NLSs were first discovered through the analysis of mutations of the gene for large T-antigen encoded by simian virus 40 (SV40).

TLS Online TPP Program

#Id: 2968

#Unit 3. Fundamental Processes

The presence of Cdc6 and Cdtl at the yeast origin allows the MCM proteins to bind to the complex in G I in the inactive state. Their presence is necessary forinitiation.

TLS Online TPP Program

#Id: 2969

#Unit 3. Fundamental Processes

In yeast,  The origin enters S phase in the condition of a prereplication complex, which contains ORC, Cdc6, Cdt1, and the inactive helicase, the MCM proteins

TLS Online TPP Program

#Id: 2970

#Unit 3. Fundamental Processes

The M C M 2 - 7 proteins form a six-member ring-shaped complex around DNA. MCM 2, 3, 5 are regulatory, while MCM4,6,7 have helicase activity.

TLS Online TPP Program

#Id: 2971

#Unit 3. Fundamental Processes

When initiation occurs, Cdc6 and Cdtl are displaced, returning the origin to the state of the postreplication complex, which contains only ORC. 

TLS Online TPP Program

#Id: 2972

#Unit 3. Fundamental Processes

In yeast, Cdc6 is rapidly degraded during S phase, and as a result it is not available to support reloading of MCM proteins. Thus, the origin crumot be used for a second cycle of initiation during S phase.

TLS Online TPP Program

#Id: 2973

#Unit 3. Fundamental Processes

in mammalian cells, Cdt1 is targeted for degradation by the action of a protein complex that is recruited to the origin of replication by PCNA, the eukaryotic counterpart of the bacterial beta-clamp.