Insertion and Excision of Ds Controlling Elements in Maize

When Barbara McClintock participated in the Cold Spring Harbor Symposium in 1951 (McClintock 1952a), she had already characterized genetically the Activator-Dissociation (Ac-Ds) controlling element system in maize. She had deduced that both Ac and Ds were small segments of chromosomes that could tranpose from one position to another, that a Ds element could do so only if a functional Ac element was present in the nucleus, and that Ac could itself transpose as well as enabling Ds elements to do so. McClintock followed the perambulations of these elements by observing their effects on genes controlling readily scoreable phenotypes. Initially, she identified a Ds element in the short arm of chromosome 9 by its property of causing chromosomal breaks that resulted, in the endosperm mitotic divisions, in a succession of dicentric-bridge breakage and reunion cycles. This property of Ds was under the control of Ac (McClintock 1947, 1948). Ds and Ac...