All four core histones show a similar type of structure in which three a-helices are connected by two loops. This highly conserved structure is called the histone fold.

TLS Online TPP Program

#Id: 6004

#Unit 3. Fundamental Processes

Licensing factor is necessary for initiation of replication at each origin. Licensing factor is present in the nucleus prior to replication, but is removed, inactivated, or destroyed by replication. Initiation of another replication cycle becomes possible only after licensing factor reenters the nucleus after mitosis.

TLS Online TPP Program

#Id: 6005

#Unit 3. Fundamental Processes

Orc2-5 binds strongly; Orc6 binds weakly to ARS and has a nuclear localization signal that must be activated by the cyclin/ CDK kinase during the G1 to S transition.

TLS Online TPP Program

#Id: 6006

#Unit 3. Fundamental Processes

ATP is required for the binding, but is not hydrolyzed until a later stage. The transcription factor ABF1 binds to the B3 element; this assists initiation by affecting chromatin structure, but it is the events that occur at the A and B1 elements that actually cause initiation

TLS Online TPP Program

#Id: 6007

#Unit 3. Fundamental Processes

Most origins are localized in regions between genes, which suggests that it may be important for the local chromatin structure to be in a nontranscribed condition.

TLS Online TPP Program

#Id: 6008

#Unit 3. Fundamental Processes

At the end of the cell cycle, ORC is bound to A–B1 elements of the origin. There is a change during G1 that results from the binding of Cdc6 and Cdt1 proteins to the ORC. 

In yeast, Cdc6 is a highly unstable protein, with a half-life of 5 minutes. It is synthesized during G1 and typically binds to ORC between the exit from mitosis and late G1. Its rapid degradation means that no protein is available later in the cycle.

TLS Online TPP Program

#Id: 6009

#Unit 3. Fundamental Processes