English: The assembly of the pre-RC during G1 readies the replication origin for firing. The activation of S–Cdks and Cdc7 triggers origin activation by promoting the formation of the preinitiation complex, a large group of proteins, including the DNA polymerases, that assembles at the origin, probably through an interaction with the Mcm2–7 complex. Some of the components of the preinitiation complex bind as dimers (Dpb1–Sld2 and Cdc45–Sld3) and tetramers (the GINS complex) and it is likely that these core proteins of the complex, together with Mcm10, bind simultaneously. Formation of the preinitiation complex results in the displacement of Cdc6 and Cdt1 from the ORC, which remains at the origin. Preinitiation complex formation activates the Mcm helicase, leading to unwinding of the DNA helix. On activation, Mcm subunits may be lost (not illustrated, for simplicity) or rearranged. RPA proteins (not shown) bind the single-stranded DNA, preventing its reannealing, and primase is recruited to begin primer synthesis and DNA replication. Synthesis of the leading strand begins first. The Mcm2–7 helicase and many components of the preinitiation complex travel out from the origin with the replication fork.^[1]