Practice question

Origins are prepared for replication during G1 through hierarchical assembly of pre-replicative complex. Origin recognition complex remains bound to DNA throughout cell cycle serving as landing pad. ATPase Cdc6 then associates in G1 in ATP-bound form undergoing conformational change that stabilizes interaction with ORC and creates composite binding site for helicase chaperone complex comprising Cdt1 and MCM2-7 double hexamer. ATP hydrolysis by Cdc6 and ORC loads MCM rings encircling double-stranded DNA in head-to-head orientation for bidirectional replication. This licensed origin remains inert until activated in S phase by DDK phosphorylation recruiting Cdc45 and CDK phosphorylation of Treslin and RecQL4 facilitating GINS association forming active CMG helicase Cdc45-MCM-GINS. After initiation, Cdc6 becomes phosphorylated by CDK2 at Ser/Thr-Pro motifs, translocates to cytoplasm and targeted for recognition by SCF-Cdc4 ligase for degradation, removing ability to load new MCMs. In yeast APC/C also contributes to clearance. Mutations impairing Cdc6 ATPase activity prevent MCM loading, causing severe replication defects, confirming its role as essential helicase loader linking G1 low CDK environment to replication competence.