A replicon is the entire region of DNA that is independently replicated from a single

origin of replication The origin of replication (also called the replication origin) is a particular sequence in a genome at which replication is initiated. Propagation of the genetic material between generations requires timely and accurate duplication of DNA by se ...

. A bacterial chromosome contains a single origin, and therefore the whole bacterial chromosome is a replicon. The chromosomes of archaea and

eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...

can have multiple origins of replication, and so their chromosomes may consist of several replicons. The concept of the replicon was formulated in 1963 by François Jacob,

Sydney Brenner Sydney Brenner (13 January 1927 – 5 April 2019) was a South African biologist. In 2002, he shared the Nobel Prize in Physiology or Medicine with H. Robert Horvitz and Sir John E. Sulston. Brenner made significant contributions to work ...

, and Jacques Cuzin as a part of their replicon model for replication initiation. According to the replicon model, two components control replication initiation: the replicator and the initiator. The replicator is the entire DNA sequence (including, but not limited to the origin of replication) required to direct the initiation of DNA replication. The initiator is the protein that recognizes the replicator and activates replication initiation. Sometimes in

bacteriology Bacteriology is the branch and specialty of biology that studies the morphology, ecology, genetics and biochemistry of bacteria as well as many other aspects related to them. This subdivision of microbiology involves the identification, classific ...

, the term "replicon" is only used to refer to chromosomes containing a single origin of replication and therefore excludes the genomes of archaea and

which can have several origins.

Prokaryotes

For most

prokaryotic A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...

chromosomes A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

, the replicon is the entire chromosome. One notable exception comes from archaea, where two ''

Sulfolobus ''Sulfolobus'' is a genus of microorganism in the family Sulfolobaceae. It belongs to the archaea domain. ''Sulfolobus'' species grow in volcanic springs with optimal growth occurring at pH 2-3 and temperatures of 75-80 °C, making them ...

'' species have been shown to contain three replicons. Examples of bacterial species that have been found to possess multiple replicons include '' Rhodobacter sphaeroides'' (two), ''

Vibrio cholerae ''Vibrio cholerae'' is a species of Gram-negative, facultative anaerobe and comma-shaped bacteria. The bacteria naturally live in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and ot ...

'', and '' Burkholderia multivorans'' (three). These "secondary" (or tertiary) chromosomes are often described as molecules that are intermediate between a true

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

and a

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; howev ...

and are sometimes called " chromids". Various '' Azospirillum'' species possess seven replicons; '' A. lipoferum'', for instance, has one bacterial chromosome, five chromids, and one plasmid.

Plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; howev ...

s and

bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bact ...

s are usually replicated as single replicons, but large plasmids in

Gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

have been shown to carry several replicons.

Eukaryotes

For

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...

chromosomes, there are multiple replicons per chromosome. Known examples range in size from 10 to 330 kilobases. A cluster of replicons replicates simultaneously. But different clusters start replicating at different times during

S phase S phase (Synthesis Phase) is the phase of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during ...

, depending on their location along the chromosomes. In general, clusters nearer the

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers ...

replicate earlier. Fine structure analysis of chromosomal origins of replication is limited to a single model eukaryote, ''Saccharomyces cerevisiae''. Therefore, no general picture of a replicon as replicator and initiator in eukaryotes has been achieved. In the case of mitochondria, the definition of replicons is somewhat confused, as they use unidirectional replication with two separate origins.

Non-cellular entities

Non-cellular entities such as

virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...

es,

transposon A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Transp ...

retrotransposon Retrotransposons (also called Class I transposable elements or transposons via RNA intermediates) are a type of genetic component that copy and paste themselves into different genomic locations (transposon) by converting RNA back into DNA through ...

viroid Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective econo ...

s, virusoids and RNA satellites are also replicons. Patrick Forterre of the

Pasteur Institute The Pasteur Institute (french: Institut Pasteur) is a French non-profit private foundation dedicated to the study of biology, micro-organisms, diseases, and vaccines. It is named after Louis Pasteur, who invented pasteurization and vaccines f ...

has coined the term "orphan replicon" to refer to those that are not viruses; i.e., that lack a

capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may ...

References

DNA replication {{genetics-stub

Prokaryotes

Eukaryotes

Non-cellular entities

See also

References