This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Basics of DNA, Subject name: Biochemistry
[{'content': "Which one of the following bonds links two nucleotides in a nucleic acid? \nSelect and explain. \nA. 3'-3' phosphodiester bond, B. 3'-5' phosphodiester bond, C. 5'-5' phosphodiester bond, D. 5'-5' phosphotriester bond", 'role': 'user'}, {'content': "Answer: B \nExplanation: 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.", 'role': 'assistant'}]