Which two enthalpic factors primarily stabilize DNA in its double-helical form at low temperature?

• A. Hydrogen bonding between base pairs and base stacking interactions
• B. Covalent bonding within the sugar-phosphate backbone and hydrophobic interactions
• C. Electrostatic repulsion between phosphate groups and hydrogen bonding with water
• D. Ionic interactions with phosphate groups and van der Waals forces between sugars

Answer: (A)

Verified step by step guidance

1. Understand that DNA stability in its double-helical form is influenced by interactions that lower the system's enthalpy, making the structure energetically favorable at low temperatures.
2. Identify that hydrogen bonding occurs specifically between complementary base pairs (adenine-thymine and guanine-cytosine), which helps hold the two strands together.
3. Recognize that base stacking interactions involve van der Waals forces and hydrophobic effects between adjacent bases along the same strand, contributing significantly to the overall stability of the helix.
4. Note that covalent bonds within the sugar-phosphate backbone provide structural integrity but do not primarily contribute to the enthalpic stabilization of the double helix since they are present regardless of the DNA being single or double stranded.
5. Conclude that the two main enthalpic factors stabilizing DNA at low temperature are hydrogen bonding between base pairs and base stacking interactions.