Much of the investigations of the classical genetics have been devoted to the localization of genes on chromosomes. As according to chromosome mapping, gene mapping depends on the assumption that genes do not move from one position to another. To a great extent, the hypothesis has been found correct. In fact most genes occupy fixed sites on chromosome and the overall structure of the genetic map is practically invariant. However in early 1940s researchers have found that some DNA sequences can actually change their position.
These mobile elements have been variously called Jumping genes, Mobile elements, Transposons etc. The formal name is Transposable elements and their movement is called transposition. The term transposon was coined by R.W. Hedges and A.E. Jacob in 1974.
These mobile elements have been variously called Jumping genes, Mobile elements, Transposons etc. The formal name is Transposable elements and their movement is called transposition. The term transposon was coined by R.W. Hedges and A.E. Jacob in 1974.
Definition
Transposable elements can be defined as small mobile DNA sequence that move around chromosome with no regard for Haemology and insertion of these elements may produce deletions inversions chromosomal fusions and even more complicated rearrangements.
Discovery of Transposable Elements
Transposable elements were discovered by Barbara McClintock in 1940s through an analysis of genetic inability in maize (corn). The instability involved chromosome breakage and was found to occur at sites where transposable elements were located. In McClintock's analysis, the events of breakage were detected by following the loss of certain genetic marker.
In some experiments, McClintock used a marker (gene) that controlled the deposition of pigmentation in the aleurone which is the outer most layer of endosperm of maize kernels. McClintock's marker was an allele of the locus C on the short arm of an chromosome I. Since this allele is dominant inhibitor of aleurone colouration any kernel possessing it should be colourless. He fertilized CC ears with pollen from C1C1 tessels producing kernels in which the endosperm was C1CC. Although many of kernels were colourless, as except some showed patches of brownish-purple pigment. McClintock guessed that in such mosaics, the inhibitory 'C' allele had been lost sometime during endosperm development leading to a clone of tissue that was capable of producing pigment. The genotype in such a clone should - CC, where the clash indicates loss of C1 allele.
