11.2 Dna replication  (Page 6/12)

Dna replication of extrachromosomal elements: plasmids and viruses

To copy their nucleic acids, plasmids and viruses frequently use variations on the pattern of DNA replication described for prokaryote genomes. For more information on the wide range of viral replication strategies, see The Viral Life Cycle .

Rolling circle replication

Whereas many bacterial plasmids (see Unique Characteristics of Prokaryotic Cells ) replicate by a process similar to that used to copy the bacterial chromosome, other plasmids, several bacteriophages , and some viruses of eukaryotes use rolling circle replication ( [link] ). The circular nature of plasmids and the circularization of some viral genomes on infection make this possible. Rolling circle replication begins with the enzymatic nicking of one strand of the double-stranded circular molecule at the double-stranded origin (dso) site . In bacteria, DNA polymerase III binds to the 3’-OH group of the nicked strand and begins to unidirectionally replicate the DNA using the un-nicked strand as a template, displacing the nicked strand as it does so. Completion of DNA replication at the site of the original nick results in full displacement of the nicked strand, which may then recircularize into a single-stranded DNA molecule. RNA primase then synthesizes a primer to initiate DNA replication at the single-stranded origin (sso) site of the single-stranded DNA (ssDNA) molecule, resulting in a double-stranded DNA (dsDNA) molecule identical to the other circular DNA molecule.

Key concepts and summary

• The DNA replication process is semiconservative , which results in two DNA molecules, each having one parental strand of DNA and one newly synthesized strand.
• In bacteria, the initiation of replication occurs at the origin of replication , where supercoiled DNA is unwound by DNA gyrase , made single-stranded by helicase , and bound by single-stranded binding protein to maintain its single-stranded state. Primase synthesizes a short RNA primer , providing a free 3’-OH group to which DNA polymerase III can add DNA nucleotides.
• During elongation , the leading strand of DNA is synthesized continuously from a single primer. The lagging strand is synthesized discontinuously in short Okazaki fragments , each requiring its own primer. The RNA primers are removed and replaced with DNA nucleotides by bacterial DNA polymerase I , and DNA ligase seals the gaps between these fragments.
• Termination of replication in bacteria involves the resolution of circular DNA concatemers by topoisomerase IV to release the two copies of the circular chromosome.
• Eukaryotes typically have multiple linear chromosomes, each with multiple origins of replication. Overall, replication in eukaryotes is similar to that in prokaryotes.
• The linear nature of eukaryotic chromosomes necessitates telomeres to protect genes near the end of the chromosomes. Telomerase extends telomeres, preventing their degradation, in some cell types.
• Rolling circle replication is a type of rapid unidirectional DNA synthesis of a circular DNA molecule used for the replication of some plasmids.

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