Difference Between Plasmid and Transposon

Main Difference – Plasmid vs Transposon

Plasmids and transposons are two types of mobile genetic elements, which are involved in the transfer of genetic material between genomes and chromosomes respectively. Insertion sequences (IS) and episomes are the other types of mobile genetic elements. A plasmid is an extra-chromosomal, self-replicative DNA molecule that naturally occurs in bacteria while transposon is a DNA sequence that moves around different positions within a genome. Plasmids are typically double-stranded, circular molecules. Transposons are called ‘jumping genes‘, and they can cause mutations and change in the amount of DNA in the genome. The main difference between plasmid and transposon is that plasmid transfer genetic material between genomes whereas transposon transfer genetic material between chromosomes within the same genome.

Key Areas Covered

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

Key Terms: Col Plasmids, Conjugative Plasmid, Degradative Plasmids, DNA Transposons, F-plasmids, Jumping Genes, Mobile Genetic Elements, Plasmid, Resistance Plasmids, Retrotransposons, Transposase, Transposon, Virulence Plasmids

What is a Plasmid

A plasmid refers to a genetic element that replicates independently of the chromosomes. Plasmids are double-stranded, circular DNA molecules, occurring in the cytoplasm of the bacteria, archaea, yeast, and protozoans. The size of a plasmid may vary from 1 – 1,000 kbp. One to thousands of different plasmid types can be identified in different types of cells. The main function of plasmids in nature is to involve in conjugation, which is a mechanism of horizontal gene transfer (HGT). HGT refers to the movement of genetic material between organisms.

Although plasmids are present in bacteria, they are not necessary for the survival of bacteria under normal conditions. They contain information necessary for antibiotic resistance, metal resistance, nitrogen fixation, and toxin production. Naturally occurring plasmids can be modified by in vitro techniques such as code transformation. Plasmids are types of vectors that are used as vehicles to carry genetic information to a second cell. A plasmid vector is shown in figure 1.

Figure 1: pBR322

Features of Plasmids

• Can self-replicate inside a cell
• Can be readily isolated from cells
• Have a unique restriction site for one or more restriction enzymes
• The insertion of a foreign DNA piece may not alter its replication properties
• Can be sequentially reintroduced into new cells and the transformants can be selected
• Do not occur freely in nature

Classification of Plasmids

Plasmids can be classified in a number of ways. Based on the mechanism of conjugation, plasmids can be classified as conjugative and non-conjugative plasmids. Conjugative plasmids consist of a set of transfer (tra) genes, encoded for sex pili, promoting conjugation (sexual reproduction) of bacteria. The transformation of plasmids from one to another bacterium takes place via sex pilli. Non-conjugative plasmids are transferred with the help of conjugative plasmids. Bacterial conjugation is shown in figure 2.

Figure 2: Conjugation

Five classes of plasmids can also be identified based on their function:

• F-plasmids – F plasmids contain tra. Thus, they are capable of expressing sex pilli during conjugation.
• Resistance plasmids – Resistance plasmids contain genes that provide resistance to antibiotics or poisons. They were historically known as R-factors before the nature of plasmids was understood.
• Col plasmids – Col plasmids contain genes that code for bacteriocins, proteins that can kill other bacteria.
• Degradative plasmids – Degradative plasmids enable the digestion of unusual substances such as toluene and salicylic acid.
• Virulence plasmids – Virulence plasmids turn the bacterium into a pathogen.

What is a Transposon

A transposon refers to a chromosomal segment that can be translocated between chromosomal, plasmid or phage DNA. Transposons are also known as transposable elements (TE). Transposition occurs in the absence of a complementary sequence in the host DNA. Transposons cause mutations in the genome. During transposition, the size of the genome can be either increased or decreased. As transposons may contain genes, they are known as jumping genes.

Classification of Transposon

The two classes of transposons are retrotransposons and DNA transposons. The life cycle of a retrotransposon is shown in figure 3.

Figure 3: Retrotransposon Cycle

Retrotransposons transpose through an RNA intermediate using a ‘copy-and-paste’ method; an RNA copy of a DNA segment is made at first, and then it is reverse transcribed into a DNA segment. Long terminal repeats (LTRs) and short terminal repeats (STRs) are the two types of retrotransposons. Most retrotransposons are LTRs. LTR transposon exhibits a similar structure and function to retroviruses. An LTR transposon is shown in figure 4.

Figure 4: LTR Transposon

DNA transposons transpose through a ‘cut-and-paste’ method; a transposon is cut out from one position of the genome and inserted into another position. Transposase is the enzyme involved in DNA transposition. A bacterial DNA transposon is shown in figure 5.

Figure 5: Bacterial DNA Transposon

DNA transposons are flanked by two terminal inverted repeats (TIRs), which are recognized by transposase for the excision. Upon insertion, the DNA in the target site is duplicated, forming target site duplications (TSDs). The mechanism of DNA transposition is shown in figure 6.

Figure 6: DNA Transposition

In both classes of transposons, the non-autonomous elements that do not encode proteins required by the mobilization of the transposon may be present. Therefore, these transposons are presumably dependent on autonomous transposons for their mobility. As an example, the miniature inverted-repeat transposable elements (MITEs) are short (80-500 bp) DNA transposon-like elements. They are mainly present in eukaryotes, particularly in plant species. Although they have TIRs and are flanked by TSDs, the MITEs lack transposase coding gene. Thus, MITEs are presumably dependent on autonomous DNA transposons for their mobilization.

Similarities Between Plasmid and Transposon

• Plasmid and transposon are two types of mobile genetic elements, involved in the transfer of genetic material between genomes and chromosomes respectively.
• Both plasmids and transposons are made up of double-stranded DNA.
• Both plasmids and transposons are naturally-occurring within the cells.

Difference Between Plasmid and Transposon

Definition

Plasmid: A plasmid refers to a genetic element that replicates independently of the chromosomes.

Transposon: A transposon refers to a chromosomal segment that can be translocated between chromosomal, plasmid or phage DNA.

Significance

Plasmid: A plasmid is an extra-chromosomal, self-replicative DNA molecule, naturally occurring in bacteria.

Transposon: Transposon is a DNA sequence that moves around different positions within a genome.

Occurrence

Plasmid: Plasmids naturally occur in bacteria and some eukaryotic cells.

Transposon: Transposons occur in bacteria and all eukaryotic cells.

Classes

Plasmid: The F plasmids, resistance plasmids, col plasmids, degradative plasmid, and virulence plasmids are the five classes of plasmids.

Transposon: Retrotransposons and DNA transposons are the two classes of transposons.

Self-replication

Plasmid: Plasmids are self-replicative inside the cell.

Transposon: Transposons are not self-replicative DNA segments.

Characteristic Features

Plasmid: Plasmids consist of an origin of replication, promoter, antibiotic resistance genes, and multiple cloning sites.

Transposon: Transposons consist of a coding region for transposase, transposable genes, and terminal repeats.

Vectors

Plasmid: Plasmids are used as vectors to produce recombinant DNA.

Transposon: Transposons are used as vectors to insert several bases in insertional mutagenesis.

Alterations of the Genome

Plasmid: Plasmids can be used to insert new genes into the genome of another organism.

Transposon: Transposons are mutagens that sometimes cause genetic diseases.

Conclusion

Plasmids and transposon are two types of mobile genetic elements that transfer DNA segments. Both plasmids and transposon are naturally-occurring within cells. Plasmids are self-replicative, circular DNA molecules, mainly found in bacteria. They can be used to transfer genes between genomes. Transposons are DNA segments that move around different positions within the genome. The main difference between plasmid and transposon is their role; plasmid transfer genetic material between genomes whereas transposon transfer genetic material between chromosomes within the same genome.

Reference:

1. “Plasmids.” Boundless Microbiology, Available here.
2. Muñoz-López, Martín, and José L. García-Pérez. “DNA Transposons: Nature and Applications in Genomics.” Current Genomics, Bentham Science Publishers Ltd., Apr. 2010, Available here.

Image Courtesy:

1. “PBR322” By Ayacop (+ Yikrazuul) – Own work (Public Domain) via Commons Wikimedia
2. “Conjugation” By Adenosine – Own work (CC BY-SA 3.0) via Commons Wikimedia
3. “Retrotransposons” By Mariuswalter – Own work (CC BY-SA 4.0) via Commons Wikimedia
4. “Composite transposon” By Jacek FH – self-made, based on Image:Composite transposon.jpg (CC BY-SA 3.0) via Commons Wikimedia
5. “DNA Transposon” By Mariuswalter – Own work (CC BY-SA 4.0) via Commons Wikimedia