Curing transcription, uracil would be used in the mRNA instead of thymine.
During transcription, the DNA code is transcribed into RNA, where each nucleotide base in DNA is paired with a complementary base in RNA. In DNA, thymine (T) pairs with adenine (A), while in RNA, uracil (U) pairs with adenine (A). Therefore, during transcription, when the DNA code is being transcribed into RNA, thymine is replaced by uracil in the RNA sequence.
Guanine and cytosine are not affected by this process and pair with each other, forming a complementary base pair in both DNA and RNA.