CONCEPT
Transposable Elements
Barbara McClintock's discovery that genes can move—the Activator/Dissociation system that showed the genome is not a fixed library but a self-rearranging system, biology's clearest natural instance of a program that edits its own code.
Transposable elements, first described by
Barbara McClintock from her work on maize chromosomes in the late 1940s, are genetic units that can physically relocate within the genome—detaching from one chromosomal position and inserting at another, often switching neighboring genes on or off in the process. McClintock named her two founding elements
Activator (Ac), which drives the transposition, and
Dissociation (Ds), which is moved under Ac’s influence. When Ds landed beside a pigment gene, it silenced the gene; when it departed, the gene reactivated; and the corn kernel recorded the entire episode as a colored sector against a silent field—a visible printout of a genome editing itself. The field dismissed the discovery for over two decades; when molecular biologists found transposable elements independently in bacteria in the 1960s and 1970s, and established that mobile DNA is universal across the living world, the discipline understood what it had missed. Today transposable elements are known to constitute a substantial fraction of the human genome, and