A replicon is a DNA molecule or RNA molecule, or a region of DNA or RNA, that replicates from a single origin of replication.
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Prokaryotes
For most prokaryotic chromosomes, the replicon is the entire chromosome. One notable exception found comes from archaea, where two Sulfolobus species have been shown to contain three replicons. Examples of bacterial species that have been found to possess multiple replicons include: Rhodobacter sphaeroides (2), Vibrio cholerae, and Burkholderia multivorans (3). These "secondary" (or tertiary) chromosomes are often described as a molecule that is a mixture between a true chromosome and a plasmid and are sometimes called "chromids". Various Azospirillum species possess 7 replicons, Azospirillum lipoferum, for instance, has 1 bacterial chromosome, 5 chromids, and 1 plasmid. Plasmids and bacteriophages are usually replicated as single replicons, but large plasmids in Gram-negative bacteria have been shown to carry several replicons.
Eukaryotes
For eukaryotic chromosomes, there are multiple replicons per chromosome. In the case of mitochondria the definition of replicons is somewhat confused, as they use unidirectional replication with two separate origins.