In eukaryotes, what is the function of the Origin Recognition Complex (ORC)?

• A. ORC marks replication origins and recruits licensing factors Cdc6 and Cdt1 to load the MCM2-7 helicase onto DNA during G1.
• B. ORC marks replication origins and initiates DNA synthesis by polymerizing nucleotides.
• C. ORC directly unwinds DNA at replication forks to begin synthesis.
• D. ORC ligates DNA after primer removal.

Answer: (B)

The function being tested is how eukaryotic origins are prepared for replication. The Origin Recognition Complex binds replication origins and coordinates the licensing step by recruiting the factors Cdc6 and Cdt1 to load the MCM2-7 helicase onto DNA during G1. This creates the pre-replication complex that marks the origin as licensed to fire later in the cycle, ensuring replication starts at the right place and only once per cell cycle. ORC itself does not synthesize DNA, unwind the DNA, or ligate strands. DNA polymerases carry out synthesis after the origin is activated, the helicase unwinds DNA when replication begins, and DNA ligase seals nicks after primer removal.