Plasmid and Episome

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Episome Meaning: François Jacob and Élie Wollman coined the word episome in 1958, to describe extrachromosomal genetic material which can reproduce independently or incorporate further into chromosomes. Nevertheless, since its inception, the term has evolved, with plasmid now being the correct terminology for autonomously replicating extrachromosomal DNA. Some attendees at a 1968 symposium in London proposed that the term episome be dropped, while others preferred to get it with a new meaning.

Most scholars now use the episome definition and define episome to apply to a plasmid that has the ability to integrate into the chromosome in prokaryotes. The integrative plasmids might be replicated and sustained in a cell for numerous generations, but they might eventually occur as their own plasmid molecule. The word episome meaning refers to a non-integrated extrachromosomal closed circular DNA molecule that can be replicated in the nucleus in eukaryotes.

Viruses, including adenoviruses, herpesviruses, and polyomaviruses, are by far the most typical types, but plasmids are also present. Aberrant chromosomal fragments, including double minute chromosomes, could also occur as a result of artificial gene amplification and otherwise pathologic processes (for example, cancer cell transformation). Like bacteriophage viruses, a few other episomes, including herpes viruses, replicate in a rolling circle. Others use a bidirectional replication mechanism to replicate (Theta type plasmids). In either case, episomes are physically separated from the chromosomes of the host cell.

A few other cancer viruses, such as Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus, are kept latent in cancer cells as chromosomally different episomes, in which they represent oncogenes that facilitate cancer cell proliferation. Definition of episome when a cancer cell divides, these episomes replicate passively alongside the host chromosomes. Whenever these viral episomes undertake lytic replication to create additional virus particles, they usually trigger the host cell's innate immune defence mechanisms, which eventually kill the cell.


Episome in bacteria is a group of extrachromosomal genetic elements known as plasmid episomes that are made up of deoxyribonucleic acid (DNA) and can provide a significant benefit to the bacteria in which they have been found.

A plasmid can be defined as a tiny extrachromosomal DNA molecule that can replicate without depending on chromosomal DNA and has been physically separated from it. Plasmids are small circular double-stranded DNA molecules that are predominantly reported in bacteria; nevertheless, they can also be found in eukaryotic and archaea organisms.

In the natural environment, episome plasmid frequently contains genes that aid the organism's survival and provide a significant benefit, including antibiotic resistance.

Unlike chromosomes, which are massive and contain all of the genetic information needed to live normally, an episome plasmid is typically small and comprises just additional genes that might have been helpful in specific circumstances and conditions.

In molecular cloning, artificial plasmids are commonly included as vectors to drive the replication of recombinant DNA sequences inside of host organisms. Plasmids can be incorporated into a cell mostly through a transformation in the laboratory. 

Properties and Characteristics

  • Plasmids require a stretch of DNA which can function as a replication origin in order to replicate totally independently inside a cell. A replicon is a self-replicating unit, such as a plasmid in this scenario. 

  • The gene for plasmid-specific replication initiation protein (Rep), repeating units named iterons, DnaA boxes, as well as a neighbouring AT-rich region could all be found in a typical bacterial replicon. 

  • Relatively small plasmids might use the host's replicative enzymes to replicate themselves, whereas bigger plasmids might contain genes particular to their replication. In prokaryotes, several kinds of plasmids could also insert further into the host chromosome, and all these integrative plasmids are known as episomes.

  • At least one gene is most often carried by plasmids. Several of the genes carried by plasmids are advantageous to the host cells, allowing them to stay alive in environments that would otherwise be fatal or restricting their growth. 

  • A few of these genes code for heavy metal resistance or antibiotic resistance, although some generate virulence factors that help a bacterium to colonise a host and tackle its defences, or metabolic functions which permit the bacterium to use a particular nutrient, such as the ability to break down recalcitrant or harmful organic compounds.

  • Plasmids can also make bacteria capable of nitrogen fixation. A few other plasmids, nevertheless, have no discernible impact on the phenotype of the host cell, and their advantage to the host cells is unknown; such plasmids have been referred to as cryptic plasmids.

Classifications and Types

Plasmids Can be Categorized in a Variety of Ways:

  • Plasmids are divided into two categories: 

  1. Conjugative Plasmids

  2. Non-Conjugative Plasmids

  • Conjugative plasmids are plasmids that include a collection of transfer genes that encourage sexual conjugation among cells. Plasmids can be transferred by one bacterium to another through sex pili encoded by a few of the transfer genes during the complicated process of conjugation.

  • Because non-conjugative plasmids cannot initiate conjugation, they could only be transported with the help of conjugative plasmids. 

  • Just a subgroup of the genes required for transfer has been carried by an intermediate class of plasmids, which are mobilizable. They might parasitize a conjugative plasmid and transfer at a greater rate only when it is present.

Plasmids Can also be Classified According to Their Function. There have been Five Major Categories:

  • F-plasmids containing tra genes for fertility. They have the ability to conjugate and express sex pili as a result.

  • Resistance (R) plasmids, that contain genes that confer antibiotic or poison resistance. Until the aspect of plasmids was recognized, they were known as R-factors.

  • Col plasmids carry genes that code for bacteriocins and these are the proteins that fight infection.

  • Degradative plasmids, such as toluene and salicylic acid, allow for the digestion of abnormal/uncommon materials.

  • Virulence plasmids are plasmids that turn bacteria into pathogens. Ti plasmid in Agrobacterium tumefaciens can be an example.

Plasmids can be classified into multiple functional groups.

FAQ (Frequently Asked Questions)

Q1. Are All Episomes Plasmids?

Ans. The size of an episome distinguishes it from other extrachromosomal DNA fragments including plasmids. In comparison to episomes, a plasmid is a single section of DNA that occurs on its own. It cannot integrate with chromosomal DNA because it contains all of the information required for its own replication.

Q2. What is the difference between a Vector and a Plasmid?

Ans. Two types of self-replicating DNA molecules are plasmids and vectors. Extrachromosomal elements, or plasmids, occur naturally and these extrachromosomal elements are found from within bacterial cells. Vectors were also DNA molecules that have been artificially incorporated into cells. Plasmids don't contain any genes that are required for bacterial cell function.