This question already has an answer here:
We all know deoxyribonucleotides form DNA while ribonucleotides form RNA, DNA is double stranded while RNA is single stranded, and RNA can transcribe from DNA. We also know that DNA use A, G, C, T, while RNA use A, G, C, U and that it's the DNA that stores genetics information, not RNA.
My question is: how does the lost oxygen atom on deoxyribonucleotides 2' site of the sugar (turning it into a ribonucleotide), make such a large difference in function? It just an oxygen atom. Why don't they just all use U or all use T?
Why deoxyribose for DNA and ribose for RNA? This answer may solve the question why is DNA double strand, RNA single strand, but it doesnt explain why does RNA uses U instead of T.