Illustration of the process of transcription of DNA (deoxyribonucleic acid, double helix, top right) to produce a complementary copy of RNA (ribonucleic acid, blue, upper centre and left). The enzyme RNA polymerase (blue) recognises a start sign on the DNA strand and then moves along the strand building the mRNA until it reaches a termination signal. mRNA is the intermediary between DNA and its protein product. Within the cell nucleus DNA is coiled around histone proteins (white), which the DNA has to temporarily unwrap from during transcription.