This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Techniques in molecular biology, Subject name: Biochemistry
[{'content': "Sanger's reagent is \nSelect and explain. \nA. Dinitrobenzene, B. Dichlorobenzene, C. Tetra-nitrobenzene, D. Tetrachlorbenzene", 'role': 'user'}, {'content': "Answer: A \nExplanation: Sanger's reagent or di-nitro-fluoro-benzene (DNFB) is used for polypeptide sequencing. Other methods of Polypeptide sequencing: Edman's degradation (Edman's reagent is Phenyl-isothiocyanate) - Determine sequence from N-terminal end and can sequence 30-60 amino acid residues. But it cannot work if N-terminal amino acid is chemically modified. Use of various chemical or enzymes to cleave peptides - method used for large polypeptides Mass spectrometry - 3 pas of mass spectrometer are ionization source, mass analyzer and ion detector.", 'role': 'assistant'}]