DNA transcription. Illustration of the process of transcription of DNA (deoxyribonucleic acid, blue) to produce a complementary copy of RNA (ribonucleic acid, green). This is done by the enzyme RNA polymerase (grey). The RNA strand is constructed from fragments of ribonucleoside tri-phosphate (rNTP, T-shaped, lower right). This illustration shows the entry and exit channels into the RNA polymerase structure. There are entry (right) and exit (centre left) channels for the DNA, an RNA exit channel (lower left), and an rNTP entry channel (lower right).