Why there is uracil in RNA instead of thymine and why not in DNA?
You're asking "why" which is something I cannot answer. But it is a possibility that it is done this way simply to differentiate between DNA and RNA so that the body can tell which is which. Changing one of the bases for example, would allow certain molecules/enzymes to attach only to RNA and not to DNA and vice versa (based on if a molecule contained Thymine or Uracil). Since RNA and DNA perform very different roles in the cell, it would make sense to differentiate them chemically in some way so that the body "knows" what is what.
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