Plasmids: Properties, Types and Functions

By Nisha Rijal •  Updated: 05/05/22 •  4 min read

Plasmids are extra-chromosomal genetic elements that replicate independently of the host chromosome. They are small, circular (some are linear), double-stranded DNA molecules (mostly) that exist in bacterial cells and in 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 plasmids may vary from none to several per bacterial cell. Different plasmids are present in a cell in a particular number called the copy number. Some plasmids are present in the bacterial cell in only 1-3 copies, whereas others may present in as many as 100 copies. This feature is controlled by the genes present on the plasmid and by interactions between the host and the plasmid.

Individual bacterial cells may contain several different types of plasmids and in some cases more than 10 at a time. Plasmids are generally isolated from the bacterial cells in the supercoiled configuration. So far, thousands of different types of plasmids have been isolated. More than 300 different types of naturally occurring plasmids have been isolated from E.coli alone.

Though, plasmids are not considered as part of the cell’s genome, when a bacterial cell divides each daughter cells receives a copy of each plasmid. Plasmids can also be transferred from one bacterial cell to another by the process called conjugation. Plasmids that govern their own transfer by conjugation are called conjugative plasmids but not all plasmids are conjugative.

Some important components of plasmids are

Types & Functions of Plasmids

As a single plasmid may carry many different genes, the classification of a plasmid in a single phenotypic category is difficult. Some of the notable types of plasmids and their functions are:

Although plasmids carry useful genes such as genes that confer “antibiotic resistance”, as mentioned above, they do not carry genes that are essential to the host under all conditions.

The presence of plasmids in a cell can also have other biological significance such as:

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

Nisha Rijal

I am working as Microbiologist in National Public Health Laboratory (NPHL), government national reference laboratory under the Department of health services (DoHS), Nepal. Key areas of my work lies in Bacteriology, especially in Antimicrobial resistance.

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