Control of chromosome duplication: Difference between revisions

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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 [[DNA replication]]|journal=Mol. Cell |volume=12 |pages=1067–1075 |year=2003 |pmid= 14636567 | doi = 10.1016/S1097-2765(03)00441-6 |issue= 5|doi-access=free }}</ref>

== The replication origin ==

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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 this is that inactivation of CDKs in nocodozole[[nocodazole]] arrested cells induced accumulation of Mcm2-7 in the nucleus.<ref name = "Diffley2008" /> Cdt1 is also exported because it binds to the Mcm complex. In Mcm depleted cells, cdt1Cdt1 did not accumulate in the nucleus. Conversely, when ana NLS[[Nuclear localization (sequence|nuclear localization signal)]] was attached to Mcm7, Mcm2-7 and Cdt1 were always found in the nucleus.<ref name = "Diffley2008" /> Export of Mcm from the [[Cell nucleus|nucleus]] prevents loading of new Mcm complexes but does not affect the complexes that have already been loaded onto the DNA.<ref name="The Cell Cycle" />

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 [[DNA replication]], is degraded during mitosis|journal=Cell |volume=93|pages=1043–1053 |year=1998 |doi=10.1016/S0092-8674(00)81209-X |issue= 6 |pmid= 9635433|doi-access=free }}</ref> Studies have shown that Geminin prevents pre_RC assembly by binding to cdt1 and preventing its association with the pre-RC.<ref name = "Dutta2000" >{{cite journal |author1=J.A. Wohlschlegel |author2=B.T. Dwyer |author3=S.K. Dhar |author4=C. Cvetic |author5=J.C. Walter |author6=A. Dutta |title= Inhibition of eukaryotic DNA replication by geminin binding to Cdt1|journal=Science |volume=290|pages=2309–2312 |year=2000 |doi=10.1126/science.290.5500.2309 |issue=5500 |pmid= 11125146}}</ref> Since geminin is degraded by the APC/C, pre-Rc assembly can proceed only when APC/C activity is high, which occurs in G1.<ref name ="Diffley2008" />

The importance of CDKs in preventing re-licensing in metazoan cells is still unclear. Some studies have showedshown that under some conditions, CDKs can also promote licensing. In G0 mammalian cells, APC mediated degradation of Cdc6 prevents licensing. However, when the cells transition into a proliferative state, CDK phosphorylates Cdc6 to stabilizes it and allow it to accumulate and bind to origins before licensing inhibitors such as geminin accumulate.<ref name = "Mailand2005" >{{cite journal |author1=Mailand, N. |author2=Diffley J.F. |title=CDKs promote DNA replication origin licensing in human cells by promoting Cdc6 from APC/C dependent proteolysis |journal=Cell |volume=122|pages=915–926 |year=2005 | doi=10.1016/j.cell.2005.08.013 |issue= 6 |pmid= 16153703|doi-access=free }}</ref>

==Activation of replication origins==