Transposable elements definition
Transposable elements (TE, transposon, or jumping gene)
Transposable elements (TE) or transposons can be defined as small mobile DNA sequences that move around chromosomes regardless of homology, and insertion of these elements can lead to deletions, inversions, chromosomal fusions, and even further rearrangements. complicated.
- Transposons are mobile genetic elements that often carry an antimicrobial resistance gene.
- These elements can be inserted randomly, moved from plasmids to the chromosome and vice versa, and can be moved from one bacterium to another by conjugation, transformation, or transduction.
- Transposable elements make up a significant fraction of the genome and are responsible for much of the DNA mass in a eukaryotic cell.
- Barbara McClintock (1965) discovered transposable elements through an analysis of genetic instability in maize (corn).
Characteristics of transposable elements
- Some outstanding characteristics of the transposable elements are:
- These are the DNA sequences that encode enzymes that result in self-duplication and insertion into a new DNA site.
- Transposons are involved in transposition events that include recombination and replication, which generally generates two copies of the original transposable elements. One of the copies remains at the main site, while the other reaches the target site on the host chromosome.
- The integrity of the target genes of these elements is invariably interrupted by the presence of those elements.
- Because transposons carry genes to initiate RNA synthesis, some previously inactive genes may be activated.
- A transposable element does not have a site for the origin of replication. As a result, it cannot replicate without the host chromosome as plasmids or phages.
- There is no homology between the transposon and its target site for insertion. These elements can be inserted at almost any position on the host chromosome or a plasmid. It seems likely that some transposons enter some specific positions (hot spots), as soon as they insert into specific target sites on the base.
Transposable Elements Types
The transposable elements are of the following two types:
Insertion Sequences (IS) or Simple Transposons
- Insertion sequences (IS) or single transposons are shorter sequences (800 to 1500 bp) and do not encode proteins.
- These sequences carry the genetic information necessary for their transposition (eg, the gene for the enzyme transposase)
- Insertion sequences have been identified in bacteriophages, in the factor F plasmid, and in many bacteria.
Transposons (Tn) or Complex Transposons
- Transposons (Tn) or complex transposons are several thousand base pairs long and have genes that encode one or more proteins that can include resistance factors in bacteria that act against antibiotics.
- A hallmark of a transposon is the presence of inverted, identical terminal repeat (IR) sequences ranging from 8 to 38 base pairs (m.p.)
- These inverted repeats are unique to different types of transposons.
- A short sequence (less than ten b.p.) is present on each side of a transposon.
- Insertion of the transposon causes duplication of a singular target sequence, which then appears as direct repeats flanking the inserted transposable element.
- Direct flanking repeats are not considered part of the transposon. These repeated sequences act as IS or IS-like segments.
Examples of transposable elements
Tn 3 transposon of E. coli
- The Tn 3 transposon is 4957 bp and contains three genes, such as tnp A, tnp R and
- Tnp A encodes for the transposase which has 1015 amino acids and is required for transposition.
- Tnp R encodes a repressor (also called resolvase), which contains 185 amino acids, which regulates transposase.
- Bla encodes a β-lactamase enzyme that confers resistance to the antibiotic ampicillin.
- The bacteriophage Mu (Mu = mutator) is a tempered bacteriophage that has the usual phage properties and could be considered as a large transposon.
- It is inserted into the coli chromosome at random locations, resulting in a mutation.
Yeast Ty elements
- These elements are found in the yeast Saccharomyces cerevisiae, which is approximately 5900 base pairs long
- Ty elements are surrounded by five direct base pair repeats that are created by duplication of DNA present at the Ty site
Applications of transposable elements
- Transposable elements can be used as a genetic tool for the analysis of gene expression and protein function.
- These are used in genetic engineering to insert or delete specific genetic sequences, and also to cause the frameshift mutation.
- The Tc1 / mariner class of the TEs Sleeping Beauty transposon system is being studied for use in human gene therapy.
Negative effects of transposable elements
- A transposable gene, when inserted into a functional gene, could disable the gene.
- Multiple copies of the same sequence hinder chromosome pairing during cell division, resulting in chromosome duplication.
- The expression of harmful, disease-causing proteins in transposons affects normal cellular function.
Mechanisms of mutagenesis
TEs are mutagens and their movements are often the causes of genetic diseases. They can damage the genome of their host cell in different ways:
A transposon or a retrotransposon that is inserted into a functional gene can disable that gene.
After a DNA transposon leaves a gene, the resulting gap may not be repaired correctly
Multiple copies of the same sequence, like Alu sequences, can hinder precise chromosomal pairing during mitosis and meiosis, resulting in unequal crossovers, one of the main reasons for chromosomal duplication.
TEs use several different mechanisms to cause genetic instability and disease in their host genomes.
Expression of proteins that cause harmful diseases that inhibit normal cellular function.
Many TEs contain promoters that drive transcription of their own transposase. These promoters can cause aberrant expression of linked genes, causing disease or mutant phenotypes