Answer to Question #51634 in Molecular Biology for DR P P SINGH
There is an enzyme, uracil DNA glycosylase, that does exactly that; it excises uracil bases from double-stranded DNA. It can safely do that as uracil is not supposed to be present in the DNA and has to be the result of a base modification.
Now, if we would use uracil in DNA it would not be so easy to decide how to repair that error. It would prevent the usage of this important repair pathway.
The inability to repair such damage doesn't matter for RNA as the mRNA is comparatively short-lived and any potential errors don't lead to any lasting damage. It matters a lot for DNA as the errors are continued through every replication. Now, this explains why there is an advantage to using thymine in DNA, it doesn't explain why RNA uses uracil.
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