DUP/Cdt1 (red) functions near replication origins (green, top), but also during elongation (bottom).

Studies on yeast have provided significant insights into eukaryotic DNA replication, but it has been difficult to examine the process in more complex organisms. On page 225, Claycomb et al. have taken advantage of a peculiar aspect of insect biology to develop a new model for studying metazoan DNA replication, and have already made one surprising finding with the system.During a specific period of oocyte development in Drosophila, the ovarian follicle cells surrounding the oocyte begin to amplify several defined clusters of genes. Using real-time PCR and a combination of microscopy techniques, the authors discovered that this amplification occurs stepwise. First, initiation takes place at the loci that will be amplified, and then, in a separate developmental stage, the replication forks elongate. The natural separation of initiation and elongation provides an ideal model for studying both processes. The system has already helped uncover one apparent difference between yeast and fly systems. The DUP/Cdt1 protein, which is thought to act only during initiation in yeast, travels with the elongating replication forks in Drosophila. Claycomb et al. are now hoping to determine how this and other metazoan replication factors are regulated in vivo. ▪