What holds the two strands of a DNA molecule together?

• A. Ionic bonds between deoxyribose sugars
• B. Peptide bonds between nucleotides
• C. Covalent bonds between phosphate groups
• D. Hydrogen bonds between complementary nitrogenous bases

Answer: (D)

Verified step by step guidance

1. Understand the structure of DNA: DNA is composed of two strands forming a double helix, where each strand consists of a sugar-phosphate backbone and nitrogenous bases extending inward.
2. Identify the bonds in the sugar-phosphate backbone: The backbone of each DNA strand is held together by covalent bonds called phosphodiester bonds between the phosphate group of one nucleotide and the sugar of the next nucleotide.
3. Recognize the role of nitrogenous bases: The nitrogenous bases (adenine, thymine, cytosine, and guanine) extend from the sugar-phosphate backbone and pair specifically (A with T, C with G) between the two strands.
4. Determine the type of bond between the two strands: The complementary nitrogenous bases on opposite strands are held together by hydrogen bonds, which are weaker than covalent bonds but crucial for the double helix structure and base pairing specificity.
5. Conclude that the two strands of DNA are held together by hydrogen bonds between complementary nitrogenous bases, not by ionic or peptide bonds.