Which proteins load the MCM2-7 helicase onto DNA during eukaryotic origin licensing?

• A. Cdc6 and Cdt1 load MCM2-7 onto DNA during origin licensing.
• B. ORC alone loads MCM2-7 onto DNA.
• C. DnaA and DnaC load MCM2-7 onto DNA.
• D. Cdc6 alone loads MCM2-7 onto DNA during origin licensing.

Answer: (D)

Loading the MCM2-7 helicase onto DNA during origin licensing requires a coordinated action of the origin-recognition complex with the licensing factors Cdc6 and Cdt1. The ORC binds to the origin and recruits Cdc6; together, Cdc6 and Cdt1 deliver the MCM2-7 helicase to the DNA, with Cdt1 acting as a chaperone to escort MCM2-7 and position it for loading. This process results in the MCM2-7 complex being loaded as a double hexamer that encircles DNA, ready for replication to begin in S phase. DnaA and DnaC are bacterial initiators and do not participate in eukaryotic origin licensing.