Chapter 19

Question 1

Denaturation

Answer 1

Complete unwinding and separation of the complementary single strands.

Double stranded DNA can be denatured by heat or a chaotropic agent.

In studies of thermal denaturation, the temperatrue of DNA is slowly increased. As the temperature is raised more and more of the bases become unstacked and hydrogen bonds between the base pairs are broken.

Question 2

Melting point Tm

Answer 2

The temperature at which half of the DNA has become single stranded.

Question 3

Stacking Interactions

Answer 3

The stacked base pairs form van der Waals contacts. Although the forces between individual stacked base pairs are weak, they are additive so in large DNA molecules the van der Waals contacts are an important source of stability

Question 4

Hydrogen bonds

Answer 4

Hydrogen bonding between base pairs is a signifigant stabilizing source.

Question 5

Hydrophobic effects

Answer 5

Burying hydrophobic purine and pyrimidine rings in the interior of the double helix increases the stability of the helix.

Question 6

Charge-Charge interactions

Answer 6

Electrostatic repulsion of the negatively charged phosphate groups of the backbone is a potential source of instability in the DNA helix. However, replusiosn is minimized by the presence of cations such as mg2+ and cationic proteins.