In eukaryotic chromosomes, how many origins of replication may exist on a chromosome?

In eukaryotic chromosomes, how many origins of replication may exist on a chromosome?

• A. Exactly one origin.
• B. Two origins.
• C. No origins; replication starts randomly.
• D. Hundreds or even thousands of origins of replication.

Answer: (D)

Eukaryotic chromosomes copy their DNA from many potential starting points. To duplicate a large linear genome within the short window of S-phase, initiation must occur at numerous sites across the chromosome. These sites are licensed in G1 by the origin recognition complex and the MCM helicase complex, creating a large pool of potential origins. When S-phase begins, many of these origins fire to start bidirectional replication forks, while others stay dormant and can be used if replication encounters problems. This redundancy and distribution mean that a chromosome typically has hundreds or thousands of origins of replication, not just one or a few. The idea that replication starts from just a single origin or that there are no origins contradicts what’s known about how eukaryotic genomes are efficiently and reliably copied.

Eukaryotic chromosomes copy their DNA from many potential starting points. To duplicate a large linear genome within the short window of S-phase, initiation must occur at numerous sites across the chromosome. These sites are licensed in G1 by the origin recognition complex and the MCM helicase complex, creating a large pool of potential origins. When S-phase begins, many of these origins fire to start bidirectional replication forks, while others stay dormant and can be used if replication encounters problems. This redundancy and distribution mean that a chromosome typically has hundreds or thousands of origins of replication, not just one or a few. The idea that replication starts from just a single origin or that there are no origins contradicts what’s known about how eukaryotic genomes are efficiently and reliably copied.