12. DNA was isolated from wild type (Gal+) and mutant (Gal-) E.coli cells and separated
by density gradient centrifugation technique. DNA from Gal- strain acquired a lower
position. This indicates that the mutation is caused by:
a. Inversion
b. Insertion
c. Missense mutation
d. Point mutation
Density gradient centrifugation separates DNA based on buoyant density, where heavier DNA bands lower in the gradient. In this E. coli experiment, wild type (Gal+) DNA bands higher than mutant (Gal-) DNA, indicating lower density in the mutant due to structural change. The correct answer is b. Insertion.
Technique Overview
Density gradient centrifugation, like CsCl equilibrium method from Meselson-Stahl, positions DNA by density: normal E. coli DNA at ~1.71 g/cm³ bands midway. Insertions add extra bases, often AT-rich, reducing average GC content and density, shifting Gal- DNA lower. Wild type remains unchanged.
Option Analysis
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a. Inversion: Rearranges segment without base addition/loss; density unchanged as composition identical. Cannot explain lower position.
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b. Insertion: Adds nucleotides (e.g., transposon), diluting GC density if AT-rich; matches lower Gal- band. Common in bacterial gal operon mutants.
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c. Missense mutation: Single base swap (e.g., GCT to GTT); negligible density impact on whole genome.
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d. Point mutation: Base substitution or small change; no detectable density shift genome-wide.
Exam Relevance
This tests mutation detection in CSIR NET Life Sciences (Unit 5: Genetics). Insertion/duplication alters density detectably, unlike point changes. Gal- phenotype links to lac/gal operon disruptions.
