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In [[cell biology]], [[eukaryotes]] possess a regulatory system that ensures that [[DNA replication]] occurs only once per [[cell cycle]].
A key feature of the DNA replication mechanism in eukaryotes is that it is designed to replicate relatively large [[genomes]] rapidly and with high fidelity. Replication is initiated at multiple [[origins of replication]] on multiple [[chromosomes]] simultaneously so that the duration of [[S phase]] is not limited by the total amount of [[DNA]].<ref name = "Diffley2008" >{{cite journal |author= Diffley, J.F. |title= Regulation of Early Events in Chromosome Replication |journal=Curr. Biol. |volume=14 |pages=R778–R786 |year=2008 |pmid= 15380092 |issue= 18 |doi= 10.1016/j.cub.2004.09.019|doi-access= free }}</ref> This flexibility in genome size comes at a cost: there has to be a high-fidelity control system that coordinates multiple replication origins so that they are activated only once during each [[S phase]]. If this were not the case, daughter cells might inherit an excessive amount of any DNA sequence, which could lead to many harmful effects.<ref name = "Kearsey2003" >{{cite journal |author1=Kearsey, S.E. |author2=Cotteril, S. |title= Enigmatic variations: divergent modes of regulating eukaryotic
== The replication origin ==
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===Yeast===
Origins in [[budding yeast]] are defined by the [[autonomously replicating sequence]] (ARS), a short stretch of DNA (100-200 bp) that can initiate replication when transferred to any sequence of DNA.<ref name="The Cell Cycle" /><ref name = "Mechali 2010" >{{cite journal |author= Mechali, M. |title=Eukaryotic DNA replication origins: many choices for appropriate answers|journal=Nature Reviews Molecular Cell Biology |volume=11|pages=728–738 |year=2010 |doi= 10.1038/nrm2976 |issue= 10 |pmid=20861881}}</ref> The ARS contains several specific sequence elements. One of these is the A element (ACS), an 11 bp consensus sequence rich in adenines and thymines that is essential for initiation. Single base-pair mutations in the ACS can abolish initiation activity.<ref name="The Cell Cycle" /><ref name = "Gilbert2001" >{{cite journal |author= Gilbert, D.M. |title=Making sense of eukaryotic replication origins|journal=Science|volume=294|pages=96–100 |year=2001 | pmid=11588251 | doi=10.1126/science.1061724 |issue= 5540 |pmc= 1255916 }}</ref> The ORC, a component of the initiation complex, binds the ACS [[in vivo]] throughout the cell cycle, and [[in vitro]] in an [[Adenosine triphosphate|ATP]] dependent manner. When a few of these sequences are deleted, DNA is still copied from other intact origins, but when many are deleted, chromosome replication slows down dramatically.<ref name="The Cell Cycle" /> Still, presence of an ACS sequence is not sufficient to identify an origin of replication. Only about 30% of ACS sequences present in the genome are the sites of initiation activity.<ref name="Mechali 2010" /> Origins in [[fission yeast]] contain long stretches of DNA rich in thymines and adenines that are important for origin function, but do not exhibit strong sequence similarity.<ref name="The Cell Cycle" />
===Animals===
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====The ORC====
The [[Origin Recognition Complex|ORC]] is a six subunit complex that binds DNA and provides a site on the chromosome where additional replication factors can assemble. It was identified in ''S. cerevisiae'' by its ability to bind the conserved A and B1 elements of yeast origins. It is a conserved feature of the replication system in Eukaryotes.<ref name = "Bell2002" /> Studies in ''[[Drosophila]]'' showed that recessive lethal mutations in multiple ''drosophila'' ORC subunits reduces the amount of [[BrdU]] (a marker of active replication), incorporated.<ref name = "Pflumm2001" >{{cite journal |author1=Pflumm, M.F. |author2=Bochtan, M.R. |title=Orc mutants arrest in metaphase with abnormally condensed chromosomes|journal=Development |volume=128|pages=1697–1707 |year=2001 |pmid= 11290306 |issue= 9}}</ref> Studies in ''[[Xenopus]]'' extracts show that immuno-depletion of ORC subunits inhibits [[DNA replication]] of ''Xenopus'' sperm nuclei. In some organisms, the ORC appears to associate with chromatin throughout the cell cycle, but in others it dissociates at specific stages of the cell cycle.<ref name = "Bell2002" />
==== Cdc6 and Cdt1 ====
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===Yeast===
In budding yeast, CDK is the key regulator of pre-RC assembly.<ref name = "The Cell Cycle" /> Evidence for this is that inactivation of CDKs in cells arrested in G2/M or in S phase drives reassembly of pre-RCs.<ref name = "Diffley2008" /> CDK acts by inhibiting the individual components of the pre-RC. CDK phosphorylates Cdc6 to mark it for degradation by the SCF in late G1 and early S phase.<ref name = "Diffley2008" /> CDK also induces export of Mcm complexes and Cdt1 from the nucleus.<ref name = "Diffley2008" /> Evidence that CDKs regulate the localization of Mcm2-7
CDK also phosphorylates ORC proteins. It has been suggested that phosphorylation affects the ability of the ORC to bind other components of the pre-RC.<ref name="The Cell Cycle" />
To get substantial re-replication of DNA, regulation of all three components, Cdc6, Mcm2-7 and the ORC has to be prevented. Having multiple mechanisms to prevent re-replication is beneficial because it the regulatory network continues to function even if one of the components fails.<ref name="The Cell Cycle" />
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===Animals===
[[Geminin]] is an important inhibitor of pre-Rc assembly is metazoan cells.<ref name="The Cell Cycle" />
Geminin was identified in a screen for APC/C substrates in ''Xenopus''.<ref name = "Kirschner1998" >{{cite journal |author1=T.J. McGarry |author2=M.W. Kirschner |title=Geminin, an inhibitor of
The importance of CDKs in preventing re-licensing in metazoan cells is still unclear. Some studies have
==Activation of replication origins==
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==References==
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[[Category:DNA replication]]
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