Ori is the DNA sequence that signals for the origin of replication, sometimes referred to simply as origin. In E. coli, ori is some 250 nucleotides in length for the chromosomal origin (oriC). The plasmid ori sequences are similar to oriC.

During conjugation

Bacterial conjugation

Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...

, the rolling circle mode of replication

Rolling circle replication

Rolling circle replication describes a process of unidirectional nucleic acid replication that can rapidly synthesize multiple copies of circular molecules of DNA or RNA, such as plasmids, the genomes of bacteriophages, and the circular RNA genome of viroids...

 starts at the oriT ('T' for transfer) sequence of the FAT plasmid.

Bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 have a single origin for replication. Eukaryote

Eukaryote

A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s have multiple replicon

Replicon (genetics)

A replicon is a DNA molecule or RNA molecule, or a region of DNA or RNA, that replicates from a single origin of replication.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...

s, each with an ori. The replicons range from 40 kb (yeast and Drosophila) to 300 kb (plants) in length.

Mitochondrial DNA

Mitochondrial DNA

Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...

in many organisms has two ori sequences. In humans, they are called oriH and oriL for the heavy and light strand of the DNA, each is the origin of replication for single-stranded replication.