MBIO 3410 lecture Sept 18 2007

MBIO 3410 lecture Sept 18 2007 - Outline- Last lecture What...

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1 Outline- Last lecture What types of nucleic acids make up genomes? DNA supercoiling Closed-circular DNA Supercoiling Energy of supercoiling Topisomerase Prokaryotic chromosome structure E. coli chromosome DNA domains DNA binding proteins Eukaryotic chromatin structure Chromatin Nucleosomes Histones Packaging DNA DNA Replication overview Semi-conservative replication Replicons, origins and termini Semi-discontinuous replication DNA replication Bacterial DNA replication Initiation Elongation
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2 Elongation Primosome (DnaB helicase and DNA primase) synthesizes RNA primers every 1000-2000 nt on the lagging strand Lagging and leading strand primers are elongated by DNA polymerase III holoenzyme This is a dimer, half synthesizes the leading strand the other half, the lagging strand In vivo DNA polymerase III holoenzyme dimer, the primosome and DNA helicase form a complex (replisome) → synthesize DNA at a rate of 900 bp per sec
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3 The leading strand has this orientation that DNA polymerase III can replicate continuously The lagging strand is looped by DNA polymerase III so that both template strands can be replicated in the correct orientation After 1000-2000 bp the Okazaki fragment is complete and the loop is released (process is repeated) DNA ligase then joins the Okazaki fragments via phosphodiester bonds Fig 1.19
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4 Termination In bacteria termination of DNA replication occurs at defined terminator sequences ~180 o opposite oriC In E. coli this the terminator sequence is bound by a protein called ‘Tus’ (inhibitor of DnaB helicase) Topoisomerase IV unlinks the replicated DNA circles
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5 DNA polymerases Bacteria and eukaryotes possess multiple DNA polymerases E. coli has 3: DNA polymerase I (DNA repair) DNA polymerase II (DNA repair) DNA polymerase III (DNA replication) DNA polymerases have 3 distinct enzymatic activities: All direct synthesis of DNA fragments in a 5’ to 3’ direction All have 3’ to 5’
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MBIO 3410 lecture Sept 18 2007 - Outline- Last lecture What...

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