XNA replaces the sugar in synthetic nucleic acids

Steps:

• Nucleotides contain three components: a five-carbon sugar, phosphate group, and nitrogenous base.
• The nitrogenous base (purine or pyrimidine) encodes genetic information.
• X is not the coding part, so it cannot be the base.
• X is an unnatural organic chemical, ruling out the inorganic phosphate; thus, it replaces the organic sugar.

Why A is correct:

• In XNA, the five-carbon sugar (ribose or deoxyribose) is replaced by a synthetic analog like threose, preserving base coding while altering the backbone.

Why the others are wrong:

• B: Phosphate group is inorganic (PO₄³⁻), not an organic chemical.
• C: Purine bases (adenine, guanine) handle coding, which is unaffected.
• D: Pyrimidine bases (thymine, cytosine, uracil) handle coding, which is unaffected.

Final answer: A