Thursday 20 November 2003
Replication forks stall or collapse at DNA lesions or problematic genomic regions, and these events have often been associated with recombination and chromosomal rearrangements.
Stalled forks generate single-stranded DNA that activates the replication checkpoint, which in turn functions to protect the stability of the fork until the replication can resume. Recombination-mediated and damage-bypass processes are the main mechanisms responsible for replication restart.
The stable propagation of genetic information requires that the entire genome of an organism be faithfully replicated only once in each cell cycle. In eukaryotes, this replication is initiated at hundreds to thousands of replication origins distributed over the genome, each of which must be prohibited from re-initiating DNA replication within every cell cycle.
Initiation of DNA replication occurs by a two-step process. In the first step initiation proteins are assembled onto the replication origin in a step-wise fashion to yield an initiation complex.
In the second step the initiation complex is activated by protein kinases, resulting in the establishment of replication forks. This process is tightly regulated such that initiation at a given replication origin occurs only once per cell cycle.
In addition, initiation is down regulated in response to agents that damage DNA or that block DNA replication.
The initiation of chromosomal DNA replication in eukaryotic cells is a highly regulated process and requires a series of complex events including recognition of origins, firing of replication origins, loading of DNA Polymerases onto origins, and elongation of newly synthesized DNA. Initiation of DNA replication takes place only at specific loci on the chromosomal DNA termed replication origins.
ORC (Origin Recognition Complexes), which are associated specifically with replication origin throughout the cell cycle serve as hallmark of the origins and is highly conserved.
Newly synthesized CDC6/CDC18 temporally associates with ORC at the G1-S boundary and is essential for assembly of preRC (pre Replication Complex) at origins of replication, before the initiation of DNA synthesis.
In the absence of CDC6, cells fail to initiate DNA replication and undergo a “reductional” mitosis, in which the unreplicated chromosomes are randomly segregated to the spindle poles. In addition to CDC6, ORC also binds to CDKs (Cyclin-Dependent Kinases).
This association is followed by the loading of MCM (Minichromosome Maintenance) protein complexes onto the replication origins, leading to the formation of preRCs. CDT1 is also required for MCM binding to chromatin.
After the MCM loading, S-phase CDKs phosphorylates CDC6/CDC18, and this phosphorylation leads to the rapid degradation of CDC6/CDC18 to prevent reinitiation.
Preceding the firing of origins, CDC7-Dbf4 kinase complexes phosphorylate MCM proteins in preRCs, and CDC45 is loaded onto preRCs at the replication origins. Finally, replication proteins, such as RPA (Replication Protein-A, single-stranded DNA-binding protein) and DNA Polymerases, are loaded onto RCs.
After DNA synthesis is initiated, some of the MCM components may leave the complexes, resulting in postRC.
Depletion of CDC6 prevents replication initiation and results in a reductional mitosis in which cells randomly segregate their unreplicated chromosomes. Under these circumstances, the G1-M checkpoint is not operating.
These checkpoints act to inhibit subsequent cell cycle events if there is a delay or error in a preceding stage.
Anomalies of DNA replication apparatus
- Anomalies of helicases
< object width="425" height="350"> < param name="movie" value="http://www.youtube.com/v/L9RjNNfgaEQ"> < /param> < param name="wmode" value="transparent"> < /param> < embed src="http://www.youtube.com/v/L9RjNNfgaEQ" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"> < /embed> < /object>
< object width="425" height="350"> < param name="movie" value="http://www.youtube.com/v/49fmm2WoWBs"> < /param> < param name="wmode" value="transparent"> < /param> < embed src="http://www.youtube.com/v/49fmm2WoWBs" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"> < /embed> < /object>
Heller RC, Marians KJ. Replisome assembly and the direct restart of stalled replication forks. Nat Rev Mol Cell Biol. 2006 Dec;7(12):932-43. PMID: 17139333
Chakalova L, Debrand E, Mitchell JA, Osborne CS, Fraser P. Replication and transcription: shaping the landscape of the genome. Nat Rev Genet. 2005 Sep;6(9):669-77. PMID: 16094312
Donaldson AD. Shaping time: chromatin structure and the DNA replication programme. Trends Genet. 2005 Aug;21(8):444-9. PMID: 15951049
Gonzalez MA, Tachibana KE, Laskey RA, Coleman N. Control of DNA replication and its potential clinical exploitation. Nat Rev Cancer. 2005 Feb;5(2):135-41. PMID: 15660109
Plosky BS, Woodgate R. Switching from high-fidelity replicases to low-fidelity lesion-bypass polymerases. Curr Opin Genet Dev. 2004 Apr;14(2):113-9. PMID: 15196456
Chakhparonian M, Wellinger RJ. Telomere maintenance and DNA replication : how closely are these two connected ? Trends Genet. 2003 Aug ;19(8):439-46. PMID : 12902162
Osborn AJ, Elledge SJ, Zou L. Checking on the fork: the DNA-replication stress-response pathway. Trends Cell Biol. 2002 Nov;12(11):509-16. PMID: 12446112
Franchitto A, Pichierri P. Protecting genomic integrity during DNA replication: correlation between Werner’s and Bloom’s syndrome gene products and the MRE11 complex. Hum Mol Genet. 2002 Oct 1;11(20):2447-53. PMID: 12351580
Blow JJ, Hodgson B. Replication licensing—defining the proliferative state? Trends Cell Biol. 2002 Feb;12(2):72-8. PMID: 11849970