Jumping Genes (Transposons)

Barbara McClintock, working with maize in the 1940s, made one of the most conceptually radical discoveries in genetics: while studying the unstable, mosaic pattern of blue, brown and red pigmentation in the aleurone layer of individual maize kernels (caused by irregular production of vacuolar anthocyanin pigments), she found that this instability was caused by DNA sequences that could physically move from one position to another within the genome. She first reported this finding in 1948, describing the responsible elements as 'mobile controlling elements'; the modern term transposable elements, or transposons, popularly nicknamed 'jumping genes' or 'hopping genes', was later coined by fellow maize geneticist Alexander Brink. In the maize genome specifically, McClintock characterised a two-component system, the Ac (Activator) and Ds (Dissociation) elements, whose transposition activity in somatic kernel cells produces the visible variegated pigmentation pattern by disrupting pigment-gene expression differently in different cell lineages as development proceeds. Transposons carry real biological significance beyond their unusual mobility: they contribute substantially to the overall spontaneous mutation rate of an organism, since a transposon landing inside or near a gene can disrupt it; over evolutionary time they contribute to genetic diversity by reshuffling genome content; and in modern genetic research they are valuable practical tools, deliberately used as mutagens to disrupt genes of interest, as molecular tags for cloning the disrupted gene, and as vehicles for inserting foreign DNA into a model organism's genome. McClintock's discovery was also historically important in …