Replication complex activity

Nasmyth We know that in G2, where HI kinase is not high, we know that there is sufficient cdk activity to inhibit pre-replication complex formation. If it has gone down at interphase, there still could be a lot of cdk present. 

In yeast, H2A.X is dephosphorylated by the phosphatase PPH3p after it has been released from chromatin. The release of y-H2A.X from repaired chromatin is independent of DNA replication, and it therefore must be assumed that a chromatin remodeling complex actively exchanges 7-H2A.X from nucleosomes. Thus far, several candidate remodeling complexes have been identified that specifically target nucleosomes containing 7-H2A.X. 

Control of origin activity occiu s via specific protein complexes that are boimd at certain times of the cell cycle to a replication origin. For replication initiation, two states of this protein complex are important, known as the pre-rephcation complex and the post-replication complex. 

Two other protein complexes also function in eukaryotic DNA replication. RPA (replication protein A) is a eukaryotic single-stranded DNA-binding protein, equivalent in function to the E. coli SSB protein. RFC (replication/actor (J) is a clamp loader for PCNA and facilitates the assembly of active replication complexes. The subunits of the RFC complex have significant sequence similarity to the subunits of the bacterial clamploading (y) complex. 

In anaphase and during Cl, a prereplication complex (pre-RC) is formed at replication initiation sites which contains the Cdc6 protein and the MCM proteins, in addition to the constitutively bound origin recognition complex (ORC). Formation of pre-RC is negatively regulated by the C2/M cyclin -CDK complex. The Cdc6 protein and the MDM proteins are removed at the Cl-S transition due to phosphorylation by the S-phase cyclin - CDK complex which allows the onset of DNA replication. The activity of the cyclin-CDK complexes is con-... 

Following activation of an origin and initiation of DNA synthesis, the Cdc6 protein becomes phosphorylated and dissociates from the origin, causing transition of the prereplication complex to a post-replication complex that is no longer able to initiate DNA replication. 

Once cyclin A-CDK2 is activated by Cdc25A and the S-phase inhibitors have been degraded, DNA replication is initiated at pre-replication complexes. The general mechanism is thought to parallel that in S. cerevisiae (see Figure... 

Additional activities that occur during replication include proofreading and DNA repair. Pol 8, which is part of the replication complex, has the 3 5 -exonuclease activity required for proofreading. Enzymes that catalyze repair of mismatched bases are also present (see section III.B.3 of this chapter). Consequently, eukaryotic replication occurs with high fidelity approximately one mis-pairing occurs for every 10 to 10 nucleotides incorporated into growing DNA chains. 

Finally a number of gioups recendy came up with an intriguing new aspect of the link between PARP-1 and aging There are a few rare genedc disorders that induce premature aging in humans. One of these examples is Werner syndrome, due to mutation in the WPiNgput. The WRN gene encodes the WRN protein, a member of the RecQ helicase family which comprises an additional exonudease activity. WRN is part of the DNA replication complex and cooperates with several replication- and repair-relat proteins. Recendy, it was shown... 

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