The DNA molecule consists of two polynucleotide strands coiled around each other to form a double helix. The two strands are held together by specific interactions between the nitrogenous bases.

Why (A) is correct:

The nitrogenous bases (adenine, guanine, cytosine, and thymine) on opposite strands pair specifically: adenine (A) always pairs with thymine (T) via two hydrogen bonds, and guanine (G) always pairs with cytosine (C) via three hydrogen bonds. These hydrogen bonds are crucial for maintaining the stability and structure of the DNA double helix.

Option Analysis:

• A) Hydrogen bonding: This is correct. Hydrogen bonds form between complementary base pairs (A-T and G-C) on opposite strands, holding the DNA double helix together.
• B) Electrostatic bonding: While there are electrostatic interactions within the DNA molecule (e.g., between the negatively charged phosphate backbone and positively charged ions), the primary force holding the base units of the two strands together is not electrostatic bonding in the sense of ionic bonds.
• C) Covalent bonding: Covalent bonds form the backbone of each individual DNA strand (phosphodiester bonds) and link the bases to the deoxyribose sugar. However, covalent bonds do not link the base units between the two strands.
• D) Van der Waals forces: Van der Waals forces (including London dispersion forces) are present between stacked base pairs within the DNA helix, contributing to its stability. However, they are not the primary force responsible for the specific coupling between base units on opposite strands; hydrogen bonding is.

Correct Answer: (A)