XNA Modifies the Sugar Backbone

Steps:

• Identify XNA as xeno-nucleic acid, a synthetic analog of DNA/RNA with an unnatural sugar component.
• Note that the coding function relies on base pairing, which remains unchanged.
• Recognize that DNA/RNA nucleotides consist of sugar, phosphate, and base; only sugar varies between them (deoxyribose vs. ribose).
• Conclude the replacement by unnatural X must be the sugar, preserving base-mediated coding.

Why A is correct:

• XNA is defined as nucleic acids with synthetic sugars replacing natural five-carbon sugars like deoxyribose or ribose, enabling novel properties while keeping genetic code intact.

Why the others are wrong:

• B: Phosphate group is inorganic and identical across natural and synthetic nucleic acids, not requiring replacement.
• C: Purine bases (A, G) are part of the unchanged coding sequence.
• D: Pyrimidine bases (C, T/U) are part of the unchanged coding sequence.

Final answer: A