In eukaryotic cells, DNA replication initiates once per cell cycle at thousands of origins. A regulatory protein accumulates in S phase and prevents reassembly of the replicative helicase on origins already fired, thereby blocking relicensing. This inhibitor is:

• A p21, induced by p53 after origin firing
• B Geminin, which binds and sequesters Cdt1 ✓
• C Retinoblastoma protein, which holds E2F in check
• D PCNA, which occupies the fork and sterically excludes MCM loading

Explanation

Origin licensing in G1 requires ORC, Cdc6 and Cdt1 to load the inactive MCM2-7 helicase. In S phase, geminin accumulates, binds Cdt1, and prevents further MCM loading, ensuring each origin fires only once per cycle; APC/C-mediated degradation of geminin in late mitosis resets licensing. p21 and RB control cell-cycle progression broadly but are not the specific relicensing inhibitor, and PCNA functions at the elongating fork, not at unloaded origins.

Reference: Molecular Biology of the Cell, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.