TransposonsMobile DNA: Transposons Mobile DNA refers to discrete segments of DNA that move as units from one location to another within other DNA molecules They are always found inserted into another DNA molecule such as a plasmid, a chromosome, or a viral genome. Transposable elements do not possess their own origin of replication. Instead, they are replicated when the host DNA molecule into which they are inserted is replicated. Transposable elements move by a process called transposition that is important both in evolution and in genetic analysis. ( in !!! toin !,!!!,!!! per generation" The two ma#or types of transposable elements in $acteria are insertion se%uences (I&" and transposons. $oth elements have two important features in common' They carry genes encoding transposase, the en(yme necessary for transposition, and they have short inverted terminal repeats at their ends that are also needed for transposition. Insertion sequences are the simplest type of transposable element. They are short DNA segments, about !!! nucleotides long, and typically contain inverted repeats of !)*! bp. +ach di,erent I& has a speci-c number of base pairs in its terminal repeats. The only gene they possess is for the transposase. &everal hundred distinct I& elements have been characteri(ed.IS2isaninsertionsequenceof1327 bpwithinverterepeatsof!1bpat its ens" Transposons are larger than I& elements, but have the same two essential components' inverted repeats at both ends and a gene that encodes transposase. The transposase recogni(es the inverted repeats and moves the segment of DNA .an/ed by them from one site to another. 0onse%uently, any DNA that lies between the two inverted repeats is moved and is, in e,ect, part of the transposon. &ome transposons (0on#ugative Tn " are capable of moving to another bacteria through con#ugation 0omposite Tn ' As in the case of Mu virus the genome itself act as an Tn. The whole structure can move as single unit. &igni-cance of transposition' +volution Mutation