Rolling circle replication
Encyclopedia
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
or RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, such as plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
s, the genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
s of bacteriophage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
s, and the circular RNA genome of viroid
Viroid
Viroids are plant pathogens that consist of a short stretch of highly complementary, circular, single-stranded RNA without the protein coat that is typical for viruses. The smallest discovered is a 220 nucleobase scRNA associated with the rice yellow mottle sobemovirus...
s. Some eukaryotic viruses also replicate their DNA via a rolling circle mechanism.
Circular DNA replication
Rolling circle DNA replicationDNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
is initiated by an initiator protein encoded by the plasmid or bacteriophage DNA, which nicks one strand of the double-stranded, circular DNA molecule at a site called the double-strand origin, or DSO. The initiator protein remains bound to the 5' phosphate end of the nicked strand, and the free 3' hydroxyl end is released to serve as a primer
Primer (molecular biology)
A primer is a strand of nucleic acid that serves as a starting point for DNA synthesis. They are required for DNA replication because the enzymes that catalyze this process, DNA polymerases, can only add new nucleotides to an existing strand of DNA...
for DNA synthesis by DNA polymerase III. Using the unnicked strand as a template, replication proceeds around the circular DNA molecule, displacing the nicked strand as single-stranded DNA. Displacement of the nicked strand is carried out by a host-encoded helicase called PcrA (the abbreviation standing for plasmid copy reduced) in the presence of the plasmid replication initiation protein.
Continued DNA synthesis can produce multiple single-stranded linear copies of the original DNA in a continuous head-to-tail series called a concatemer. These linear copies can be converted to double-stranded circular molecules through the following process:
First, the initiator protein makes another nick to terminate synthesis of the first (leading) strand. RNA polymerase
RNA polymerase
RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...
and DNA polymerase III then replicate the single-stranded origin (SSO) DNA to make another double-stranded circle. DNA polymerase I
DNA polymerase I
DNA Polymerase I is an enzyme that participates in the process of DNA replication in prokaryotes. It is composed of 928 amino acids, and is an example of a processive enzyme - it can sequentially catalyze multiple polymerisations. Discovered by Arthur Kornberg in 1956, it was the first known...
removes the primer, replacing it with DNA, and DNA ligase
DNA ligase
In molecular biology, DNA ligase is a specific type of enzyme, a ligase, that repairs single-stranded discontinuities in double stranded DNA molecules, in simple words strands that have double-strand break . Purified DNA ligase is used in gene cloning to join DNA molecules together...
joins the ends to make another molecule of double-stranded circular DNA.
Rolling circle replication has found wide uses in academic research and biotechnology
Biotechnology
Biotechnology is a field of applied biology that involves the use of living organisms and bioprocesses in engineering, technology, medicine and other fields requiring bioproducts. Biotechnology also utilizes these products for manufacturing purpose...
, and has been successfully used for amplification of DNA from very small amounts of starting material.
External links
- DNA replication systems used with small circular DNA molecules Genomes 2, T. Brown et al., at NCBINational Center for Biotechnology InformationThe National Center for Biotechnology Information is part of the United States National Library of Medicine , a branch of the National Institutes of Health. The NCBI is located in Bethesda, Maryland and was founded in 1988 through legislation sponsored by Senator Claude Pepper...
Books - MicrobiologyBytes: Viroids and Virusoids