Ooocx

mutations changes in DNA sequences il telomeres tips chromosome

Function(s)

Bacteria

Eukaryotes

single-stranded DNA binding, stimulates DNA polymerase, promotes origin unwinding

SSB (one subunit)

RPA (three subunits)

clamp loader

y8/8'r (5 subunits)

RFC (five subunits)

sliding clamp, holds DNA polymerase on DNA

g (two identical subunits)

PCNA (three identical subunits)

replicative DNA polymerase, proofreading exonuclease

DNA polymerase III

DNA polymerase 8 (two subunits) DNA polymerase e (four subunits)

DNA primase

DnaG

DNA polymerase a (four subunits)

Okazaki fragment processing

DNA polymerase I RNase DNA ligase H

Dna2 FEN1 RNase H DNA ligase I

DNA helicase

DnaB

?

Swivel ahead of replication fork

DNA gyrase

Topoisomerase I Topoisomerase II

Initiator protein

DnaA

Origin Recognition Complex (six subunits)

DNA replication proteins. controlling the assembly of complexes at replication origins. In bacteria, the accumulation of the initiator protein, dnaA, seems to be an important factor in determining when replication begins.

In eukaryotes, DNA replication and cell division are separated by two "gap" cell cycle phases (G1 and G2), during which neither DNA replication nor nuclear division occurs. DNA replication occurs during the S (or synthesis) phase, but ORC is thought to bind replication origins throughout the cell cycle. During the G1 phase of the cell cycle, ORC helps to assemble other replication initiation factors at replication origins to make so-called pre-replicative-complexes (pre-RCs) that are competent to initiate replication during S phase. These other initiation factors include a protein called Cdc6 and a family of six related MCM ("mini-chromosome maintenance") proteins. The functions of these proteins are not yet known; however, the MCM proteins are currently the best candidate for the eukaryotic replica-tive helicase, and Cdc6 is necessary for MCM proteins to bind DNA. DNA polymerase a also assembles on origins during this time.

Replication initiation is actually triggered at the beginning of S phase by the phosphorylation (addition of a phosphate group to) of one or more proteins in the pre-RC. The enzymes that phosphorylate proteins in the pre-RC are called protein kinases. Once they become active, they not only trigger replication initiation, but they also prevent the assembly of new pre-RCs. Therefore, replication cannot begin again until cells have completed cell division and entered G1 phase again. see also Cell Cycle; Chromosome, Eukaryotic; Chromosome, Prokaryotic; DNA; DNA Polymerases; DNA Repair; Mutation; Nucleases; Nucleotide; Nucleus; Telomere.

Carol S. Newlon

Bibliography

Baker, T. A., and S. P. Bell. "Polymerases and the Replisome: Machines within Machines." Cell 92 (1998): 295-305.

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