Simultaneous replication of both DNA strands. (a) Replication stages of a circular, double-stranded DNA plasmid, based on autoradiography and electron microscopic observations. Blue strands are parental DNA; red strands are new DNA. The two replication forks proceed in opposite directions (bidirectionally) around the circular plasmid until they meet at the opposite side, forming two duplex circles. (b) Autoradiographs viewed in the electron microscope, showing ³H-dTTP-labeled plasmid DNA undergoing replication from a single origin. The origin is located in the center of the small DNA bubble in the first micrograph.