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.   

1.
NoteThe main sources for reconstructing this story come from the annual reports which McClintock made in the Carnegie Institution of Washington Year Book. These references are listed first in order, but they are not specifically attributed in the text or in the figures since these annual reports covered several aspects of her work.
2.
McClintock B: Carnegie Institution of Washington Year Book
3.
Maize genetics. 41:181–186 (1942).
4.
Maize genetics. 42:148–152 (1943).
5.
Maize genetics. 43:127–135 (1944).
6.
Cytogenetic studies of maize and Neurospora. 44:108–112 (1945).
7.
Maize genetics. 45:176–186 (1946).
8.
Cytogenetic studies of maize and Neurospora. 46:146–152 (1947).
9.
Mutable loci in maize. 47:155–169 (1948).
10.
Mutable loci in maize. 48:142–154 (1949).
11.
Mutable loci in maize. 49:157–167 (1950).
12.
Mutable loci in maize. 50:174–181 (1951).
13.
Mutable loci in maize. 51:212–219 (1952).
14.
Mutation in maize. 52:227–237 (1953).
15.
Mutations in maize and chromosomal aberrations in Neurospora. 53:254–260 (1954).
16.
Controlled mutation in maize. 54:245–255 (1955).
17.
Mutation in maize. 55:323–332 (1956).
18.
Genetic and cytological studies of maize. 56:393–401 (1957).
19.
The suppressor-mutator system of control of gene action in maize. 57:415–429 (1958).
20.
Genetic and cytological studies in maize. 58:452–456 (1959).
21.
Further studies of the suppressor-mutator system of control of gene action in maize. 60:469–476 (1961).
22.
Topographical relations between elements of control systems in maize. 61:448–461 (1962).
23.
Further studies of gene-control systems in maize. 62:486–493 (1963).
24.
Aspects of gene regulation in maize. 63:592–601 (1964).
25.
Components of action of the regulators Spm and Ac. 64:527–534 (1965).
26.
Regulation of pattern of gene expression by controlling elements in maize. 65:568–576 (1966).
27.
The states of a gene locus in maize. 66:664–672 (1967).
28.
The contribution of one component of a control system to versatility of gene expression. 70:5–17 (1970–1971).
29.
Fedoroff NV: Controlling elements in maize, in Shapiro JA (ed): Mobile Genetic Elements, Chapter 1, pp 1–63 (Academic Press, New York 1983).
30.
Fedoroff NV: Transposable genetic elements in maize. Sci Am 250:64–76 (1984).
31.
Fincham JRS, Sastry GRK: Controlling elements in maize. Ann Rev Genets 8:15–50 (1974).
32.
McClintock B: The behaviour in successive nuclear divisions of a chromosome broken at meiosis. Proc Natl Acad Sci USA 25:405–416 (1939).
33.
McClintock B: The stability of broken ends of chromosomes in Zea mays. Genetics 26:234–282 (1941).
34.
McClintock B: The fusion of broken ends of chromosomes following nuclear fusion. Proc Natl Acad Sci USA 28:458–463 (1942).
35.
McClintock B: Chromosome organization and gene expression. Cold Spring Harbor Symp Quant Biol 16:13–47 (1951).
36.
McClintock B: Controlling elements and the gene. Cold Spring Harbor Symp Quant Biol 21:197–216 (1956).
37.
McClintock B: The control of gene action in maize. Brookhaven Symposium in Biology 18:162–184 (1965).
38.
McClintock B: Mechanisms that rapidly reorganize the genome. Stadler Symposium 10, pp 25–48, University of Missouri, Columbia (1978).
39.
Rhoades MM: On the genetic control of mutability in maize. Proc Natl Acad Sci USA 31:91–95 (1945).
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