Practice question

Eukaryotic genomes are orders of magnitude larger than bacterial, ranging from 12 Mb in yeast to 3 Gb in humans, requiring many origins to complete duplication within S phase timeframe. Mammalian chromosomes contain 30,000-50,000 origins, each defining replicon of 30-300 kb. Origin firing is staggered into early and late domains reflecting chromatin state, with euchromatic gene-rich regions replicating early and heterochromatic regions late. Licensing through ORC, Cdc6, Cdt1 and MCM assembly in G1 restricts initiation to once per cycle. Multirepliconic organization prevents excessive replication fork travel distance and ensures timely S phase completion.