A plasmid is a piece of DNA, small and circular, that is not the same as chromosomal DNA since this one is the one that contains all the genetic material found in the chromosomes of an organism. It replicates separately from chromosomal DNA.

These plasmids are mostly found in organisms like bacteria, but they have also been known to exist in archaea and multicellular beings too. They usually contain one gene at least, and many of those are beneficial to their host organism. While they have separate genes from them, they aren’t considered to be independent.

What is their function?

Plasmids serve several purposes since they contain genes that can improve the survival of an organism. These genes might do their job by killing other organisms or by defending the host cell through toxins. They can also help in the process of replication in the bacteria.

Because they are so tiny, the number of genes they contain is quite small which usually have a specific function. One cell can also house multiple plasmids, and they each could perform a different function.

What are the types of plasmids?

Plasmids can be divided into three major types or categories, and they are:

• Conjugative and Non-Conjugative: this basically tells apart the plasmids that allow bacteria to reproduce by sexual conjugation, where the transfer of genetic material occurs between bacteria through direct contact or a bridge between the cells. Some plasmids have a type of gene that allows this conjugation called transfer genes. Those that can’t perform this conjugation process, are non-conjugative plasmids.
• Incompatibility: another type of plasmid are those that can co-exist with other plasmids only if they are compatible with each other. If a plasmid is incompatible, it will be ultimately expelled from the bacterial cell. Plasmids are incompatible if their reproduction strategy coincides with the others.

We can also classify plasmids into five specific groups:

• Fertility F-plasmids: also known as F-plasmids, carry specific transfer genes that will then allow genes to be transferred between two bacteria through conjugation. They make up for the category of conjugative plasmids.They are also episomes, which means that they can go inside chromosomal DNA. Bacteria that have these plasmids are called F positive (F+) and those that do not have this type of plasmid are called F negative (F-). There can only be one F-plasmid inside a bacteria, and when F+ conjugates with an F-, the result is two F+ bacterium.
• Resistance plasmids: these plasmids are also called R-plasmids and their job is to aid a bacterial cell to defend itself when threatened by environmental factors such as poisons or antibiotics. Some of these plasmids can transfer to another bacterium through conjugation, resulting in a strain of bacteria that can become resistant to antibiotics.
• Virulence plasmids: when a bacterium has a virulence plasmid inside, the bacterium becomes a pathogen or an agent of disease. This type of bacteria can spread and replicate among affected individuals.
• Degradative plasmids: this kind of plasmid is in charge of digesting compounds that aren’t usually found in nature. They contain genes that are specific for enzymes that are able to break down certain compounds. These degradative plasmids are conjugative.
• Col plasmids: col plasmids contain genes that produce colicins or bacteriocins, these proteins kill other bacteria that might harm the host bacterium, so they protect it.

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How do we use plasmids?

We have found ways to use plasmids and record the DNA sequences so they can be used in different areas. Genetic engineering, molecular cloning, protein replication, and gene therapy are just some of the fields in which the use of plasmids is becoming more and more popular and important.