Element transposable

A element transposable , called sometimes wrongly transposon , is a sequence of DNA able to move and to multiply in an autonomous way in a Génome, by a mechanism called transposition . Present at all the living organisms, the elements transposable are one of the most important components of the genomes Eucaryote S.

History

The discovery was made by it by Barbara McClintock with the beginning of the year 50 and which received the Nobel Prize of medicine in 1983.

Leaves in the genomes

Relatively not very frequent in small the Genome S (Bacterium S, Yeast S), the elements transposable however were always found there of small number (few different families and little copies by family). A contrario , the elements transposable constitutes most of DNA repeated large genomes. As example, 45% of the genome of the man are elements transposable or sequences derived from elements transposable, against approximately 15% in the fly Drosophile ( Drosophila melanogaster ) and more than 70% at the Maïs ( Zea but ), as at the majority of the Angiosperme S. Alone, they can explain important differences in the faces of the genome of organization S close relations.

Types

Elements with ARN

These elements with ARN are known as of class I. Functioning on the principle of “copy-sticking”, the elements of class I, called retro transpose (for the elements with LTR) or retro pose (for the elements without LTR) transpose via an intermediary ARN, obtained by a traditional transcription of the sequence of the element, and retro-transcribed before their reintegration. One usually distinguishes the elements called “to LTR” (for L ong T erminal R epeats), which is framed by long not reversed repetitions, and the elements “without LTR”, which do not have any. Their mechanism of transposition is completely different, the cycle of the elements with LTR resembling enormously that of a Rétrovirus. Besides one suspected for a long time the elements with LTR of being old Virus having lost their capacity to be left the cell, but the recent discovered ones tend to show that on the contrary the retroviruses are old elements applicable to LTR having gained this infectious capacity.

The elements “with LTR” code the manufacture of various proteins whose one Transcriptase reverses (taking part in the replication of the DNA), a Intégrase (allowing the integration of the transposon retorted the DNA of the chromosome) and a Protéase (separating the transposon from the chromosomal DNA). The elements “without LTR”, also called LINE ( Long Interspersed Nuclear Element ), code a single protein allowing separation, the replication and the integration of the transposon.

The elements of class I are majority at many vegetable (of 10% at Arabidopsis thaliana 95% of the genome of some Liliaceae and Triticeae has), at the Levure and the Mammifère S.

Elements with DNA

These elements with DNA are known as of class II. The elements of class II, or let us transpose , transpose on the mode of the " to cross-coller" (ex: Tn10, Tn5 Mos1, element P) or " to copy-coller" (ex: IS911), i.e. their transposition is coupled with their excision of their site of origin (to cross) or their replication (to copy). Some are autonomous (Transpose DNA per has, coding an enzyme, the Transposase , allowing its own transfer after replication) and the other non-self-governing ones, having to use the machinery of the autonomous elements (Mites, Miniature Inverted repeat Transposable Elements ).

The elements with DNA are numerous at the Insecte S but are found in all the organizations (20% of the genome of common wheat or rice are made of these elements). They are also the only elements transposable known at the Bactérie S or the archée S, where they are called Séquences of insertion (or English IS).

Evolution of the elements transposable

In spite of the existence of specific examples of the role of sequences derived from elements transposable in certain genetic functions, the elements are for the majority useless with the alive cell, and sometimes even unfavourable. Their mobility is source of changes, creative of genetic diversity, but also causes genetic diseases. For these reasons, the presence of the elements transposable in the genomes is for a long time discussed: it can with difficulty be justified by their potential role in the evolution of the organizations; they are often seen like genetic parasites, whose activity is only used to ensure their own persistence during the generations.

But currently the theory of the DNA " égoïste" is rather in withdrawal. Indeed, several experiments lean in favor of a role prevailing of these elements transposable in the evolution of the species, by the creation of new genes. They would be as a source of damping of the changes due to the environment, these changes more often intervening in the zones not-coding as in genes if these not-coding zones are majority.

Elements transposable and diseases

Let us transpose seem to intervene in various human diseases:

Certaines forms of Hémophilie S would be due to the insertion of an element transposable within gene coding the factor VIII whose deficit in production deteriorates the possibilities of blood Coagulation.
Some let us transpose were accused in the genesis of the Cancer S by their insertion on the level of a gene Répresseur.

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