f4 - DNA Replication Semi-conservative nature Prokaryotic...

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• Prokaryotic DNA Replication • Eukaryotic DNA Replication Readings: Ch. 4: pp 131-136; Ch. 10: pp 435-437 DNA Replication: Semi-conservative nature
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Eukaryotic DNA Replication Initiation, Elongation and Termination
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Helicase Helicase Eukaryotic DNA Replication… Initiation 1) Replication occurs at the origin (Ori)…in yeast autonomous replicating sequences ( ARSs ) - This sequence serves as a recognition site for the Origin Recognition Complex (ORC) that associates with other proteins to load the helicase (i.e. Large T-antigen in SV40). (The Prereplication complex consists of the following: Helicase, topoisomerase, primase/DNA pol α )
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Eukaryotic DNA Replication: Initiation 2) Helicases use energy from ATP to unwind parental DNA creating a “replication bubble,” with one replication fork at each end. - Topoisomerase relaxes any supercoiling - ssDNA is bound by Replication Protein A (RPA)…preventing ssDNA from reforming a duplex. 3) Primase-DNA Pol α complex synthesizes short RNA-DNA primers on each strand, which mark the starting points to be replicated. Replication forks Replication bubble
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Okazaki Fragment Primer Eukaryotic DNA Replication: Elongation 4) Pol δ -PCNA-Rfc complex replaces the primase-Pol α complex and extends the RNA-DNA hybrid primer, generating the leading strand For the lagging strand • As the helicase further unwinds the parental strands, the primase-Pol α complex makes primer for the lagging strand and Pol δ -PCNA-Rfc complex synthesize the Okazaki fragment 5) On the lagging strand, RNA-DNA primers are removed by RnaseH and FenI…Pol δ then fills the gap with dNTPs 6) DNA ligase joins Okazaki fragments
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Replication fork in Eukaryotes: Elongation • It is believed that the two DNA pol δ complexes on both strands work together and move in the same direction • The lagging strand loops back to accommodate…
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Replication fork in Eukaryotes: Termination • Termination occurs when DNA replication forks meet one another or run to the end of a linear DNA molecule .
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Telomeres • Telomeres are the physical ends of a linear chromosome -They consist of tandem arrays of a short DNA sequence, TTAGGG in vertebrates. -Telomeres are non-coding DNA (no genes) -The 3’ end of the G-rich strand extends 12- 16 nucleotides beyond the 5’end of the complementary C-rich strand. • This 3’ overhang region is bound by specific proteins that serve to protect the ends of the linear chromosomes from attack by exonucleases. 5’-… TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG -3’ 3’-… AATCCC AATCCC AATCCC AATCCC -5’
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Telomere replication: a special case • They are the last regions of DNA to be replicated…and are not replicated in the same way as the rest of the chromosome. A problem arises when we try to replicate the ends of the linear DNA molecule Can you think what this problem might be?
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