Which one of the following bonds links two nucleotides in a nucleic acid?: (A) 3'-3' phosphodiester bond, (B) 3'-5' phosphodiester bond, (C) 5'-5' phosphodiester bond, (D) 5'-5' phosphotriester bond
Answer is B. Nucleic acids are directional molecules: 5' end is the end with free (unesterified) phosphate group. 3' end is the end with free OH group. Nucleic acid sequences are written in 5'-3' direction. During the synthesis of nucleic acids, 3' OH group of existing nucleotide is esterified to the 5' phosphoryl group of the incoming nucleotide 3'-5' phosphodiester bond.