6.6
S-Cdk initieert DNA-replicatie
De celcyclus is een reeks gebeurtenissen die leidt tot DNA-duplicatie, gevolgd door de verdeling van de celinhoud om twee dochtercellen te vormen. De…
In eukaryotes, DNA replication takes place during the S-phase of the cell cycle. The cell cycle control system, consisting of a network of regulatory proteins, governs the progression of the cell cycle.
The S-phase cyclin-dependent kinase, or S-Cdks, are enzyme complexes involved in cell cycle control during S-phase. This control system ensures that every nucleotide in the genome is copied only once during replication.
DNA replication begins at the origins of replication, present at multiple locations in the chromosomes. Before the initiation of replication, multiprotein complexes called origin recognition complexes bind to the DNA and serve as docking sites for other proteins.
During early G1-phase of the cell cycle, the regulatory proteins Cdc6 and Cdt1 bind to the ORCs, and aid in the assembly of a set of proteins called the MCM proteins into inactive ring complexes on the adjacent DNA, thereby forming large multiprotein complexes called the pre-replicative complexes or pre-RCs. MCMs function as DNA helicases for replication.
At the onset of S-phase, S-Cdks are activated and trigger origin firing, or initiation of DNA replication, by phosphorylating specific initiator proteins. Phosphorylated initiator proteins promote the recruitment of helicase activator complexes. These complexes activate the DNA helicases and recruit the DNA polymerase, leading to replication.
S-Cdks not only initiate replication but also prevent re-replication from occurring at the same origin.
S-Cdks phosphorylate the Cdc6 and Cdt1 proteins, promoting their release from the ORCs, leading to degradation, and the disassembly of the pre-RCs.
After DNA replication, when the helicases disengage from the DNA strand, the S-Cdks phosphorylate the helicases, triggering their export from the nucleus.
This cell cycle control mechanism involving S-Cdks prevents re-initiation of replication, thereby ensuring that the DNA replication occurs only once per cell cycle.
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Q1: What role do S-Cdks play in initiating DNA replication?
S-Cdks are enzyme complexes activated at the onset of S-phase that trigger origin firing by phosphorylating specific initiator proteins. These phosphorylated proteins promote recruitment of helicase activator complexes, which activate DNA helicases and recruit DNA polymerase, leading to replication initiation at multiple chromosomal origins.
Q2: How do pre-replicative complexes form during the cell cycle?
During early G1-phase, regulatory proteins Cdc6 and Cdt1 bind to origin recognition complexes on DNA. These proteins aid assembly of MCM proteins into inactive ring complexes on adjacent DNA, forming large multiprotein structures called pre-replicative complexes that prepare origins for replication.
Q3: What mechanism prevents re-replication at the same origin?
S-Cdks prevent re-replication by phosphorylating Cdc6 and Cdt1 proteins, promoting their release from origin recognition complexes and triggering their degradation. This disassembles pre-replicative complexes, converting origins to a post-replicative state where high Cdk activity persists throughout S-phase and mitosis, ensuring DNA replicates only once.
Q4: How does Cdk activity control each round of DNA replication?
Low Cdk activity at the end of M-phase permits pre-replicative complex assembly, creating a replication-competent state. During the G1-S transition, increased Cdk activity triggers replication initiation and disassembles pre-replicative complexes. Persistent high Cdk activity throughout S-phase prevents re-assembly until mitosis ends and activity reduces.
Q5: What happens to MCM proteins after DNA replication is complete?
MCM proteins function as DNA helicases during replication. After replication completes and helicases disengage from the DNA strand, S-Cdks phosphorylate these helicases, triggering their export from the nucleus and preventing them from re-initiating replication at the same origin during the cell cycle.
Q6: Why do origins of replication exist in two distinct states?
Origins exist in two states to regulate replication timing and prevent re-replication. The pre-replicative complex state in G1-phase creates a replication-competent origin. The post-replicative complex state from S-phase through M-phase, maintained by high Cdk activity, prevents re-replication and ensures DNA duplicates only once per cell cycle.
Q7: What is the relationship between origin recognition complexes and replication initiation?
Origin recognition complexes bind to DNA at multiple chromosomal locations and serve as docking sites for regulatory proteins like Cdc6 and Cdt1. These complexes anchor the assembly of pre-replicative complexes, which become competent for replication when S-Cdks are activated and phosphorylate initiator proteins during S-phase.