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.
23. According to Figure 2, as the heated DNA solution was cooled from 90°C to 30°C, the absorbance:
Your Answer is
Correct Answer is D
Explanation
Look at the cooling curve in figure 2. When the temperature is from 90-30°C, the absorbance is flat first, then drops sharply, and then stays flat