17.6: S-Cdk Initiates DNA Replication

The cell cycle is a series of events leading to DNA duplication followed by the division of cell content to form two daughter cells. The cell cycle progresses in four stages—the cell increases in size (gap 1 or G1-phase), duplicates its DNA (synthesis or S-phase), prepares to divide (gap 2 or G2-phase), and divides (mitosis or M-phase).

Two states at the origin of replication

In eukaryotes, the initiation of replication occurs at many sites on the chromosomes, called the origins of replication. During the progression of the cell cycle, the origins of replication exist in two states. The first state exists in the G1-phase when a multiprotein complex called the pre-replicative complex (pre-RC) assembles on the origin. The second state exists from the initiation of the S-phase to the end of the M-phase when a complex with fewer components called the post-replicative complex (post-RC) remains on the origin DNA.

Cdk activity controls each round of DNA replication

At the end of the M-phase, cyclin-dependent kinase (Cdk) activity is low within the cells, which permits the pre-RC assembly, resulting in a replication-competent state. During the G1—S phase transition, Cdk activity increases, triggering DNA replication initiation. The increased Cdk activity also causes the disassembly of the pre-RC complex, converting the origin to a post-RC state. Throughout the S-phase and the remainder of the cell cycle, persistent high Cdk activity prevents the re-assembly of pre-RC until the end of mitosis when the Cdk activity again reduces. This control mechanism inhibits re-replication.

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.