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| Modern Genetic Analysis Chapter 13. Transposable Genetic Elements Insertion Sequences Figure 13-1. Mutation by insertion is demonstrated with phage l particles carrying the bacterial gene for galactose utilization (gal+) or the mutant gene gal. The viruses are centrifuged in a cesium chloride solution. The gal particles are found to be denser. Because the virus particles all have the same volume and their outer shells all have the same mass, the increased density of gal particles shows that they must contain a larger DNA molecule: the gal mutation was caused by the insertion of DNA. (From S. M. Cohen and J. A. Shapiro, "Transposable Genetic Elements." Copyright © 1980 by Scientific American, Inc. All rights reserved.)

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