A plasmid is a small, circular piece of DNA that is different than the chromosomal DNA, which is all the genetic material found in an organism’s chromosomes. It replicates independently of chromosomal DNA. Plasmids are mainly found in bacteria, but they can also be found in archaea and multicellular organisms. Plasmids usually carry at least one gene, and many of the genes that plasmids carry are beneficial to their host organisms. Although they have separate genes from their hosts, they are not considered to be independent life.
Functions of Plasmids
Plasmids have many different functions. They may contain genes that enhance the survival of an organism, either by killing other organisms or by defending the host cell by producing toxins. Some plasmids facilitate the process of replication in bacteria. Since plasmids are so small, they usually only contain a few genes with a specific function (as opposed to a large amount of noncoding DNA). Multiple plasmids can coexist in the same cell, each with different functions. The functions are further detailed in the section “Specific Types of Plasmids” below.
General Types of Plasmids
Conjugative and Non-Conjugative
There are many ways to classify plasmids from general to specific. One way is by grouping them as either conjugative or non-conjugative. Bacteria reproduce by sexual conjugation, which is the transfer of genetic material from one bacterial cell to another, either through direct contact or a bridge between the two cells. Some plasmids contain genes called transfer genes that facilitate the beginning of conjugation. Non-conjugative plasmids cannot start the conjugation process, and they can only be transferred through sexual conjugation with the help of conjugative plasmids.
Another plasmid classification is by the incompatibility group. In a bacterium, different plasmids can only co-occur if they are compatible with each other. An incompatible plasmid will be expelled from the bacterial cell. Plasmids are incompatible if they have the same reproduction strategy in the cell; this allows the plasmids to inhabit a certain territory within it without other plasmids interfering.
Specific Types of Plasmids
There are five main types of plasmids: fertility F-plasmids, resistance plasmids, virulence plasmids, degradative plasmids, and Col plasmids.
Fertility plasmids, also known as F-plasmids, contain transfer genes that allow genes to be transferred from one bacteria to another through conjugation. These make up the broad category of conjugative plasmids. F-plasmids are episomes, which are plasmids that can be inserted into chromosomal DNA. Bacteria that have the F-plasmid are known as F positive (F+), and bacteria without it are F negative (F–). When an F+ bacterium conjugates with an F– bacterium, two F+ bacterium result. There can only be one F-plasmid in each bacterium.
Resistance or R plasmids contain genes that help a bacterial cell defend against environmental factors such as poisons or antibiotics. Some resistance plasmids can transfer themselves through conjugation. When this happens, a strain of bacteria can become resistant to antibiotics. Recently, the type bacterium that causes the sexually transmitted infection gonorrhea has become so resistant to a class of antibiotics called quinolones that a new class of antibiotics, called cephalosporins, has started to be recommended by the World Health Organization instead. The bacteria may even become resistant to these antibiotics within five years. According to NPR, overuse of antibiotics to treat other infections, like urinary tract infections, may lead to the proliferation of drug-resistant strains.
When a virulence plasmid is inside a bacterium, it turns that bacterium into a pathogen, which is an agent of disease. Bacteria that cause disease can be easily spread and replicated among affected individuals. The bacterium Escherichia coli (E. coli) has several virulence plasmids. E. coli is found naturally in the human gut and in other animals, but certain strains of E. coli can cause severe diarrhea and vomiting. Salmonella enterica is another bacterium that contains virulence plasmids.
Degradative plasmids help the host bacterium to digest compounds that are not commonly found in nature, such as camphor, xylene, toluene, and salicylic acid. These plasmids contain genes for special enzymes that break down specific compounds. Degradative plasmids are conjugative.
Col plasmids contain genes that make bacteriocins (also known as colicins), which are proteins that kill other bacteria and thus defend the host bacterium. Bacteriocins are found in many types of bacteria including E. coli, which gets them from the plasmid ColE1.
Applications of Plasmids
Humans have developed many uses for plasmids and have created software to record the DNA sequences of plasmids for use in many different techniques. Plasmids are used in genetic engineering to amplify, or produce many copies of, certain genes. In molecular cloning, a plasmid is a type of vector. A vector is a DNA sequence that can transport foreign genetic material from one cell to another cell, where the genes can be further expressed and replicated.
Plasmids are useful in cloning short segments of DNA. Also, plasmids can be used to replicate proteins, such as the protein that codes for insulin, in large amounts. Additionally, plasmids are being investigated as a way to transfer genes into human cells as part of gene therapy. Cells may lack a specific protein if the patient has a hereditary disorder involving a gene mutation. Inserting a plasmid into DNA would allow cells to express a protein that they are lacking.