Transposable Elements- Definition, Types, Examples, Applications
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Transposable elements definition
Transposable elements (TE) or transposons are short, mobile DNA sequences that travel about chromosomes with little regard for homology, and their insertion can result in deletions, inversions, chromosomal fusions, and even more intricate rearrangements.
- Transposons are genetic components that move and frequently carry an antibiotic resistance gene.
- These elements can insert at random, travel from plasmids to the chromosome, and be transferred from one bacteria to another by conjugation, transformation, or transduction.
- Transposable elements constitute a considerable portion of the genome and account for the majority of the DNA mass in a eukaryotic cell.
- Barbara McClintock (1965) discovered transposable elements while studying genetic instability in maize (corn).
Characteristics of transposable elements
The following are some notable characteristics of transposable elements:
- These are the DNA sequences that encode enzymes that cause self-duplication and insertion into a new DNA location.
- Transposons participate in transposition activities that include recombination and replication, resulting in two copies of the original transposable element. One copy remains at the parent site, while the other travels to the target location on the host chromosome.
- The presence of these elements inevitably disrupts the integrity of the target genes of those elements.
- Because transposons contain the genes for RNA synthesis start, certain previously dormant genes may be awakened.
- A transposable element lacks a location for replication origin. As a result, it cannot replicate as plasmids or phages without the host chromosome.
- There is no similarity between the transposon and its intended insertion site. These elements can insert into the host chromosome or plasmid at practically any location. Although certain transposons appear to be more likely to enter at specific locations (hot spots), they seldom insert at base-specific target sites.
Types of Transposable elements
There are two types of transposable elements:
1. Insertion Sequences (IS) or Simple Transposons
- Insertion sequences (IS) or simple transposons are shorter (800 to 1500 bp) sequences that do not code for proteins.
- These sequences include the genetic information required for transposition (e., the gene for the enzyme transposase)
- Insertion sequences have been found in bacteriophages, F factor plasmids, and several bacteria.
2. Transposons (Tn) or Complex Transposons
- Transposons (Tn) or complex transposons are several thousand base pair long and contain genes that code for one or more proteins, which may include antibiotic resistance factors in bacteria.
- A transposon is distinguished by the presence of identical, inverted terminal repeat (IR) sequences spanning from 8 to 38 base pairs (b.p.)
- Distinct types of transposons have different inverted repeats.
- A transposon has a short (less than ten base pairs) sequence on either side.
- Transposon insertion results in the replication of a single target sequence, which appears as direct repetitions flanking the inserted transposable element.
- The transposon does not include the flanking direct repeats. These repetitive sequences function as IS or IS-like segments.
Examples of Transposable elements
1. Tn 3 transposon of E. coli
- Tn 3 transposon has three genes: tnp A, tnp R, and Tnp A codes for transposase, which has 1015 amino acids and is essential for transposition.
- Tnp R encodes a 185-amino acid repressor (also known as resolvase) that controls the transposase.
- Bla is a gene that encodes a B-lactamase enzyme that gives resistance to the antibiotic ampicillin.
2. Bacteriophage Mu
- Mu (mutator) is a temperate bacteriophage with typical phage features that may be considered a big transposon.
- It inserts itself at random sites on the coli chromosome, causing a mutation.
3. Yeast Ty elements
- These elements are found in yeast Saccharomyces cerevisiae and are around 5900 base pairs long. Ty elements are surrounded by five base-pair direct repeats formed by DNA duplication at the Ty site.
Applications of transposable elements
- Transposable elements have the potential to be employed as a genetic tool for studying gene expression and protein function.
- In genetic engineering, they are employed to insert or delete certain genetic sequences, as well as to produce frameshift mutation.
- The Tc1/mariner-class TEs Sleeping Beauty transposon system is being researched for potential use in human gene therapy.
Negative effects of transposable elements
- When a transposable gene is placed into a functioning gene, it has the potential to deactivate the gene.
- Multiple copies of the same sequence obstruct chromosomal pairing during cell division, causing chromosome duplication.
- The expression of detrimental disease-causing proteins in transposons has an impact on normal cellular function.
