Passage V

When DNA is heated to its melting temperature (Tm) it denatures (breaks apart) into 2 strands. When cooled, the strands can relink to form less stable DNA, indicated by a lower Tm(see Figure 1).

Figure 2 shows how the absorbance of light by a solution of DNA molecules, each initially composed of 2 completely linked strands, varied as it was heated and then cooled. When the DNA began to denature, the absorbance increased sharply. When the DNA began to renature, the absorbance decreased sharply.

Figure 3 shows how Tm for heating varied for identical solutions of DNA previously exposed to UV light for different amounts of time.

Figures 1-3 adapted from Charles M. Lovett, Jr., Thomas N. Fitzgibbon, and Raymond Chang, "Effect of UV Irradiation on DNA as Studied by Its Thermal Denaturation." ©1989 by Division of Chemical Education, Inc., American Chemical Society.

26. A heating/cooling curve was determined for a new solution containing only completely linked, doublestranded DNA molecules. The Tm for heating this solution was found to be 10°C lower than the Tm for heating the solution represented in Figure 2. Which of the following best shows the heating/cooling curve of the new solution?

Answer and Explanation

Your Answer is

Correct Answer is F

Explanation

According to the question stem, Tm is 10°C lower than Tm in figure 2, so both curves in figure 2 are shifted to the left by 10°C That's it, so it's option F