Cell Bio - Exam 3

Cell Bio - Exam 3 - Exam 3 Notes 4.5 DNA Replication - The...

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Exam 3 Notes 4.5 DNA Replication - The precise copying of DNA sequences in preparation for cell division. - Using the existing ( parental ) strands as templates in the formation of new ( daughter ) strands that are complementary to the parental strands - Modified Fig 4-29a o Meselson and Stahl - DNA Replication o DNA synthesis proceeds 5’ to 3’ o Chain growth results from formation of phosphodiester bond between 3’O of growing strand and α phosphate of a dNTP o Unlike RNA polymerase, DNA polymerase cannot initiate chain synthesis without a pre-existing RNA strand called a primer - DNA Synthesis Starts with an RNA Primer o From pg 141 o 3’ OH on the last nucleotide is needed for condensation synthesis o Primase lays down the RNA primer o DNA polymerase reads the template strand 3’ to 5’ to synthesize the daughter strand 5’ to 3’ - DNA Replilcation o Double-stranded helix must be un-woundd to make the bases available for base-pairing iwht the dNTP bases for the new strand o This is done by helicases o Then, a specialized RNA polymerase called a primase forms a primer complementary to the template strand o DNA polymerase then synthesizes a new daughter strand o The region where all of this occurs is called the replication fork o As replication proceeds, Topoisomerase 1 must relieve supercoils that form ahead of the replication fork from unwinding of DNA strands by helicases - Complications in Replication o Due to 2 key factors: 2 parental strands in DNA duplex are anti-parallel DNA polymerase can only add nucleotides in the 5’ to 3’ direction o Synthesis of the leading strand can proceed from a single RNA primer 5’ to 3’, the direction that the replication fork is moving. o The problem occurs in the lagging strand , where copying occurs opposite of the movement of the fork o So, a new primer is made every few hundred base pairs as the DNA duplex is unwound. o Synthesis occurs 5’ to 3’, resulting in discontinuous segments called Ozaki fragments o The RNA primers are removed and the adjacent fragments are joined together by DNA ligase
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- Leading versus Lagging Strand Synthesis o Fig 4-30 - SV40 DNA o Most of what we know about the replication process comes from studying circular small viral DNAs o SV40DNA – from a virus that infects monkeys o Virus infected cells replicate large numbers of viral genome in a very short period of time using the DNA replication machinery of their hosts 1. Large T-antigen acts as helicase . Single un-wound strands bound by multiple copies of replication protein A ( RPA ), a heterotrimeric protein 2. Primers made by a complex of primase and DNA polymerase α (mixed DNA-RNA primer) 3. Primer is extended into daughter-strand DNA by DNA polymerase δ (has proofreading mechanism, unlike α) 4. Pol δ forms a complex with replication factor C ( Rfc ) and proliferating cell nuclear antigen ( PCNA ), and replaces Pol α-primase complex after primer synthesis - SV40 DNA replication o PCNA - homotrimeric complex with a hold through which the daughter
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Cell Bio - Exam 3 - Exam 3 Notes 4.5 DNA Replication - The...

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