Question

Undergoing two rounds of DNA replication in a single S-phase is lethal to the cell. One of the ways that cells help prevent “re-replication” is by ensuring that pre-RCs do not re-form on origins of replication more than once per cell cycle, thus they suppress pre-RC formation after G1-phase. What are the mechanisms by which the cells ensure that pre-RCs are not re-assembled during S-phase or beyond?

Answer 1

Regulation of licensing factors and the activation of cell cycle checkpoints are involved in preventing the assembly of Pre-RC.

Usually, the assembly of pre-replication complexes (pre-RCs) is a tightly regulated process and because of this, the replication occurs once per cell cycle. The activation of pre-RC at replication origins can only occur during the period of low concentrations of CDK (cyclin-dependent kinases ) and high APC/C (anaphase-promoting complex/cyclosome) period from late mitosis through the early G1 phase. The firing of the origin can only occur if APC/C is inactivated and there is reaccumulation or activation of CDKs. Due to this reason the pre-RCs cannot be assembled at origin during S phase and beyond that is G₂ and M.

The pre-RC assembly reaction is also called licensing, this reaction involves ATP dependent loading of the enzyme helicase and the Mcm2–7 complex. The reaction also requires the origin recognition complex (ORC) associated with two essential factors, Cdc6 and Cdt1.