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Transcript
DNA Replication and Repair The Central Dogma of Molecular Biology • genetic information is passed on to the next generation • semi-conservative DNA Replication T A T A T A C G C G C T A T A T A A T A T A T G C G C G C G A T A T A T C G C G C G T A T A T A T A T A T C G C G C A G Parent molecule with two complementary molecules Parental strands separate Each parental strand is a template Each daughter DNA molecule consists of one parental and one new strand Overview of replication Initiation • DNA is unwound and stabilized • Origins of replication: Replication bubble and replication fork Priming • RNA primers bind to sections of the DNA and initiate synthesis Elongation • Leading strand (5’ 3’) synthesized continuously • Lagging strand synthesized discontinuously then fragments are joined • RNA primer replaced by DNA Proofreading • Mismatch repair by DNA polymerase • Excision repair by nucleases Review of DNA structure • double helix • each strand has a 5’ phosphate end and a 3’ hydroxyl end • strands run antiparallel to each other • A-T pairs (2 H-bonds), G-C pairs (3 H-bonds) STEP 1 Initiation at origins of replication separation sites on DNA strands • Depend on a specific AT-rich DNA sequence – Prokaryotes – one site – Eukaryotes – multiple sites • Replication bubble • Replication fork • Proceeds in two directions from point of origin The proteins of initiation 1. Helicase – unwinds double helix 2. Singlestrand binding proteins – holds DNA apart STEP 2 Priming initiation of DNA synthesis by RNA RNA primers bind to unwound sections through the action of primase – leading strand – only 1 primer – lagging strand – multiple primers – replaced by DNA later STEP 3 Elongation of a new DNA strand lengthening in the 5’ 3’ direction DNA polymerase III can only add nucleotides to the 3’ hydroxyl end Leading strand - DNA pol III – adds nucleotides towards the replication fork; - DNA pol I - replaces RNA with DNA Lagging strand - DNA pol III - adds Okazaki fragments to free 3’ end away from replication fork - DNA pol I - replaces RNA with DNA - DNA ligase – joins Okazaki fragments to create a continuous strand STEP 4 Proofreading correcting errors in replication Mismatch repair • DNA pol III – proofreads nucleotides against the template strand Excision repair • nuclease – cuts damaged segment • DNA pol III and ligase – fill the gap left Telomeres at 5’ ends of lagging strands • no genes, only 100 – 1000 TTAGGG sequences to protect genes • telomerase catalyzes lengthening of telomeres