Plasmids

<p ><b>Plasmids.</b> Small bits of circular DNA. Often in E Coli. Distinct from bacterial chromosome but an autonomously replicating molecule. Also distinct from Bacteriophages though both can be used as vectors for DNA modification. Every plasmid has its own ‘origin of replication’ – a stretch of DNA that ensures it gets replicated (copied) by the host bacterium. For this reason, plasmids can copy themselves independently of the bacterial chromosome, so there can be many copies of a plasmid – even hundreds – within one bacterial cell.</p>

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A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Plasmids naturally exist in bacterial cells, and they also occur in some eukaryotes. Often, the genes carried in plasmids provide bacteria with genetic advantages, such as antibiotic resistance.</p>

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Once a gene is inserted into a plasmid, you obviously don't want to lose it. Wouldn't it be great if the bacteria did all the work maintaining the plasmid for us? Actually, that's one of the main reasons scientists put a gene into a bacterial plasmid. As we learned previously, the origin of replication is the place where the process of DNA replication begins. Therefore, if our plasmid possesses an origin of replication, the bacteria will automatically make a new copy of the plasmid during the replication process.</p>

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DNA replication creates copies of genetically-engineered plasmids

<b>Resistance plasmids</b>

These plasmids contain genes that provide resistance against antibiotics or poisons.

<b>Col plasmids</b>

These plasmids contain genes that code for bacteriocins, proteins that can kill other bacteria.

<b>F-plasmid</b>

Fertility F-plasmids contain tra genes and are capable of conjugation resulting in the expression of sex pilli.</p>

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An <b>episome</b> is distinguished from other pieces of DNA that are independent of the chromosome (i.e.,plasmids) by their large size.</p>

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<b>Plasmids are different from episomes</b>, as plasmid DNA cannot link up with chromosomal DNA. The plasmid carries all the information necessary for its independent replication. While not necessary for bacterial survival, plasmids can be advantageous to a bacterium. For example, plasmids can carry genes that confer resistance to antibiotics or toxic metals, genes that allow the bacterium to degrade compounds that it otherwise could not use as food, and even genes that allow the bacterium to infect an animal or plant cell. Such traits can be passed on to another bacterium.</p>

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