<prompt> Which one of the following bonds links two nucleotides in a nucleic acid?3-3 phosphodiester bond3-5 phosphodiester bond5-5 phosphodiester bond5-5 phosphotriester bond  <response>: 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. 