Why can't DNA read uracil during transcription?

[charliemagne]

What is in DNA why can it not read the uracil nitrogenous base during transcription? May I have answer in the molecular level?

 

[Ygggdrasil]

Uracil can function perfectly fine in DNA. DNA containing uracil can be read by DNA and RNA polymerases. The problem is that a common type of DNA damage, deamination, converts cytosine to uracil. Therefore, if uracil were used as a base in DNA, one could not distinguish between uracil bases that are supposed to be present and cytosines that had been deaminated. By using thymine instead of uracil, the cell will recognize uracil bases in DNA as deaminated cytosines, remove them, and allow repair enzymes to restore a cytosine at that position in order to prevent a mutation from occurring.