What is a key difference in origin activation between prokaryotes and eukaryotes?

• A. Prokaryotic origin activation is less complex and often single-origin; eukaryotic origins require licensing, licensing factors, and cell-cycle cues to fire.
• B. Prokaryotic origin activation is more complex; eukaryotes use a single origin.
• C. Both types use licensing and cell-cycle cues.
• D. Eukaryotic origins can fire without any licensing.

Answer: (D)

Origin activation is regulated very differently between these two groups. Prokaryotes typically start replication at a single origin on their circular chromosome, using straightforward initiation factors, so activation is simpler and more centralized. Eukaryotes, with linear chromosomes and many possible origins, rely on a licensing system: origins must be prepared in G1 by loading the MCM helicase complex with the help of Origin Recognition Complex and licensing factors like Cdc6 and Cdt1. This licensing ensures origins can fire only once per cell cycle, in S phase, when kinases (CDKs and DDK) trigger their activation. Because of this, eukaryotes coordinate the firing of many origins with the cell cycle, while prokaryotes don’t depend on such licensing and generally initiate at a single origin.