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 of the following base is usually not present in DNA? \nSelect and explain. \nA. Adenine, B. Guanine, C. Cytosine, D. Uracil', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Uracil is not usually present in DNA, instead thymine is present. However in DNA, spontaneous deamination of cytosine can produce uracil. But, this uracil is promptly recognized by Uracil DNA-glycosylase and replaced with cytosine. If uracil were present in DNA, DNA machinery could not differentiate the original uracil and the deaminated cytosine. Thus, presence of thymine/absence of uracil gives stability to DNA. Uracil is present in RNA in place of Thymine in DNA', 'role': 'assistant'}]