why does RNA contain uracil instead of thymine, and/or why does uracil take the place of thymine in RNA???? Please help me this is an extra credit problem i need to do to get an "A".

The biosynthesis of dTTP requires the methylation of the uracil of one dUMP:

dUMP -> dTMP -> dTTP.

 

So based on this one can see that having dUTP is easier and less costly to synthesize than dTTP (minus one methylation).

 

Moreover in some textbooks (a good source for this kind of trivia btw.) you can also find that uracil is less stable and eases the degradation of mRNA, which is of advantage for regulatory processes of course.

Now the real question that I would ask is: why does DNA have T instead of U?

 

The answer is equally trivial, btw.