DNA transcription. Illustration of the process of transcription of deoxyribonucleic acid (DNA, blue) to produce a complementary copy of ribonucleic acid (RNA, green). This is done by the enzyme RNA polymerase (purple). The RNA strand is constructed from fragments of ribonucleoside tri-phosphate (rNTP, lower right). For this artwork with labels, see C029/3252. For more details of the entry and exit channels into the RNA polymerase structure, see image C029/3253.