Control of chromosome duplication: Difference between revisions

Revision as of 13:08, 19 October 2012

Revision as of 22:59, 30 December 2012

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===Components of the Pre-Rc===

===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 |author= Pflumm, M.F and 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" />

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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 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, cdt1 did not accumulate in the nucleus. Conversely, when an [[NCS]]{{dndisambiguation needed|date=November 2011}} tag 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" />