Jennifer, a 16 year old female from Brooklyn asks on April 13, 2003,Why does uracil replace thymine in RNA?
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RNA came first as a primitive way of encoding information. DNA is a more refined version of RNA. As such, DNA has features that make it more immune to errors of information coding and more long lasting. One of these features is the use of Thymine instead of Uracil. Thymine is chemically the same as uracil but with a methyl group (CH3) added to the pyrimidine ring. The presence of this methyl group acts as a sort of "blocker" that both protects the entire DNA molecule from attack by certain enzymes AND also prevents thymine from combining with any other base pairs except adenine. (Uracil, lacking this methyl group, can combine with any base pair so RNA has the potential for more errors or mutations than DNA.) You can find out more about all this at the Mad Scientists Network.
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