Mechanism: How Polymerase Find Promoter Sequences

TLS Online TPP Program

#Id: 6000

#Unit 3. Fundamental Processes

The origin recognition complex (ORC) is a highly conserved complex found in all eukaryotes.  It is composed of six proteins with a mass of 400 kD.  ORC binds to the yeast A and B1 elements on the A-T–rich strand and is associated with ARS elements throughout the cell cycle.

TLS Online TPP Program

#Id: 6001

#Unit 3. Fundamental Processes

By counting the number of sites to which ORC binds, we can estimate that there are about 400 origins of replication in the yeast genome. This means that the average length of a replicon is 35,000 bp.

TLS Online TPP Program

#Id: 6002

#Unit 3. Fundamental Processes

ORC was first found in S. cerevisiae (where it may be called scORC), but similar complexes have now been characterized in Schizosaccharomyces pombe (spORC), Drosophila (DmORC), and Xenopus (XlORC). All of the ORC complexes bind to DNA.

TLS Online TPP Program

#Id: 6003

#Unit 3. Fundamental Processes

A eukaryotic genome is divided into multiple replicons, and the origin in each replicon is activated once and only once in a single division cycle.

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.