription DNA-dependent RNA polymerase from E.Coli is large (6 subunits) MW Number/pol Function  ’155 kD1 DNA binding  151 kD1 Catalytic site for RNA pol  36.5 kD2 Interacts with reg proteins  70 kD1 Recognizes promoter Transcription initiation

RNA Metabolism Transcription

RNA Metabolism Transcription RNA synthesis initiated at promoters (specific DNA sequence) Typical E.Coli promoters: Pribnow box -25 TATA box (TATAAA) EUKARYOTES: -75 CAAT box

RNA Metabolism Transcription - frequency of transcription regulated by inhibitors (repressors) and activators of RNA Polymerase Repressor = trp repressor Activator = CAP (catabolite activator protein) DNA Trp dimer Trp DNA CAP dimer cAMP Interacts with RNAP

RNA Metabolism Transcription - termination Termination sites have a GC-rich region followed by 4-10 A:T bp Stem-loop structure in RNA being synthesized induces pausing of RNA Polymerase Weak U:A bp cause a conformational change in RNA Polymerase

RNA Metabolism Transcription - Inhibitors of RNAP Rifampcin - blocks initiation by binding to  subunit Actinomycin, acridine - intercalates into DNA, prevents RNAP moving