This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pathology, Subject name: Pathology
[{'content': 'Cells are most radio-resistant in \nSelect and explain. \nA. S phase, B. M phase, C. G1 phase, D. G2 phase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Cell cycle Correct sequence: G0-G1-S-G2-M Most impoant checkpoint: G1-S Cellular content doubled in: S phase Cyclins D,E,A,B appears sequentially Cyclin D - first to increase in cell cycle Cyclin D - controls G1-S transition P16 (CDKN2A), P53& RB - Induce cell cycle arrest at G1-S Cyclin B - controls G2-M transition Fixed time is required for: S & M phase Most radio-resistant phase: S Most radio-sensitive phase: M', 'role': 'assistant'}]