Conventional Dna Replication in Insects

Insect chromosomes provide wonderful model systems to unravel the mysteries of DNA replication. Data from studies of conventional replication of insect chromosomes are compatible with a mode of semidiscontinuous replication by a set of enzymes similar to those in mammals. Normally, replication initiates just once per S phase in each replicon. During various stages of insect development, however, origins of DNA replication (ORI) at certain loci become the targets of complex developmental signals that lead to their selective activation or repression. As a result, insect chromosomes exhibit several molecular phenomena such as DNA amplification and underrepresentation that are in direct violation of the "rule of DNA constancy." We believe that these phenomena may yield insights into more general principles for DNA replication control; understanding how a given OR1 is driven to fire more than once (or not at all) during S phase may define the molecular mechanisms that direct an OR1 to fire once and only once per cell cycle during conventional replication. In this chapter, we focus on the use of differential DNA replication in insect chromosomes as a model system to identify ORIs and explore the regulation of their activation. Many insect tissues become polyploid or polytene, and certain cellcycle controls are overridden during these processes. The chapter by Carminati and Orr-Weaver (this volume) describes studies on Drosophila mutants that have provided insights into controls for progression through the cell cycle.