Plays a crucial role in forming the pre-replicative complexes. Interacts with the origin recognition complex (ORC) and MCM2-7 helicase complex leading to the linking of those complexes and loading of the replicative helicase MCM2-7 onto the pre-replicative complexes. Required for the initiation of DNA replication and then actively participates in the suppression of nuclear division. Associates with the ORC complex and the MCM2-7 helicase complex. Interacts with CDC4, CDC28, DIA2, ORC1, and TOM1. Transcribed at the end of mitosis, but in cells with a prolonged G1 phase there is a second burst of transcription in late G1. Ubiquitinated by the E3 ubiquitin ligase complex SCF(CDC4), DIA2, and TOM1; and targeted to the 26S proteasome for degradation. Phosphorylated by CDC28. Phosphorylation is a prerequisite to ubiquitination and degradation. Belongs to the CDC6/cdc18 family.