Which statement about nucleic acids is true?

Which statement about nucleic acids is true?

• A. The phosphate group links nucleotides through a 3' to 5' phosphodiester bond.
• B. RNA lacks a hydroxyl group at the 2' position.
• C. Uracil is found in RNA and DNA.
• D. Xylose can replace ribose as the sugar in nucleic acids.

Answer: (A)

Think about how nucleic acids are built. Nucleotides are linked by phosphodiester bonds that connect the 3' hydroxyl of one sugar to the 5' phosphate of the next, forming a sugar–phosphate backbone that runs from the 5' end to the 3' end. This 3'-to-5' linkage is the defining connection in both DNA and RNA, so that statement is true. RNA does have a hydroxyl group at the 2' position on the ribose, whereas DNA lacks it, so the claim about lacking a 2' OH is false. Uracil is used in RNA, but thymine is used in DNA, making that statement incorrect as well. Xylose isn’t used as the sugar in nucleic acids; the sugars are ribose in RNA and deoxyribose in DNA, so that option isn’t correct either.

Think about how nucleic acids are built. Nucleotides are linked by phosphodiester bonds that connect the 3' hydroxyl of one sugar to the 5' phosphate of the next, forming a sugar–phosphate backbone that runs from the 5' end to the 3' end. This 3'-to-5' linkage is the defining connection in both DNA and RNA, so that statement is true.

RNA does have a hydroxyl group at the 2' position on the ribose, whereas DNA lacks it, so the claim about lacking a 2' OH is false. Uracil is used in RNA, but thymine is used in DNA, making that statement incorrect as well. Xylose isn’t used as the sugar in nucleic acids; the sugars are ribose in RNA and deoxyribose in DNA, so that option isn’t correct either.