Chapter 18, Problem 6EQ

Richard Boyce and Paul Howard-Flanders conducted an experiment that provided biochemical evidence that thymine dimers are removed from DNA by a DNA repair system. In their studies, bacterial DNA was radiolabeled so the amount of radioactivity reflected the amount of thymine dimers. The DNA was then subjected to UV light, causing the formation of thymine dimers. When radioactivity was found in the soluble fraction, thymine dimers had been excised from the DNA by a DNA repair system. But when the radioactivity was in the insoluble fraction, the thymine dimers had been retained within the DNA. The following table illustrates some of the experimental results involving a normal strain of E. coli and a mutant strain that was very sensitive to killing by UV light:

Strain	Treatment	Radioactivity in the Insoluble Fraction (cpm*)	Radioactivity in the Soluble Fraction (cpm)
Normal	No UV	<100	<40
Normal	UV-treated, incubated 2 hours at 37°C	357	940
Mutant	No UV	<100	<40
Mutant	UV-treated, incubated 2 hours at 37°C	890	<40

Source: Adapted from R. P. Boyce and P. Howard-Flanders (1964), Release of ultraviolet light–induced thymine dimers from DNA in E. coli K-12. Proc Natl Acad Sci USA 51, 293–300.

*The abbreviation cpm stands for “counts per minute,” which is a measure of the number of radioactive emissions from the sample.

Explain the results found in this table. Why is the mutant strain sensitive to UV light?