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Wikipedia says that "in RNA, thymine is replaced with uracil in most cases." I'm curious what are the cases when this does not happen? Does this ever occur in normal functioning, or is it an error? Do T and U ever appear in the same strands of RNA together, or does a single strand just use T or just use U?

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tRNA molecules contain a T in the T arm. I believe that this results from post-transcriptional modification of a U by uridine methylase.

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just curious: can RNA polymerase incorporate thymine from TTP against Adenine ? – WYSIWYG Jul 4 '13 at 15:14
Do cells make TTP? (or 5-methyl UTP, as it is usually referred to) – Alan Boyd Jul 4 '13 at 17:53
I am not sure.. UMP is the first product of de-novo pyrimidine synthesis. The ribose is reduced and uracil is methylated to produce dTTP. I dont know if the order of the reactions is unchangeable ? I guess if artificially provided, Thymine can be incorported in RNA. (Sorry, i should have looked it up instead of commenting) – WYSIWYG Jul 5 '13 at 5:34

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