In a prokaryotic circular chromosome, replication initiates at a single origin. How does this compare to eukaryotic origins?

• A. Prokaryotes Have Many Origins; Eukaryotes Have One
• B. Prokaryotes Have One Origin; Eukaryotes Have Many
• C. Both Have Equal Numbers of Origins
• D. Neither Has an Origin

Answer: (B)

The concept being tested is how origin usage differs between prokaryotic and eukaryotic genomes. In bacteria, the chromosome is circular and has a single origin of replication. Replication begins at that one site and proceeds bidirectionally around the circle until the two replication forks meet, copying the entire genome. This works efficiently for the relatively small, compact prokaryotic genome.

In contrast, eukaryotic chromosomes are linear and much larger. To replicate quickly within the cell cycle, each chromosome contains many origins of replication. These origins fire at different times during S phase, creating many replication forks that work in parallel and ensure the entire genome is copied in a timely manner. Licensing mechanisms help ensure origins fire once per cycle.

So, the correct statement reflects that prokaryotes have one origin while eukaryotes have many. The other possibilities don’t fit because they misstate the number of origins or imply no origin at all.