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Lecture 4

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BIOL 205

DNA synthesis is fast and accurate! • 2000 nucleotides/second- in E. coli two forks moving at 1000 nucleotides/second • E.coli genome 5 million base pairs- 40 minutes • 1 nucleotide error in 10 inserted (thanks to 3’-5’ exonuclease proofreading (Pol I and III) Initiation of Replication • The origin of replication in E. coli is termed oriC – origin of Chromosomal replication • Three types of DNA sequences in oriC are functionally significant – AT-rich region – DnaA boxes – GATC methylation sites • Many proteins are required Unwinding the double helix • Two enzymes open helix and prevent overwinding – HELICASE and TOPOISOMERASE(=DNA gyrase) – HELICASE- clamps to DNA –breaks Hydrogen bonds ahead of DNA synthesis – SSB –single strand DNA binding proteins –bind to unwound DNA and prevent it from re-forming • . • DNA replication is initiated by the binding of DnaA proteins to the DnaA box sequences – This binding stimulates the cooperative binding of an additional 20 to 40 DnaA proteins to form a large complex Composed of six subunits Travels along the DNA in the 5’ to 3’ direction Uses energy from ATP Bidirectional replication Prokaryotic initiation of replication Figure 7-20 Topoisomerase/ DNA gyrase removes extra twists Figure 7-19a DNA gyrase removes extra twists –relaxes DNA Figure 7-19b DNA Replication Complexes • DNA helicase and primase are physically bound to each other to form a complex called the primosom
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