Question

1. Why must DNA have thymine instead of Uracil?
2. Why is it necessary for RNA to have uracil instead of thymine?

Answer 1

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Because thymine sticks to arganine better than uracil does. You want the double-stranded DNA to stick together pretty well most of the time. RNA has uracil so it doesn't stay stuck to the DNA after transcription or get stuck to complementary RNA strands as easily.

Answer 2

Good question.
It happens so that uracil does not allow binding of transcription factors (regulatory proteins that assist with transcription) as well as thymine does.
In fact, it is this point that is used in the treatment of some kinds of cancer.
You can look up 5-FU (5-fluorouracil) which prevents methylation of U in the formation of T. So U is incorporated into the DNA strand and compromises transcription, killing cancer cells.