Practice question

DNA unwinding at origin creates bubble structure where parental duplex separates into single strands coated by single-strand binding proteins. At each boundary where double-stranded DNA transitions to single-stranded, replication fork assembles with helicase, primase, polymerases and clamp. Origin bubble therefore contains two oppositely oriented forks moving outward, one clockwise and one counterclockwise, each synthesizing leading strand continuously toward bubble center and lagging strand discontinuously via Okazaki fragments. Bidirectional bubbles expand, eventually fusing with adjacent bubbles. Electron microscopy of replicating SV40 chromosomes visualizes two forks per bubble.