What you already know:
There is two strains of DNA in most mammals including humans. There is a templete strain and a complementary strain. The base letters of DNA is ACGT. Because of the chemical make up of these molecules each base can only pair with one other molecule. A pairs with T, and C pairs with G.
When we translate to mRNA the base T is replaced with U. The pairing is now A&U and C&G
