How Barbara McClintock discovered transposable elements in maize


The paper describes the early part of Barbara McClintock`s work on DNA transposons in maize, in which she discovered the Ac-Ds family of mobile "controlling elements". An account is first given of the cytology of the system that was used to generate intact chromosomes having "sticky" (broken) ends. Cytogenetical aspects of the chromatid and chromosome breakage-fusion-bridge cycles, deriving from breakage, are then described, which leads on to the way in which variegation in phenotypes of the maize kernels could be "read" in terms of chromosome breakage. The "genetic earthquake" event of 1944, triggered by introducing broken chromosomes into a zygote from both parents, lead to the discovery of Ds and Ac. Finding mobility of Ds from one chromosomal location to another was pure serendipity: the transposition showed itself while experiments were being undertaken to accurately map Ds. A similar chance observation revealed transposition of Ac as well, and then the relationship between the two elements was elucidated in terms of their autonomous and non-autonomous nature.

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