Activator-Dissociation system

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A pair of interacting genetic elements in maize discovered and analyzed by Barbara McClintock. Ac is an autonomous element that is inherently unstable. It has the ability to excise itself from one chromosomal site and to transpose to another. Ac is detected by its activation of Ds. Ds is nonautonomous and is not capable of excision or transposition by itself. Ac need not be adjacent to Ds or even on the same chromosome in order to activate Ds. When Ds is so activated, it can alter the level of expression of neighboring genes, the structure of the gene product, or the time of development when the gene expresses itself, as a consequence of nucleotide changes inside or outside of a given cistron. An activated Ds can also cause chromosome breakage, which may yield deletions or generate a breakage-fusion-bridge cycle (q.v.). It is now known that Ac is a 4,500 bp segment of DNA that encodes a transposable element (q.v.) which contains within it the locus of a functional transposase (q.v.). The transposase gives Ac the ability to detach from one chromosome and then insert into another. The excision of Ac may cause a break in the chromosome, and this is what generated the breakage- fusion-bridge cycles that McClintock observed. Ds is a defective transpon that contains a deletion in its transposase locus. Therefore the Ds transposon can move from chromosome to chromosome only if Ac is also in the nucleus to supply its transposase. Ac and Ds were originally classified as mutator genes, since they would sometimes insert into structural genes and modify their functioning. See Chronology, 1950, McClintock; 1984, Pohlman et al.; Dotted, genomic instability, mutator gene, terminal inverted repeats (TIRs), transposon tagging.

Subjects: Genetics and Genomics.

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