Theta (θ) replication occurs in
Theta replication describes intermediate structures resembling Greek letter θ arising when circular double-stranded DNA undergoes bidirectional replication from single origin. As forks proceed, parental circle remains topologically linked to two newly synthesized circles forming bubble that expands until termination, producing intertwined catenated products separated by topoisomerase IV. This mode is characteristic of circular bacterial chromosomes, many plasmids, bacteriophage lambda and mitochondrial DNA. Linear chromosomes replicate via Y-shaped forks rather than theta, and rolling circle produces sigma structure with single-stranded tail. Visualization by electron microscopy confirmed Cairns theta model.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 5: Theta Replication Occurs in Circular DNA