These mutations caused by IS elements were called polar mutations, since transcription from all sequences downstream from insertion site was inhibited. Since these mutations did not revert back, and because different mutagens did not affect frequency of reversion, these were neither deletions, nor frameshift or point mutations (for mutations including frameshift, consult Mutations : 1. Morphological Level (Including Lethal Mutations), Mutations : 2. Biochemical Level (Biochemical and Microbial Genetics) and Mutations: 3. Molecular Level (Mechanism)). In order to study the mechanism of induction of polar mutations, lambda (λ)phage (which picks up gal region after inserting in this region) was used. λdgalm was isolated from mutant galm and its -DNA was used for synthesis of radioactive RNA. Certain fragments of this RNA hybridized with galm, but not with gal+, suggesting that an extra piece of DNA was present in galm.
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