The Topography of Chromosome Replication in Yeast

Abstract

Replication begins at multiple sites in eukaryotic chromosomes. This observation was first recorded in DNA fiber autoradiographs and later confirmed by electron microscopy. The tracing of an electron micrograph of replicating DNA (Fig. 1a) from the yeast Saccharomyces cerevisiae depicts a chromosomal molecule that is being replicated from four origins. This image illustrates two additional features of replication: Origins are found at an average spacing of from 50 kb to 100 kb, and not all origins are activated at the same time in S phase (Fig. 1b). The interpretations from electron micrographs such as this are based on several important assumptions. For example, the deductions made from this image rely on the assumptions that there was a single origin at the center of each of the four bubbles, that bidirectional replication is established at each origin, and that all forks move at constant