Here's why:

* DNA and RNA have different bases: DNA uses the bases Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). RNA uses the same bases except it replaces Thymine with Uracil (U).

* Base pairing rules: The bases pair up in a specific way:

* Adenine (A) pairs with Thymine (T) in DNA, or Uracil (U) in RNA.

* Guanine (G) pairs with Cytosine (C).

Therefore, if you have DNA A, it will pair with U in mRNA during transcription.