DNA Replication and Repair

1. DNA Replication and Repair

2. The Central Dogma of Molecular Biology

3. T A A A A T A T T T T A G C G C C C C G G G G C A A T T T T A A A A T T T A A A A T A T T T T A C G G G G C G C C C C G DNA Replication • genetic information is passed on to the next generation • semi-conservative

4. Overview of replication

5. 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)

6. STEP 1 Initiation at origins of replicationseparation 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

7. The proteins of initiation • Helicase – unwinds double helix • Single-strand binding proteins – holds DNA apart

8. STEP 2 Priminginitiation 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

9. STEP 3Elongation of a new DNA strandlengthening 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

11. STEP 4Proofreadingcorrecting 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