Here's why:

* DNA uses thymine (T) instead of uracil (U). Both thymine and uracil are pyrimidines, but they differ in their chemical structure.

* Uracil pairs with adenine (A) in RNA, just like thymine pairs with adenine in DNA. This difference in base pairing is one of the key distinctions between RNA and DNA.

So, while both DNA and RNA have adenine (A), guanine (G), and cytosine (C), only RNA contains uracil.