The genome is not static: genes move
      Phenomenon identified in 1940s by B. McClintock in maize: Nobel Prize 1983
                    [Keller: "A Feeling for the Organism"]

       Ac - Ds system in maize (Zea) described 1950s
       Ds (Dissociator) locus causes chromosome breaks
            only when Ac (Activator) locus is present in trans configuration
                    Break affects expression of other genes in cis configuration
       Ds element may also transpose ("jump") to a locus elsewhere in genome:
                    effect on expression of other genes may be unstable & reversible,
                         depending on position of Ds

       Ac & Ds turn out to be transposable DNA elements
            inverted terminal repeats (ITRs) - dsDNA reads the same in both orientations

     5' - ATCCG ~~~~~(ORF)~~~~~CGGAT - 3'
     3' - TAGGC~~~~~~~~~~~~~~~~GCCTA - 5'

 
           elements may move from one position to another in genome, with similar ITRs

       Ac  has an ORF for a 'Transposase' gene
                     Ds is an Ac with a defective Transposase gene

       Transposable elements account for 'classical' genetic mutants
           'wrinkled' (rr) peas (Pisum)
                        defect of starch-branching enzyme (SBEI)
           'white eyes' in fruit flies (Drosophila)
                        insertion of copia sequences
            Alu sequences in Homo
                  300 bp  element,  9 x 10⁵ copies 10% of human genome is mobile