Transposable Elements PDF

Transposable elements  Transposable elements are mobile DNA sequences found in the genomes of all organisms.  Transposable elements frequently cause mutations and DNA rearrangements.  Many transposable elements regulate their own transposition, either by controlling the amount of transposa...

Transposable elements  Transposable elements are mobile DNA sequences found in the genomes of all organisms.  Transposable elements frequently cause mutations and DNA rearrangements.  Many transposable elements regulate their own transposition, either by controlling the amount of transposase or by direct inhibition of the transposition event.  Transposable elements were first discovered in the 1940s by Barbara Mc Clintock during her studies on maize genetics.  General characters There are many different types of transposable elements, most generate short , flanking direct repeats at the target site as they insert. Many transposable elements also posses short terminal inverted repeats. Flanking direct repeats From 3 to 12 bp long are present on both sides of most transposable elements. They are not a part of a transposable element and do not travel with it. Rather , they are generated in the process of transposition, at the point of insertion. Terminal inverted repeats At the ends of many, but not all, transposable elements are terminal inverted repeats, which are sequences from 9 to 40 bp in length that are inverted complements of one another.  Transposition Transposition is the movement of a transposable elements from one location to another. Several different mechanisms are used for transposition in both prokaryotic and eukaryotic cells. Nevertheless, all types of transposition have several features in common. 1. staggered breaks are made in the target DNA. 2. The transposable element is joined to single stranded ends of the target DNA. 3. DNA is replicated at the single strand gaps,  Mechanisms of Transposition Transposition may take place through a DNA molecule or through the production of an RNA molecule that is then reverse transcribed into DNA. [ some transposable elements transpose as DNA and are referred to as DNA Transposons also called class I transposable elements] [Other transposable elements transpose through an RNA intermediate. (In this case , RNA is transcribed from the transposable element (DNA) and is then copied back in to DNA by the use of special enzyme called reverse transcriptase]. Elements that transpose through an RNA intermediate are called retrotransposons and also called class II transposable elements. Transposition may be replicative, in which the transposable element is copied and the copy moves to a new site , or non replicative, in which the transposable elements excises from the old site and moves to a new site.  Characteristics of two major classes of transposable genetic elements Class I Structure : short, terminal inverted repeats; short flanking direct repeats at target site. Genes encoded : Transposase gene ( and sometimes others) Transposition : Through DNA (replicative or non replicative) Examples : ISI (E.Coli); Tn3 (E.Coli); Ac, Ds ( Maize); p elements (Drosophila). Class II : ( Retrotransposon) Structure : Long, terminal direct repeats; short flanking direct repeats at target site. Genes encoded: Reverse transcriptase gene (and sometimes others) Transposition : By RNA intermediate. Examples : Ty (Yeasts); copia(Drosophila); Alu(Human)  Structure of Transposable elements Bacteria and eukaryotic organisms possess a number of different types of transposable elements, the structures of which vary extensively. Transposable elements in Bacteria: The DNA Transposons found in bacteria constitute two major groups. I ). Simple transposable elements that carry only the information required for movement ( insertion sequence). II ). More complex transposable elements that contain DNA sequences not directly related to transposition (Composite transposons).  I. Insertion sequences  The simplest type of transposable element in bacterial chromosomes and plasmids is an insertion sequence.  This type of element carries only the genetic information necessary for its movement.  They are typically from 800 to 2000 bp in length and possess the two hall marks of transposable elements : terminal inverted repeats and the generation of flanking direct repeats at the site of insertion. Most insertion sequences contain one or two genes that code for transposase.  II. Composite Transposons  Segment of DNA that becomes flanked by two copies of an insertion sequence(Consists of a central region containing the extra genes, flanked on both sides by IS elements that are identical or very similar in sequence).  Contain genes other than those required for transposition.  Transposable elements which lack insertion sequences are referred to as non composite transposons.  EG: Tn3 ( 5000 b p - Possesses terminal inverted repeats of 38 b p and generates flanking direct repeats that are 5 b p in length).  Effects of Transposable Elements 1. They can insert with in a gene to cause a mutation or stimulate DNA rearrangement leading to deletions of genetic material. 2. Transposons may be used to purify genes and study their function (Transposable element can fragment the genome and isolate the mutated fragment, there by purifying the gene). 3. Eukaryotic genes as well as prokaryotic genes can be turned on and off by transposon movement. 4. Transposons also are located with in plasmids and participate in such processes as plasmid fusion and the insertion of F plasmids into E.Coli chromosome. 5. Transposable elements play an extremly important role in the generation and transfer of new gene combinations. Evolutionary significance of Transposable Elements 1. The cellular – function hypothesis suggests that transposable elements provide some important function for the cell. 2. The genetic- variation hypotheses proposes that transposable elements provide evolutionary flexibility by inducing mutations. 3. The selfish – DNA hypothesis suggests that transposable elements do not benefit the cell but are widespread because they can replicate and spread.