When the anomeric hydroxyl of a D-hexose is on the same side of the ring as C-6, the structure is β a by definition; when it is on the opposite side from C-6, the structure is α.

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

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.