Evidence for defined lengths of DNA replication units in 'satellite' DNA from D. ordii.

EUKARYOTTC DNA can be isolated as long fibres on which replication has occurred at several locations1–9. These replicating units are arranged in tandem and spaced at intervals which vary in length from 1–> 100 µm2–5,7–9. Although it can be determined that at any time replication has been initiated from a particular position on a fibre, it is not known whether that position remains fixed from one round of replication to the next, nor whether replication terminates at fixed locations on the DNA fibre. Attempts to resolve these questions have used experiments in which replicated units are visualised and their length and spatial distribution reconciled with models of fixed4,5 or varying3,8 sites of initiation and termination. Different studies have reached the different conclusions that these sites are variable3,8 or fixed4,5 (particularly with respect to termination of replication). All of these studies have been complicated by the observation that the length of replicated regions seems to vary in a manner consistent with a broad but continuous distribution for the length of the replicating unit. Such a distribution could reflect variation in the position of fixed termination sites or the fact that termination is not at a fixed position on the DNA fibre.