In the DNA double helix, the two antiparallel strands are held together primarily by which type of interaction?

• A. Covalent phosphodiester bonds between bases on opposite strands
• B. Ionic bonds between the sugar-phosphate backbones of the two strands
• C. Hydrogen bonds between complementary nitrogenous bases
• D. Peptide bonds linking nucleotides across the helix

Answer: (C)

Verified step by step guidance

1. Understand the structure of the DNA double helix: it consists of two strands running in opposite directions (antiparallel) with a sugar-phosphate backbone on the outside and nitrogenous bases on the inside.
2. Recall the types of chemical bonds and interactions in DNA: covalent bonds (phosphodiester bonds) link nucleotides within a single strand, while interactions between strands are non-covalent.
3. Identify that phosphodiester bonds are covalent bonds connecting the sugar of one nucleotide to the phosphate of the next within the same strand, not between strands.
4. Recognize that the two strands are held together by hydrogen bonds formed specifically between complementary nitrogenous bases (adenine with thymine, guanine with cytosine).
5. Conclude that the primary interaction holding the two antiparallel strands together is hydrogen bonding between complementary bases, not covalent or ionic bonds.