In the phosphodiester linkage between two nucleotides, the phosphate group of one nucleotide is attached to which carbon of the first nucleotide's sugar?

• A. 5' carbon
• B. 3' carbon
• C. 2' carbon
• D. 1' carbon

Answer: (A)

The bond between nucleotides is a phosphodiester linkage that forms when a phosphate group connects to the sugar of the neighboring nucleotide. Specifically, the phosphate group from the incoming nucleotide is esterified to the 3' hydroxyl of the sugar on the preceding nucleotide, meaning the attachment occurs at the 3' carbon of the first sugar. This setup gives nucleic acid strands their 5' to 3' directionality: the 5' end carries the phosphate on its 5' carbon, and each new bond forms at the 3' carbon of the growing strand.