When the mRNA strand is being formed, what base matches up to adenine?

Uracil

Explanation:

Transcription is the first stage of gene expression in a living cell and it involves the synthesis of a mRNA molecule using a DNA template. The process of transcription is carried out by an enzyme called RNA polymerase, which initially binds to the PROMOTER region of a DNA, unwind its double-helix and begins to add nucleotide bases to the growing RNA strand based on the nucleotides it reads on the DNA.

The basis of nucleotide addition by RNA polymerase is COMPLEMENTARY BASE PAIRING RULE, which states that Adenine (A) always pairs with Thymine (T) and Guanine (G) with Cytosine (C). RNA polymerase adds to the RNA strand, nucleotides complementary to the ones it reads on the DNA. Since, Uracil base replaced Thymine in the RNA molecule, RNA polymerase adds Uracil to the RNA strand, for every Adenine it reads on the DNA template.