Final answer:
In transcription, a T on the DNA template strand pairs with a U in the mRNA, and an A on the template pairs with a U in the mRNA. RNA polymerase synthesizes mRNA by following base pairing rules where the mRNA is complementary to the DNA template strand.
Step-by-step explanation:
During transcription, a T on the template DNA strand is paired with a U (uracil) in the mRNA, and an A on the template DNA strand is paired with a U (uracil) in the mRNA. The process of transcription involves RNA polymerase reading the template DNA strand and synthesizing the mRNA strand by pairing the complementary nucleotides. This means that where the DNA template has an adenine (A), the mRNA will have a uracil (U), and where the DNA has a thymine (T), the mRNA will have an adenine (A). The mRNA molecule is a complementary copy of the DNA template strand, and this strand is also referred to as the antisense strand. The other DNA strand, which has the same nucleotide sequence as the mRNA except for the substitution of uracil (U) for thymine (T), is the coding or sense strand.