How do prokaryotic origins differ from eukaryotic origins?

• A. Prokaryotes typically have a single origin of replication with a defined sequence; eukaryotes have multiple origins with licensing and cell-cycle-regulated firing.
• B. Prokaryotes have multiple origins; eukaryotes have a single origin with no licensing.
• C. Prokaryotes lack defined origins and begin replication randomly; eukaryotes have defined origins.
• D. Prokaryotes rely on RNA primers for origin firing; eukaryotes do not.

Answer: (A)

The main idea here is how replication origins differ in number and regulation between prokaryotes and eukaryotes. Prokaryotes typically have a single origin of replication on their circular chromosome, a defined sequence (often called oriC) that recruits initiator proteins to start bidirectional replication. Because there’s only one origin, there isn’t a licensing system to prevent reinitiation; replication proceeds once per cell cycle.

Eukaryotes, with their linear chromosomes, use many origins. These origins must be licensed in G1 by assembling a pre-replicative complex (involving licensing factors) so they’re ready to fire, and then are activated in S phase in a cell-cycle–regulated way. This licensing and controlled firing ensure each origin fires once and only once per cycle, coordinating replication across the genome.

So the best statement reflects one origin with a defined sequence in prokaryotes versus multiple, licensed origins with cell-cycle–regulated firing in eukaryotes. The other options mix up the numbers or omit the licensing mechanism.