Transposons: Types, Examples and Significance

Transposons are genetic elements with the ability to move around. It can relocate within a genome. As a result, it's also known as the jumping gene. They make up a large portion of the genome and thus are responsible for genome function. It also has an impact on evolution.

• Barbara McClintock discovered transposons for the first time in 1940.
• They changed the location of the gene within the chromosomes.
• Transposons are classified into three types- class II transposons, class III transposons, and retrotransposons (class I transposons).
• Transposons have the ability to cause or reverse mutations.

Read More: Law of Limiting Factor

Key Terms: Transposons, jumping genes, mutation, chromosome, DNA, RNA

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What are Transposons?

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Transposons are DNA segments that are present in the genomes of all living things with the capability of moving from one place to another place within a genome. It does not use an independent element, such as a phage or plasmid, but instead jumps from one site to another.

• They carry genes for antibacterial resistance, infection, symbiotic factors, and xenobiotic catabolism.
• Transposons cause a wide range of DNA rearrangements that have far-reaching implications for cell survival and evolution.
• Transposons have diverse structures and are highly regulated by a variety of mechanisms.
• They are discovered to encode a special protein called transposase, which catalyzes the transposition process.
• Transposons are unique to different types of organisms.
• Transposons have a significant impact on changing an organism's genetic composition

Read More: Genetic Engineering

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Various names of Transposons

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There are various names for Transposons like 

1. Jumping genes
2. Transposable elements
3. Cassettes
4. Mobile elements
5. Insertion sequences.

Read More: Miller Urey Experiment

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Discovery of Transposons

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Barbara Mcclintock discovered transposable elements in 1965 while studying genetic instability in maize and earned a Nobel Prize in 1983. 

• The instability involved chromosome breakage and was found to occur at transposable sites. 
• Before this discovery, it was widely assumed that genes lined up in a chromosome in a consistent manner and occupied specific positions. 
• She discovered that certain genes known as transposons can move to different locations on the chromosome. 
• She made this discovery on corn kernels and discovered that the offspring plants can differ in color due to activated or deactivated genes.

Read More: Biofortification

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Characteristics of Transposons

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A few mentionable characteristics of transposons are– 

• They are DNA sequences that code for enzymes that cause an identical copy of themselves to be inserted into a new DNA site.
• Transposition events involve both recombination and replication processes, which frequently result in the production of two daughter copies of the original transposable elements. 
• One copy is kept at the parent site, while others are displayed at the target site.
• Transposable element insertion invariably compromises the integrity of their target genes.
• Transposable elements carry signals for the start of RNA synthesis and can occasionally activate previously dormant genes.
• Because a transposon is not a replicon, it cannot replicate independently of the host chromosome as plasmids and phages can.
• There is no homology between the transposons and the target site for their insertion.

Read More: Animal Cell

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Types of Transposons

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Transposons are classified into the following types based on their transposition mechanism:

1. Cut-and-Paste Transposons:

• They transpose by removing the transposable sequence from one location in the genome and inserting it into another location in the genome (pasting) (Fig. 1).

Cut-and-Paste Transposons

• Two transposase subunits are involved in the cut-and-paste transposition. Each transposase submission binds to specific sequences at the transposon's two ends. These transposase protein subunits then interact, resulting in transposon excision.
• This 'transposon-Transposase Complex' is then excised and integrated into the target recipient site. The transposon is thus cut from one site and pasted on another via a mechanism mediated by transposase protein (Fig. 2)

• IS-elements, P-elements in maize, hobo-elements in Drosophila, and other cut-and-paste transposons are examples.

2. Replicative Transposons:

• They transpose through a mechanism that involves the replication of transposable sequences, and the resulting copy of DNA is inserted into the target site while the donor site remains unchanged (Fig. 3). 

Replicative Transposons

• In this type of transposition, one copy of the transposon is gained, and both the donor and recipient DNA molecules have one transposable sequence each after transposition.
• Bacterial Tn3-elements are a good example of this kind of transposon.

3. Retro Elements:

• Their transposition is done through a process that uses elements of RNA as the template to synthesize DNA by reverse transcription (i.e., RNA DNA) (Fig. 4).

Reverse transcription

• In this kind of transposition, the transposable DNA is transformed to create an RNA molecule as an intermediate. The reverse transcriptase enzyme then uses this RNA as a template to create complementary DNA.

Once formed, this single-stranded DNA copy is double-stranded before being inserted into the target DNA site.

• Retro transposons are transposable elements that move only when reverse transcriptase is active.
• Some retro-transposons are related to retroviruses. Only Eukaryotic organisms contain retro-transposon.
• The retro components could be viral or non-viral. Non-viral retroelements are the more important of the viral retroelements and can be further divided into.

4. Retrovirus-like elements: Retrovirus elements carry long terminal repeats(LTR). Copia and gypsy elements are two examples of drosophila. 
5. Retroposons: In this LTR are not present. Human LINEs and SINEs are two examples.

Read More: Discovery of Cells

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Examples of Transposons

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1) Transposase - a protein with 1015 amino acids that are required for transposition.

2) Repressor (resolvase) - a 185 amino acid protein that regulates the transposase.

3) Beta-lactamase - an enzyme that confers antibiotic resistance to ampicillin. 

Read More: Microbiology

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Significance of Transposable Element

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The important significances of transposable elements are– 

• Transposons, by changing their position in the genome, can alter the structural and functional characteristics of the genome.
• Transposable elements cause mutation through insertion, deletion, and so on.
• It contributes positively to evolution because it has a large impact on the changes in the genetic organization of organisms.
• They can also be used as cloning vectors in gene cloning. P-elements, for example, are frequently used as vectors for introducing transgenes into Drosophila.
• Transposons may also be used as genetic markers in genome mapping.
• Transposon-mediated gene tagging is used to find and isolate a specific gene.

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Things to Remember

• A Transposon is a genetic element that can move.
• Transposons are classified into three types- class II transposons, class III transposons, and retrotransposons (class I transposons).
• Barbara McClintock discovered transposons for the first time in 1940 on maize.
• Transposons cause a wide range of DNA rearrangements that have far-reaching implications for cell survival and evolution.
• Transposons are DNA sequences that code for enzymes that cause an identical copy of themselves to be inserted into a new DNA site.
• Different types of Transposons are as follows: Cut-and-Paste Transposons, Replicative Transposons, and Retro Elements.
• Examples of Transposons are Transposase, Repressor (resolvase), and Beta-lactamase.