The chapter describes the early part of Barbara McClintock’s work on DNA transposons in maize, in which she uncovered the Ac-Ds family of mobile “controlling elements”. As a basis to understanding the work there is a preamble on the reproductive biology of maize, which explains the particular qualities of the maize plant as an experimental organism for genetics. An account is then given of the cytology of the system that was used to generate intact chromosomes having “sticky” (broken) ends, which is the starting point of the story. Cytogenetic aspects of the chromatid and chromosome breakage-fusion-bridge cycles, deriving from breakage, are then described; this leads on to a discussion about the way in which variegation phenotypes of the maize kernels can be “read” in terms of chromosome breakage. The “genetic earthquake” event of 1944, triggered by introducing broken chromosomes into a zygote from both parents, is then described; and the sequence of events leading to the discovery of Ds and Ac is traced. Finding mobility of Ds from one chromosomal location to another was pure serendipity, and 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. The chapter concludes with a brief reference to the molecular cloning of Ac and Ds.
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