The primary effort has focused on replication of the SV 40 (Simian Virus 40) chromosome; these studies have progressed so rapidly in recent years that the SV40 chromosome now can be replicated in-vitro using only eight purified components from mammalian cells. Eukaryotic DNA replication is unidirectional.
The specific functions of these proteins are highly reminiscent of proteins required for replication of plasmids carrying “Ori.C”. The DNA molecules in eukaryotic cells are considerably larger than those in bacteria and are organized into complex nucleoprotein structure. There are Eukaryotic DNA Replication origins many choices for appropriate answers.
Mechanism of Eukaryotic DNA Replication:
There are three Eukaryotic DNA replication steps required to synthesize new DNA molecule.
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Initiation of Eukaryotic DNA replication is similar to E.Coli, replication, in Eukaryotic replication, it is initiated at a unique location on the SV40 DNA by inter activation of a virus-encoded, site-specific DNA binding protein called “T.antigen”.
- Prokaryotic DNA Polymerases
- Fidelity of DNA Replication
This multifunctional protein locally unwinds duplex at the SV40 origin also requires ATP and replication factor-A (RF-A). A host cell single-strand binding protein with a function similar to that of SSB Protein in E.Coli cells.
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As in E.coli, eukaryotic DNA replication occurs “bidirectionally from RNA primers made by a “Primase” synthesis of the leading strand is continuous, while synthesis of lagging strand is discontinuous.
Two distinct ‘Polymerases’ – a and d”, appear to function at the eukaryotic growing fork. Polymerase d (pol. d) is largely responsible for leading synthesis; polymerase a (pol. a), which is tightly associated with a “Primase”, is thought to synthesize the lagging strand. RNA primers, formed by the action of “Primase”, these are elongated for a short stretch by “Pol. a”, whose activity is stimulated by replication factor.C.
Binding of “PCNA (Proliferating Cell Nuclear Antigen)” at the primer-template terminus then displaces Pol.a, thus interrupting leading-strand synthesis. PCNA increases the “Processivity of the enzyme”. The function of PCNA thus appears to be highly analogous to that of the b-subunit of E.Coli polymerase-III.
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The termination of replication on linear eukaryotic chromosomes involves the synthesis of special structures called “Telomeres” at ends of the chromosome.
The telomeres consist of repetitive “Oligomeric sequences”. The enzyme that prevents this progressive shortening of the lagging strand is a “modified reverse transcriptase” called “Telomerase”, which can elongate the lagging-strand template from it’s 3’-hydroxyl end.
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This unusual enzyme contains a catalytic site that polymerizes deoxyribonucleotides directed by an RNA template as well as the RNA molecule that functions as that template.