TRANSPOSONES
Transposons also known as ‘Jumping Genes’ are the sequences of DNA that can move or transpose themselves to new positions within the genome of a single cell.
The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create
· phenotypically significant mutations and
Transposons make up a large fraction of the C-value of eukaryotic cells. Transposons are often considered "junk DNA". In Oxytricha, which has a unique genetic system, they play a critical role in its development. Transposons are very useful to researchers as a means to alter DNA inside a living organism.
Types of Transposones:
According to their mechanism of transposition, this can be described as either “copy or paste” (class I) or “cut and paste” (class II), transposones are assigned to one of two classes.
· Class I transposons. These are Retrotransposons that :
- first transcribe the DNA into RNA and then
- Use reverse transcriptase to make a DNA copy of the RNA to insert in a new location.
- Class II transposons. or DNA Transposones : These consist of DNA that moves directly from place to place.
DDNA Transposones:
Some of the simplest transposones, called DNA Transposones, consist of DNA sequences which code for an enzyme called transposase. Transposone is able to cleave a transposone from the genome, transport it to the new location, and reinsert it into the genome.This process is called conservative transposition. All prokaryote transposones as well as eukaryotic transposones utilize conservative transposition.
Example of DNA Transposone:
Ac(activator) transposable Elements:-
Ac transposones were one of the first transposable elements discovered by Barbara McClintock within the Maize corn genome. McClintock discovered that Ac transposones are self-sufficient in that they code for their own transposase enzyme, and, therefore can transpose independently.
Dc(dissociator)Transposable Elements:
McClintock discovered that Dc transposons are non-autonomous, as they are unable to transpose independently. Dc transposons do not code for their own transposase enzyme as they must depend on excess transposase produced by nearby Ac transposons. Therefore, Dc transposons remain immobile without the assistance of Ac transposons.
Retrotransposons:
The majority of Eukaryotic transposable elements are more complex transpsons called Retrotransposons. The host cell recognizes the retrotransposon as a normal DNA sequence within its genome, and synthesizes an RNA copy of it. However, the retrotransposon codes for an enzyme, called reverse transcriptase which is able to convert RNA copy of retrotransposon into an exact DNA duplicate of the original. Transposase then finds a suitable location to insert the new retrotransposon into the genome. This process is called Replicate Transposition.
There are 2 main types of retrotransposons:
Non VIRAL RETROTRANSPOSONS:-
Non viral transposons comprise the largest of mammalian transposons.
EXAMPLE:
LINEs: There are roughly 850,000 LINEs, or long interspersed element, within each human genome. Some cases of hemophilia are caused by the insertion of a LINE transposon into the gene of the X chromosome, which is partly responsible for the clotting of the blood.
SINEs: There are roughly 1500,000 SINEs , or short interspersed elements, within each human genome. Among these are the Alu elements.
Alu Elements:
Alu elements are the most common transposable elements within the human genome, making up more than 5%. It is believed that approximately 30-50 million years ago, Alu elements spread rapidly throughout the primate genome. Although they had lost the majority of their activity before the evolution of the human-like apes, they are still the popular source of research. Approximately 0.1% of human genetic diseases are caused by the transition of Alu elements within the genome.
Viral Transposon: Viral transposon has properties very similar to that of retrovirus.
Examples:
Ty Transposable Element: The best known example Ty transposon elements can be found within the yeast genome. During transposition, the Ty element is very prone to mutation. As a result, there are many variations of Ty transposons passed down through generations. One of these variations, the Tyl transposons is present in 35 copies throughout the yeast genome. The Ty transposable element causes harmful mutation by being inserted into, and disrupting, various gene within the yeast genome.
Drosophilacopia Transposable Elements: The drodophilacopia transposable element is found within the drosophila(fruit fly). There are seven known families of Drosophilacopia within the fruit fly genome, each of which has 10-100 copies within the genome. All together, Drosophilacopia transposons comprise 15% of the drosophila genome. The Drosophilacopia transposon is very similar to the Ty transposon in yeast. When inserted into genome, the Drosophilacopia transposon duplicates a number of DNA bases from the Drosophila genome at either of its ends. The Drosophilacopia transposon cause mutation by being inserted into functioning genes and disrupting their function and regulation. An example of such a mutation is the White Apricot Mutation which causes the variation the in colour of the fruit fly’s eye.
The Effect of Transposons:
Functional transposon pose an enormous risk to the genome because of their mobility and instability. When a transposon relocated within the genome it may land on a vital gene, disrupting its function. Or, the transposon may land near vital gene, causing them to be activated or shut down. No matter its defences, no organism is capable of restricting every transposon. In humans, genetic diseases are sometimes caused by the transfer of transposon from one location to another within the genomeduring sperm/egg production. As a result of transposon’s movement, a genetic mutation often arises, which may trigger the development of disease.
However, the danger of transposon today is not often seen as an enormous concern. With all of the precautions taken by our cell, transposition within the genome has become a rather infrequent event.
