Transposable Elements in Eukaryotes

Transposable Elements in Eukaryotes:

This system of transposable elements in maize was analysed and given by Barbara Mc. Clintock. Here Ac stands for Activator and Ds for Dissociation.
Barbara found that Ds and Ac genes were sometimes mobile and moved to different chromosomal locations thus resulting in different kernel phenotypes.
Ds element is activated by Ac and on activation it serves as the site provider for breakage in chromosome.
Ac can move autonomously while Ds can move only in the presence of Ac (Fig. 6). The transposition involving this Ac-Ds system produces altered kernel phenotypes.

Other transposable elements of maize are:

i. spm (suppressor mutator) system,

ii. dt (dotted) system,

iii. Mu (Mutator) system, etc.

A number of transposable elements are found in Drosophila which are of different types and account for a quite high fraction of Drosophila genome.
Some of these transposons are given below:

These were discovered during the study of ‘hybrid-dysgenesis’ which is a sterility causing condition.
They are 2.9 kb long and contain 31 bp long inverted terminal repeats High rate of P-element transposition causes hybrid dysgenesis.
P-elements encode transposase enzyme which helps in their transposition.
These are also useful as vectors for introducing foreign genes into Drosophila.

Their transposition causes mutations for eye-colour in Drosophila.
They are of size approximately 5-8 kb with direct terminal repeat (DTR) of about 276 bp at each end.
Within each of this direct repeats is present short inverted repeat (IR) of about 17 bp length.
About 10-80 copia- elements are present in cell-genome (Fig. 7).

These are the fold back elements present in Drosophila genome.
These have ability to fold back to form a stem and loop structure due to the presence of long inverted terminal repeats.
Their transposition results into a changed expression by causing mutation by insertion or by affecting the normal gene expression.
Other important types of transposable elements found in Drosophila are:

i. I elements,

ii. Mariner elements,

iii. Gypsy elements,

iv. Hobo elements, etc.

Transposons in humans are in the form of repetitive DNA which consists of sequences that are interspersed within the entire human genome.
These sequences are transposable and can move to different locations within the genome.
These are of following two types:

They are ~ 300 bp long and may be present about 5 lakh times in human genome.
Alu sequences are the best characterized SINEs in humans.
These are termed as ‘Alu’ elements because they contain specific nucleotide sequences which are cleaved by the restriction enzyme named Alul.
Alu elements contain Direct Terminal Repeats (DTR) of 7-20 bp length.
These DTRs help them in the insertion process during transposition.

They are ~ 6400 bp long and are present about 1 lakh times in the human genome. Most prominent example is LI sequence.
These transposable elements are some of the most abundant and common families of moderately repeated sequences in human DNA.

Transposons may change the structural and functional characteristics of genome by changing their position in the genome.
Transposable elements cause mutation by insertion, deletion, etc.
Transposons make positive contribution in evolution as they have tremendous impact on the alteration of genetic organisation of organisms.
They are useful as cloning vectors also, in gene cloning. For example, P-elements are frequently used as vector for introducing transgenes into Drosophila.
Transposons may also be used as genetic markers while mapping the genomes.
Transposon-mediated gene tagging is done for searching and isolation of a particular gene.

Transposons are inserted within genes affecting their function, thus cause disruption of their functions.
When they are inserted within the regulatory sequence of genes, they cause change in their expression.
They are most common source of mutation. Transposons may insert stop codons thus producing truncated proteins.
In drosophilla, majority of spontaneous mutations are caused by transposons jumping into a gene.
The mutant white-eyed drosphilla is produced by a transposable element inserted into the gene, which normally produces red pigment.
In human beings, transposons cause many genetic diseases.
Transposons lead to the development of functional immune system in vertebrates.
In bacteria, the transposable elements are present on extra chromosomal DNA called plasmid.
Transposable elements on plasmids carry genes for proteins that nullify the effects of antibacterial drugs and toxins.