Pre-replication complex

A pre-replication complex (pre-RC) is a protein complex that forms at the origin of replication during the initiation step of DNA replication. The proteins involved in the pre-RC are essential for DNA replication.

The Prokaryotic Pre-RC
In prokaryotes, the pre-RC is made up of the following factors :
 * A helicase such as dnaA, which unwinds the DNA ahead of the replication fork.
 * A primase such as dnaG, which generates an RNA primer to be used in DNA replication.
 * A DNA holoenzyme, which is actually a complex of enzymes that performs the actual replication.

The Eukaryotic Pre-RC
In eukaryotes, the pre-RC is made up of the following factors:
 * A six-subunit complex called Origin Recognition Complex (ORC) which binds to the origin.
 * Two regulatory proteins called Cdc6 and Cdt1 which are recruited by ORC.
 * The MCMs (Minichromosome Maintenance proteins), the helicase complex.

These proteins assemble on cellular origins in G1 phase of the cell cycle. Once these proteins are assembled, the MCMs are phosphorylated and DNA replication begins.