Brenner here reported progress in his research with acridine mutants--mutants produced by chemicals of the acridine family, such as proflavine, which are chemically similar to the bases of DNA. Acridines slip in and out between the bases of one chain of the DNA molecule, leading to the insertion or deletion of a base on the complementary strand, thereby causing a mutation. Research with acridine mutants led Brenner, Crick, and their collaborators to conclude later in 1961 that the genetic code was to be read three bases at a time from a fixed starting point. This research involved the use of suppressors, also discussed by Brenner, which are second mutations, at a different site on the DNA chain, that mask the expression of a first mutation.
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