What is the Difference Between Plasmid and Episome

The main difference between plasmid and episome is that plasmid does not integrate into the genome, whereas episome can integrate into the genome. Also, plasmids mainly occur in prokaryotes while in eukaryotes, episomes behave as plasmids in prokaryotes.  

Plasmid and episome are two types of DNA elements which exist independently of the genome. In general, both of them can undergo autonomous replication. 

Key Areas Covered 

1. What is a Plasmid
     – Definition, Structure, Importance
2. What is an Episome
     – Definition, Structure, Importance
3. What are the Similarities Between Plasmid and Episome
     – Outline of Common Features
4. What is the Difference Between Plasmid and Episome 
     – Comparison of Key Differences

Key Terms

Autonomous Replication, Episome, Extra-chromosomal DNA Elements, Integration into Genome, Plasmid

What is a Plasmid 

A plasmid is an extrachromosomal genetic element that mainly occurs in prokaryotes. Moreover, it can replicate independently of the genome. Generally, plasmids are small, circular, double-stranded DNA molecules. Furthermore, they do not contain genes required for the survival of bacteria. However, they contain some genes important for the selection of the host. Their size can vary from 1-200 kbp. Moreover, the number of identical plasmids can range 1-1000. That means different types of plasmids can occur inside the same cell. On the other hand, due to the autonomous replication, plasmids are known as replicons. They serve as a single-replicating unit due to the presence of an origin of replication.

Figure 1: Plasmids

Furthermore, there are two types of plasmids classified based on the ability of conjugation. Generally, conjugative plasmids contain a set of transfer or tra genes, promoting sexual conjugation. In contrast, non-conjugative plasmids are unable to initiate conjugation. Moreover, there are five classes of plasmids classified by function. They include F-plasmids, which are capable of conjugation, R-plasmids containing genes for antibiotic resistance, Col plasmids, containing genes for bacteriocins and proteins and kill other bacteria, degenerative plasmids enabling the digestion of unusual substances, and virulence plasmids, which turn bacteria into pathogens.  

What is an Episome 

An episome is an integrative plasmid, which is a non-essential, extrachromosomal genetic element same as plasmids in prokaryotes. Generally, plasmids capable of integrating into the genome in prokaryotes are also known as episomes. However, the integration into the genome allows stable maintenance of the episomal DNA over several generations. As an example, DNA in some viruses such as herpesviruses, adenoviruses, and polyomaviruses serve as episomes. 

Figure 2: Plasmids vs Episomes

Furthermore, the F factor, which is an incompatibility group of plasmids, is another example of episomes. Generally, it exists in three states. Here, the cells with the autonomous, extrachromosomal state are known as F⁺ cells. Also, Hfr cells are the cells with F factors integrated into the genome. On the other hand, in the F prime state, F factor exists outside the chromosome, but with a section of chromosomal DNA attached to it. Moreover, episomes can be distinguished from other extrachromosomal elements by its large size, which is around 62 kbp. Also, episomes can replicate autonomously in the cytoplasm. In contrast, in eukaryotes, episomes refer to the non-integrative extrachromosomal genetic element.  

Similarities Between Plasmid and Episome 

• Plasmid and episome are extrachromosomal DNA elements. 
• Both can exist in the cytoplasm. 
• They can replicate autonomously. 
• Furthermore, they are accessory DNA elements which can occur in both prokaryotes and eukaryotes. 
• Both are circular, double-stranded DNA. 
• Both contain genes. 

Difference Between Plasmid and Episome 

Definition 

A plasmid refers to a genetic structure in a cell, which can replicate independently of chromosomes, typically a small circular DNA strand in the cytoplasm of prokaryotes while episome refers to a genetic element, which can especially replicate in association with a chromosome with which it becomes integrated. Thus, this is the main difference between plasmid and episome.

Introduced By 

The plasmid was first introduced by Joshua Lederberg in 1952, while episome was first introduced by François Jacob and Élie Wollman in 1958.  

Occurrence 

While plasmids mainly occur in prokaryotes, episomes occur in both prokaryotes and eukaryotes. 

Size 

Another difference between plasmid and episome is that plasmids are generally small while episomes are large.  

Replication 

Plasmids replicate autonomously while episomes replicate either autonomously or in association in with a chromosome. 

Importance 

Furthermore, plasmids are used as an element for genetic manipulation while DNA in some lysogenic bacteriophages acts as episomes, integrating into the genome and persisting as prophages. 

Conclusion 

Basically, a plasmid is an extrachromosomal genetic element that mainly occurs in prokaryotic cells. Generally, it contains genes for the selective advantages of host cells. Significantly, it replicates autonomously in the cytoplasm. On the other hand, an episome is an extrachromosomal genetic element that mainly occurs in eukaryotes, behaving as plasmids in prokaryotes. However, the main feature of episomes is that they can integrate into the genome for the replication and can be excluded totally out from the cell. Therefore, the main difference between plasmid and episome is their ability to integrate into the genome and their occurrence. 

References:

1. “Episomes, Plasmids, Insertion Sequences, and Transposons.” World of Microbiology and Immunology, Encyclopedia.com, 2019, Available Here.

Image Courtesy:

1. “Plasmid (english)” By User:Spaully on English wikipedia – Own work (CC BY-SA 2.5) via Commons Wikimedia  
2. “Plasmid replication (english)” By User:Spaully – Own work (CC BY-SA 2.5) via Commons Wikimedia