What is a plasmid and what do plasmids do?

The takeaway: Plasmids perform important duties, and those new to a lab might be asked to design, modify, or build one. Let’s look at plasmids in more detail.

What is a plasmid?

A plasmid is a small circular piece of double stranded DNA that can replicate independently from a host’s chromosomal DNA. A plasmid is distinct from a cell’s chromosomal DNA and naturally exists in bacterial cells while also being found in some eukaryotes. The genes that some plasmids can carry may have genetic advantages, such as antibiotic resistance. They can vary widely in length, from 1,000 DNA base pairs to several hundred thousand DNA base pairs.

Where are plasmids found?

Plasmids are mostly found in bacteria but may also be found in yeast and plants. When found naturally, plasmids provide functions for a host such as resistance to antibiotics and virulence. Natural plasmids contain an origin of replication that controls the host range and copy number of the plasmid. Plasmids used in a lab are typically constructed and may be used to introduce foreign DNA into another cell; these lab-created plasmids contain an origin of replication, a selection marker, and a cloning site.

How is a plasmid constructed in the lab?

In the lab, plasmids are often called “vectors” or “constructs.” To get a particular gene into a vector, a researcher may use a cloning method such as a restriction enzyme or ligation. Once the cloning is completed, the vector with the new gene transforms into a bacterial cell and can then be grown on an antibiotic plate.

How are plasmids used?

To get a full plasmid definition it is important to understand how plasmids are used. Plasmids are used by research scientists in a variety of ways, though mostly they are used to manipulate gene expression in target cells. They are generally flexible, versatile, and cost-effective to use, meaning plasmids are used in many different types of applications. Plasmids can also be used to close, transfer, and manipulate genes.

These applications include:

• Producing a large amount of a protein so a researchers can purify and study it in a controlled setting
• Producing synthetic viruses that can be used in therapeutics research
• Creating enzymes that will make specific changes in a genome
• Producing proteins that clog, such as green fluorescent protein and luciferase, so researchers can track their location or quantity inside a cell
• Monitoring the level of a chemical in a setting

What types of plasmids are there?

The different types of plasmids include cloning plasmids, expression plasmids, gene knock-down plasmids, reporter plasmids, viral plasmids, and genome engineering plasmids.

A cloning vector is a small piece of DNA that can be maintained in an organism in a stable manner and that a foreign DNA fragment can be put into for cloning.

Expression plasmids are plasmids that are designed to facilitate gene expression in a cell. Also called an expression vector, it introduces a specific gene in a target cell and can be used to produce the protein that is encoded by the gene.

A gene knock-down plasmid is a plasmid that can be used to replace genes that are found in yeast or mice.

Reporter plasmids are plasmids that contain the reporter gene. This material identifies and characterizes the promoter and enhancer element functions since reporter gene expression corresponds with the transcriptional activity that the reporter gene undertakes.

A viral plasmid is a plasmid engineered to deliver a virus to a host; this mimics how natural viruses are delivered to a host. They can be modified in research settings so they are able to create viruses with modified viral genomes.

Genome engineering plasmids are a group of two or three plasmids used in CRISPR genome editing systems; they express the gRNA and Cas9 nuclease.

What is the difference between genome DNA and plasmid DNA?

Both genomic DNA and plasmid DNA are found in living organisms. Genomic DNA are chromosomal DNA in living organisms that contain genetic information, while plasmid DNA are extrachromosomal DNA found in bacteria, archaea, and some eukaryotes—any diverse domain of organisms whose cells have a nucleus.

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