mcb450 exam 3 - Lecture 23 1. Helicases unwind the two...

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Lecture 23 DNA replication and enzymes involved: 1. Helicases unwind the two parental strands of DNA. Topoisomerases prevent supercoiling ahead of replicating fork. 2. A specialized RNA polymerase, called primase, lays down a short run of complememtary RNA, reading each parental strand 3′→5′ and synthesizing RNA 5′→3′ . 3. DNA polymerase III begins extending each of the RNA primers. 4. fork opens and DNA polymerase extends one of the DNA chains, the so-called continuous, or leading, strand. 5. On the other parental strand, a second RNA primer is synthesized. 6. DNA polymerase III extends the new RNA primer on the lagging, or discontinuous strand. 7. DNA polymerase I chews RNA ahead and synthesizes new DNA. The lagging strand then has runs of DNA called Okazaki fragments. 8. DNA ligase seals the gaps in the Okazaki fragments. general features of E. coli polymerases 1-3 - Polymerase I : catalyzes n(dATP, dTTP, dGTP, dCTP) = DNA + nPPi .attaches incoming nucleotide to the free 3’-OH group of the primer .reads the template strand in 3’ 5’ direction .not the major enzyme involved in DNA replication, but necessary for replication : serves as 3’ 5’ exonuclease (3’ exonuclease) .important in proofreading : serves as 5’ 3’ exonuclease .important in DNA repair, can remove distorted segments ahead of the DNA it is laying down. -
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mcb450 exam 3 - Lecture 23 1. Helicases unwind the two...

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