Parts of a Plasmid – Ori, Resistance Gene, Cloning Site

All 7 parts on the diagram

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Start here: 2 parts of the cell 2

Plasmid A small, usually circular ring of double-stranded DNA. It is separate from the chromosome and copies itself on its own.
Chromosome The bacterium's main DNA, tangled in the cell. Much larger than a plasmid.

On the plasmid map: 5 5

Origin of replication The stretch of DNA where copying of the plasmid begins. Without it, the plasmid cannot copy itself.
Antibiotic resistance gene A gene that lets the bacterium survive an antibiotic. Scientists use it as a selectable marker: only bacteria with the plasmid grow when the antibiotic is added.
Multiple cloning site A short stretch full of restriction sites. New DNA can be cut in there. It is also called a polylinker.
Restriction sites Places where restriction enzymes cut the DNA. A new piece can then be joined in.
Gene of interest A gene inserted into the cloning site. The bacteria then copy it along with the plasmid.

How this list was compiled: Read 2026-09-27. Wikipedia, Plasmid, https://en.wikipedia.org/wiki/Plasmid : a small extrachromosomal DNA molecule, physically separated from chromosomal DNA, that replicates independently, most commonly small circular double-stranded DNA in bacteria; plasmids need an origin of replication to replicate; they often carry genes such as antibiotic resistance; cloning plasmids carry a selectable marker, mostly an antibiotic resistance gene, an origin of replication and a multiple cloning site or polylinker with several restriction sites where DNA fragments can be ligated; a gene of interest is inserted there. Licence: own work

The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work

How it works

These are the parts of a plasmid: first where plasmids sit in a bacterium, then one drawn as a map with a gene put into it. Start with the 2 parts of the cell.

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In the bacterium

A bacterium's main DNA is its large chromosome. Plasmids are small separate rings of double-stranded DNA that copy themselves on their own.

On the map

Copying starts at the origin of replication. The antibiotic resistance gene is the selectable marker. New DNA goes into the multiple cloning site, cut open at a restriction site, like the gene of interest here.

Only bacteria that took up the plasmid survive the antibiotic, so the resistance gene shows scientists which bacteria have it.

Questions and answers

What are the main parts of a plasmid?

An origin of replication, where copying starts; a selectable marker, mostly an antibiotic resistance gene; and, in plasmids made for cloning, a multiple cloning site where new DNA is put in.

What is the origin of replication?

The stretch of DNA where copying of the plasmid begins. A plasmid needs one to copy itself inside the cell.

How is a plasmid different from the chromosome?

A plasmid is a small, usually circular piece of DNA separate from the bacterium's much larger chromosome, and it copies itself independently.