The nucleus uses the nucleotide sequence of DNA as a template to make mRNA. The enzyme RNA polymerase II catalyzes this activity, which requires nucleotide triphosphates as substrates. Transcription is the process of producing mRNA from DNA, and it takes place in the nucleus. The DNA of a gene serves as a template for complementary base-pairing during transcription, and an enzyme known as RNA polymerase II catalyzes the synthesis of a pre-mRNA molecule, which is then processed to generate mature mRNA.