Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular mechanisms of genetic information transfer
DNA replication and transcription
Eukaryotic DNAs have multiple origins of replication

Obviously, the Replication of eukaryotic DNA, which is organized into nucleosomes and packaged into Chromatin fibers, must be considerably more complex than the replication of a bacterial chromosome. To determine how METABOLISM/36.html">DNA replication proceeds in eukaryotes, autoradiographic experiments similar to Cairns's studies on E. coli replication were conducted. These experiments showed that eukaryotic DNA also replicates bidirectionally, but the replication forks move very slowly—about 10 times slower than in E. coli. If each chromosome had only a single pair of replication forks, the large size of eukaryotic DNA molecules means their replication would take nearly two months. It turned out that eukaryotic DNA replication initiates simultaneously at many points (the number of which likely exceeds a thousand). From each such origin, two replication forks move simultaneously in opposite directions (Fig. 28-4), allowing the replication of an entire eukaryotic chromosome to be completed even faster than that of a bacterial chromosome. Since a Introduction/5.html">Eukaryotic Cell contains many Chromosomes, all of them must replicate concurrently. Thus, many thousands of replication forks operate simultaneously within The Nucleus of a eukaryotic cell.