Which statement best describes how origin licensing prevents re-initiation in the same cell cycle in eukaryotes?

• A. Origins re-license themselves automatically in S-phase.
• B. Origins licensing factors are degraded immediately after firing.
• C. Origins fire multiple times without licensing.
• D. Origins are licensed in G1 and fired once per S-phase; licensing factors prevent re-licensing.

Answer: (D)

Origin licensing happens in G1 when factors load the MCM helicase onto replication origins, forming the pre-replicative complex and marking those sites as ready to fire. Once S-phase starts, the licensed origins activate and initiate DNA synthesis at most once. After firing, the licensing machinery is suppressed or removed—for example, licensing factors like Cdt1 are inhibited or degraded and inhibitors such as geminin prevent new licensing from occurring in the same cycle. This combination guarantees that an origin cannot be licensed again and re-fired within the same cell cycle, so each origin fires only once during S-phase. That’s why the statement describing licensing in G1, a single firing in S-phase, and the prevention of re-licensing by licensing factors best captures the mechanism. The other options don’t fit because licensing does not occur in S-phase, origins cannot fire multiple times without re-licensing, and while some licensing factors are degraded after firing, the essential concept is the controlled timing and inhibition that prevents re-licensing in the same cycle.