OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

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How are the two strands of a DNA double helix held together?

Covalent bonds
Hydrogen bonds
Disulfide bridges
Van der Waals forces

The correct answer is: Hydrogen bonds

The two strands of a DNA double helix are primarily held together by hydrogen bonds formed between the nitrogenous bases of each strand. Each base pairs specifically with another base: adenine pairs with thymine, and cytosine pairs with guanine. Hydrogen bonds, though relatively weak individually, collectively provide significant stability to the structure of the DNA molecule due to the vast number of these interactions present in a typical DNA strand. Covalent bonds play a crucial role in the structure of DNA as well, but they primarily form the backbone of the DNA strands, linking the sugar and phosphate groups. Disulfide bridges are a type of strong covalent bond mainly found in proteins, linking cysteine residues, and are not relevant in the context of DNA structure. Van der Waals forces are weak attractions that can occur between molecules but do not specifically account for the stable pairing of bases in DNA.