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': 'About DNA, true is: \nSelect and explain. \nA. Two strands are held together by peptide bonds, B. Non-covalent bonds in sugar-phosphate backbone, C. Most common DNA is Z- DNA, D. Melting point of DNA is closely related to cytosine guanine content of DNA', 'role': 'user'}, {'content': "Answer: D \nExplanation: DNA 2 strands are held together by hydrogen bonds. There are covalent bonds (3'-5' phosphodiester bonds) in sugar-phosphate backbone. Melting point of DNA is closely related to the cytosine-guanine content of DNA as C & G have 3 bonds, as compared to the 2 bonds present between A & T. Most common DNA is B- DNA. A single turn of B-DNA about the long axis of the molecule contains 10 bp. The distance spanned by one turn of B-DNA is 3.4 nm (34 Ao). The width (helical diameter) of the double helix in B-DNA is 2 nm (20Ao).", 'role': 'assistant'}]