Plasmids are extra-chromosomal genetic elements that replicate independently. They are small, circular (some are linear), double-stranded DNA molecules (mostly) that exist in bacterial cells and some eukaryotes. The sizes of plasmids range from roughly one to more than 1000 kilobase pairs.
A typical plasmid is a circular, double-stranded DNA molecule less than 1/20 the size of the chromosome.
The number of plasmid may vary from none to several per bacterial cell. The copy number is the particular number of plasmids present in the cell. Some are present in the bacterial cell in only 1-3 copies, whereas others may present in as many as 100 copies. The genes on the plasmid and interactions between the host and the plasmid control the copy number. The plasmids are not a part of the cell’s genome, but during cell division but each daughter cell receives a copy of it.
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Types of Plasmids with Their Functions
Individual bacterial cells may contain several different types of plasmid and in some cases more than 10 at a time. They are generally isolated from the bacterial cells in the supercoiled configuration. So far, there has been the isolation of thousands of its types.
Did you know? E.coli alone has more than 300 different types of naturally occurring plasmids.
As a single plasmid may carry many different genes, classifying a plasmid in a single phenotypic category is difficult. Some of the notable types of plasmids and their functions are:
Conjugation also plays a role in transferring it from one bacteria to another. Those formed by conjugation are called conjugative plasmids. Examples include the F plasmid in E.coli and the conjugative P plasmid of Vibrio cholerae.
Resistance plasmid (R plasmid)
R plasmids help in developing resistance to antibiotics and various inhibitors of growth.
- It carries a variety of antibiotic resistance genes which encode proteins that either inactivate the antibiotic or affect its uptake into the cell. Example: Plasmid R100 has resistance genes for sulfonamides, streptomycin, fusidic acid, chloramphenicol, and tetracycline.
- R100 also carries several genes that confer resistance to mercury.
- Resistant strains can transfer resistance to sensitive strains via cell-to-cell contact. R100 can transfer itself between enteric bacteria of the genera Escherichia, Klebsiella, Proteus, Salmonella, and Shigella but does not transfer to the non-enteric bacterium Pseudomonas.
Plasmid that code for virulence characteristics & toxins
Some plasmid code proteins increase the ability to attach and colonize specific sites within the host, increasing the bacteria’s virulence factor. e.g., colonization factor antigen (CFA) of E.coli.
Toxin production in various pathogenic bacteria is found to be linked with the presence of plasmids. For example:
- Hemolysin (lyse RBCs) and enterotoxin (induces extensive secretion of salt and water in the bowel) properties of Enteropathogenic Escherichia coli (EPEC) are governed by plasmids.
- Production of coagulase, hemolysin, fibrinolysin, and enterotoxin properties of S.aurues are linked to the plasmid’s presence.
Many bacteria produce peptides that inhibit or kill closely related species or even different strains of the same species. The plasmids codes the gene responsible for this peptide and its post-translational modification. E.g., Col plasmid codes colicin of E.coli.
Although plasmids carry functional genes such as genes that confer “antibiotic resistance,” as mentioned, they do not have genes that are essential to the host under all conditions.
Artificial Plasmid or Recombinant Plasmid
Artificial plasmid or recombinant plasmid is the plasmid formed by recombining the bacterium’s DNA with desired DNA fragments. The process of transformation helps in introducing the DNA fragments to the gene. Then rapid replication of bacteria contributes to making numerous copies of the recombinant plasmid. The recombinant plasmid has uses in various fields.
- Most importantly, these have great use in research and development as cloning the gene complex organism becomes easier using bacteria.
- Similarly, gene therapy is another use of recombinant plasmids.
- Likewise, these can help modify crops to yield good quality products.
Important Parts of Plasmids
- Origin of replication (Ori): A DNA sequence allows bacteria to make more copies of the plasmid as they grow and divide.
- Antibiotic resistance gene: It is a specific gene that allows bacteria with the plasmid to grow in the presence of an antibiotic specific to the gene.
- Gene: A DNA sequence encoding a particular protein that a researcher has inserted into the plasmid to study.
- Promoter: A DNA sequence that allows the cell to produce the protein encoded by the gene.
- Restriction sites: DNA sequences that allow a researcher to cut and paste components of plasmids together.
Significance of Plasmids
The presence of plasmids in a cell can also have other biological significance such as:
- Nodulation and symbiotic nitrogen fixation: Rhizobium
- Transfer genetic information for a biochemical pathway for the degradation of organic compounds such as octane, camphor, naphthalene, salicylate etc: Pseudomonas.
- Pigment production: Erwinia, Staphylococcus
- Lactose, sucrose, urea utilization, nitrogen fixation: Enteric bacteria
- Plasmids can be constructed artificially (artificial plasmids are called vectors) and are used to introduce foreign DNA into another cell of interest. Plasmids play crucial roles in genetic engineering, molecular cloning and various areas of Biotechnology.
This plasmid rap video is awesome…
References and further reading
- Plasmids 101 (addgene blog)
- Madigan MT, Martinko JM. Brock Biology of Microorganisms. Pearson Educational International.