A model mechanism for the replication of DNA molecules, so named because the growing point can be imagined as rolling around a circular template strand. The circular DNA is shown here in A. In B, a nick opens one strand, and the free 3′-OH end is extended by DNA polymerase. The newly synthesized strand displaces the original parental strand as it grows (C, D). By E, the polymerase has completed one revolution, and by F, two revolutions. The result is a molecule containing three unit genomes, one old and two new. The displaced strand can then serve as a template for a complementary strand. This mechanism is used to generate concatemeric duplex molecules (e.g., phage lambda, amplified rDNA in amphibian oocytes, etc.). This type of DNA replication is sometimes called sigma replication because the structure produced by the rolling circle resembles the Greek lower case sigma (σ). The rolling circle mechanism also occurs in viroids (q.v.). Here the infectious (+) viroid RNA serves as a template for an RNA polymerase, which generates a concatomeric, complementary (−) strand. In turn, this serves as template for synthesis of a (+) concatomeric RNA, which is later cleaved into genomic units that are then ligated to form circles. See Chronology, 1968, Gilbert and Dressler; hammerhead ribosome, plus (+) and minus (−) viral strands, theta replication.
Subjects: Genetics and Genomics — Chemistry.