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.

27. Based on Figure 1, how did the DNA molecules in the solution that was heated and cooled to produce Figure 2 compare with the DNA molecules in the solution prior to heating? Prior to heating, each DNA molecule was composed of:

Answer and Explanation

Your Answer is

Correct Answer is A

Explanation

Look at figure 1, before heating is a whole double-stranded DNA, so it is 2 completely linked strands;

After cooling and further cooling, it is longer DNA forms as other strands linked to unlinked portions , that is, more strands are connected, so it is option A