Areplicon is a region of an organism's genome that is independently replicated from a singleorigin of replication[citation needed]. A bacterial chromosome contains a single origin, and therefore the whole bacterial chromosome is a replicon. The chromosomes ofarchaea andeukaryotes can have multiple origins of replication, and so their chromosomes may consist of several replicons[citation needed]. The concept of the replicon was formulated in 1963 byFrançois Jacob,Sydney Brenner, 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.[1]
Sometimes inbacteriology, the term "replicon" is only used to refer to chromosomes containing a single origin of replication and therefore excludes the genomes ofarchaea andeukaryotes which can have several origins.[2]
For mostprokaryoticchromosomes, the replicon is the entire chromosome. One notable exception comes fromarchaea, where twoSulfolobus species have been shown to contain three replicons. Examples of bacterial species that have been found to possess multiple replicons includeRhodobacter sphaeroides (two),Vibrio cholerae,[3] andBurkholderia multivorans (three). These "secondary" (or tertiary) chromosomes are often described as molecules that are intermediate between a truechromosome and aplasmid and are sometimes called "chromids". VariousAzospirillum species possess seven replicons;A. lipoferum, for instance, has one bacterial chromosome, five chromids, and one plasmid.[4]Plasmids andbacteriophages are usually replicated as single replicons, but large plasmids inGram-negative bacteria have been shown to carry several replicons.[5]
Foreukaryotic chromosomes, there are multiple replicons per chromosome. Known examples range in size from 10 to 330kilobases. A cluster of replicons replicates simultaneously. But different clusters start replicating at different times duringS phase, depending on their location along the chromosomes. In general, clusters nearer thecentromere replicate earlier. Fine structure analysis of chromosomal origins of replication is limited to a singlemodel eukaryote,Saccharomyces cerevisiae. Therefore, no general picture of a replicon as replicator and initiator in eukaryotes has been achieved.
In the case ofmitochondria, the definition of replicons is somewhat confused, as they use unidirectional replication with two separate origins.
Non-cellular entities such asviruses,plasmids,transposons,retrotransposons,viroids,virusoids andRNA satellites are also replicons.Patrick Forterre of thePasteur Institute has coined the term "orphan replicon" to refer to those that are not viruses; i.e., that lack acapsid.[6]
