Fig. 5.

Chorion gene amplification. Six locations in the genome amplify the DNA encoding major and minor chorion proteins. Shown here is the portion of DAFC-66D that contains s18 and s15, genes encoding the late chorion proteins Cp18 and Cp15. DNA replication initiation occurs at discrete origins associated with open areas of chromatin, which are indicated by curved arrows bracketing each gene. The winged helix-turn-helix E2F transcription factor and the Dbf4-like zinc finger protein encoded by chiffon (so named for the translucent eggshells of mutants; Landis and Tower 1999) facilitate the binding of origin recognition complex (ORC), which in turn recruits several additional winged helix-turn-helix proteins (Cdc6, double-parked/Cdt1) and the mini-chromosome maintenance (MCM) DNA helicase complex, allowing association of the helicase CDC45. Two kinases, Cyclin-dependent kinase (Cdk2), with its partner Cyclin E, and Dbf4-dependent kinase (Cdc7; Stephenson et al. 2015), activate this prereplication complex. Activation facilitates binding of proliferating cell nuclear antigen (PCNA) and the DNA polymerase complex (Pol), thereby initiating bidirectional fork movement. SUUR limits fork movement to ∼50 kb in each direction. Several rounds of reinitiation create branched duplexes such that DNA levels are highest at the origins and gradually decrease on each side (reviewed by Tower 2004; Claycomb and Orr-Weaver 2005; Nordman and Orr-Weaver 2012).