replicons

Replicon is a DNA or RNA molecule or a region of DNA that replicates as a single, functional unit in a cell, starting from an origin of replication. The term emphasizes autonomous replication and can refer to whole chromosomes, plasmids, viral genomes, or defined regions within larger genomes. A key feature of a replicon is the presence of sequences that recruit the cell's replication machinery and regulate copy number and timing.

In bacteria and many archaea, chromosomes and plasmids are replicons. Replication begins at an origin (ori) and proceeds bidirectionally, yielding a copy number controlled by initiator proteins and regulatory elements. Bacterial chromosomes often behave as a single replicon, while plasmids are smaller replicons with defined copy numbers. Some organisms carry multiple replicons that together form the genome.

In eukaryotes, replication is organized into many origins of replication across chromosomes. These origins define replicons of variable sizes; replication is licensed to occur once per cell cycle to prevent re-replication. Replication timing programs lead to temporal organization of replicons during S phase, producing replication domains.

Organelles such as mitochondria and chloroplasts contain their own circular genomes that act as replicons, with distinct replication machineries and control elements separate from the nuclear genome.

In biotechnology, replicons form the basis of cloning vectors. Plasmids, viral vectors, and artificial replicons carry an origin of replication and selectable markers, enabling propagation in host cells and controlled copy number for expression or analysis.