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Rolling circle replication (RCR) is a process of unidirectional nucleic acid replication that can rapidly synthesize multiple copies of circular molecules of DNA or RNA, such as plasmids, the genomes of bacteriophages, and the circular RNA genome of viroids. Some eukaryotic viruses also replicate their DNA or RNA via the rolling circle mechanism.
A rolling circle mechanism that produces linear strands while progressing in a loop around the circular genome is also common. [8] Some dsDNA viruses use a strand displacement method whereby one strand is synthesized from a template strand, and a complementary strand is then synthesized from the prior synthesized strand, forming a dsDNA genome. [9]
The circular fragments are copied by rolling circle replication resulting in many single-stranded copies of each fragment. The DNA copies concatenate head to tail in a long strand, and are compacted into a DNA nanoball. The nanoballs are then adsorbed onto a sequencing flow cell.
Concatemers are frequently the result of rolling circle replication, and may be seen in the late stage of infection of bacteria by phages. As an example, if the genes in the phage DNA are arranged ABC, then in a concatemer the genes would be ABCABCABCABC and so on (assuming synthesis was initiated between genes C and A).
Each D-loop contains an origin of replication for the heavy strand. Full circular DNA replication is initiated at that origin and replicates in only one direction. The middle strand in the D-loop can be removed and a new one will be synthesized that is not terminated until the heavy strand is fully replicated, or the middle strand can serve as a primer for the heavy strand replication.
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The term plasmid was coined in 1952 by the American molecular biologist Joshua Lederberg to refer to "any extrachromosomal hereditary determinant." [11] [12] The term's early usage included any bacterial genetic material that exists extrachromosomally for at least part of its replication cycle, but because that description includes bacterial viruses, the notion of plasmid was refined over time ...
A circular chromosome, showing DNA replication proceeding bidirectionally, with two replication forks generated at the "origin". Each half of the chromosome replicated by one replication fork is called a "replichore".