Helicase loading occurs at all replicators during G1.

While activation occurs at S-phase.

Eukaryotic helicase loading requires four separate proteins to act at each replicator

Eukaryotic helicase loading does not lead to the immediate unwinding of origin DNA

Loaded helicases are activated by two protein kinases:

1. CDK (cyclindependent kinase)2. DK (Dbf4-dependent kinase)

How is this regulation achieved?

Budding yeast S. cerevisiae

CDKs has the power of regulation

CDKs are required to activate loaded helicases to initiate DNA replication.

CDK activity inhibits helicase loading.

Eukaryotic cells focus regulation on the initial loading of the MCM helicase onto the origin DNA.

Bacterial cells focus regulation on the binding of the DnaA initiator protein to the DNA (DnaA.ATP Levels and SeqA).

Prokaryotic Problem

Eukaryotic Problem

“priming protein” binds to the lagging-strand template and uses an amino acid to provide an OH(tyrosine)

certain bacterial and animal viruses.

Most eukaryotic cells use an entirely different solution

Telomerase is an enzyme that includes multiple subunits and an RNA component. (ribonucleoprotein) RNP

“Telomerase RNA” (TER)

Telomerase includes an RNA.DNA helicase activity.

Activator

Elimination of these proteins leads to the recognition of the telomeres as normal DNA breaks.

Telomeres isolated from human cells were observed by electron microscopy and found to form a loop rather than a linear structure