Which statement best describes covalent bonds in DNA?

Which statement best describes covalent bonds in DNA?

• A. Covalent bonds form the sugar-phosphate backbone, whereas hydrogen bonds hold base pairs together
• B. Hydrogen bonds form the sugar-phosphate backbone, whereas covalent bonds hold base pairs together
• C. Covalent bonds hold base pairs together
• D. Hydrogen bonds form the backbone of DNA

Answer: (A)

DNA’s two strands stay connected by a sturdy outer backbone made of sugars and phosphate groups, joined together by covalent phosphodiester bonds. These bonds link the 5′ phosphate of one nucleotide to the 3′ OH of the next, creating a continuous sugar–phosphate backbone along each strand. The bases between the strands pair up across the helix via hydrogen bonds, which are weaker and allow the strands to separate when DNA is copied or read. So, covalent bonds form the backbone, while hydrogen bonds hold the base pairs together. The other statements mix up these roles: hydrogen bonds don’t form the backbone, and covalent bonds don’t hold base pairs together.

DNA’s two strands stay connected by a sturdy outer backbone made of sugars and phosphate groups, joined together by covalent phosphodiester bonds. These bonds link the 5′ phosphate of one nucleotide to the 3′ OH of the next, creating a continuous sugar–phosphate backbone along each strand. The bases between the strands pair up across the helix via hydrogen bonds, which are weaker and allow the strands to separate when DNA is copied or read. So, covalent bonds form the backbone, while hydrogen bonds hold the base pairs together. The other statements mix up these roles: hydrogen bonds don’t form the backbone, and covalent bonds don’t hold base pairs together.