Chemical mutagens can be divided into two groups:

those that are mutagenic to both replicating and nonreplicating DNA, such as the alkylating agents and nitrous acid; and 

those that are mutagenic only to replicating DNA, such as base analogs— purines and pyrimidines with structures similar to the normal bases in DNA.

TLS Online TPP Program

#Id: 6009

#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 6010

#Unit 3. Fundamental Processes

In mammalian cells, Cdc6 is controlled differently; it is phosphorylated during S phase, and as a result it is degraded by the ubiquitination pathway.  Cdt1 is initially stabilized by the protein Geminin, which prevents its degradation, and subsequent Geminin binding prevents its reuse

TLS Online TPP Program

#Id: 6011

#Unit 3. Fundamental Processes

Hence Cdc6 and Cdt1 are the key licensing factors and helicase loading proteins.

TLS Online TPP Program

#Id: 6012

#Unit 3. Fundamental Processes

Cdc6 has an ATPase activity that is required for it to support initiation.

TLS Online TPP Program

#Id: 6013

#Unit 3. Fundamental Processes

The presence of Cdc6 and Cdt1 at the yeast origin allows the MCM proteins to bind to the complex in G1 in the inactive state.

TLS Online TPP Program

#Id: 6014

#Unit 3. Fundamental Processes

Helicases are loaded during G1 are only activated to unwind DNA and initiate replication after cells pass from the G1 to the S phase of the cell cycle in effect with S phase CDKC