RNA polymerase binds to the DNA at the start of a gene * Hydrogen bonds between base pairs break, causing DNA to unwind and strands to separate * One strand acts as the template (antisense strand) * Complementary RNA nucleotides bind by base pairing (A–U, C–G) * RNA polymerase joins nucleotides together forming phosphodiester bonds → mRNA strand forms * mRNA detaches and leaves the nucleus through nuclear pores