What is the central difference between chromosomal DNA replication in prokaryotes and eukaryotes?

• A. Prokaryotes have circular chromosomes with a single origin; eukaryotes have linear chromosomes with multiple origins.
• B. Prokaryotes replicate in the nucleus.
• C. Eukaryotes have no replication origins.
• D. Prokaryotes replicate in mitochondria.

Answer: (A)

The key idea is how the genome is organized and where replication starts. Prokaryotes typically have a circular chromosome that begins replication at a single origin and copies bidirectionally around the circle until the whole genome is duplicated. Eukaryotes, on the other hand, have linear chromosomes and use many origins of replication along each chromosome; these origins fire in S phase so the entire genome can be copied efficiently in a larger, more complex genome. This difference fits the biology: prokaryotes lack a nucleus and reproduce from a compact, circular DNA, while eukaryotes compartmentalize DNA in the nucleus and need multiple initiation sites to handle long linear chromosomes. So describing a single origin on a circular chromosome in prokaryotes versus multiple origins on linear chromosomes in eukaryotes best captures how chromosomal replication fundamentally differs between the two.