RNA polymerase II. Computer model showing the secondary structure of the enzyme RNA polymerase II. The molecule comprises 12 subunits. This enzyme synthesises a complementary mRNA (messenger ribonucleic acid) strand from a strand of DNA (deoxyribonucleic acid) during a process called transcription. It recognises a start sign on the DNA strand and then moves along the strand building the mRNA until it reaches a stop sign. mRNA is the intermediary between DNA and its protein product.