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5.5: DNA Replication

Last updated

Mar 17, 2025
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- 5.4: Base Pairing in DNA and RNA
- 5.6: The Meselson - Stahl Experiment

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Diagram of DNA replication fork, showing template strands, leading and lagging strands, DNA polymerase, DNA ligase, and Okazaki fragments. Arrows indicate direction of replication. — Figure 5.5.3: Control of Replication

With their multiple origins, how does the eukaryotic cell know which origins have been already replicated and which still await replication?

An observation: When a cell in G₂ of the cell cycle is fused with a cell in S phase, the DNA of the G₂ nucleus does not begin replicating again even though replication is proceeding normally in the S-phase nucleus. Not until mitosis is completed, can freshly-synthesized DNA be replicated again.

Two control mechanisms have been identified — one positive and one negative. This redundancy probably reflects the crucial importance of precise replication to the integrity of the genome.

Licensing: positive control of replication

In order to be replicated, each origin of replication must be bound by:

an Origin Recognition Complex of proteins (ORC). These remain on the DNA throughout the process.
Accessory proteins called licensing factors. These accumulate in the nucleus during G₁ of the cell cycle. They include:
- Cdc-6 and Cdt-1, which bind to the ORC and are essential for coating the DNA with
- MCM proteins. Only DNA coated with MCM proteins (there are 6 of them) can be replicated.

Once replication begins in S phase,

Cdc-6 and Cdt-1 leave the ORCs (the latter by ubiquination and destruction in proteasomes).
The MCM proteins leave in front of the advancing replication fork.

Licensing: positive control of replication

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