DNA is transcribed by the enzyme RNA polymerase. The RNA polymerase (pale blue) moves stepwise
along the DNA, unwinding the DNA helix in front of it. As it progresses, the polymerase adds nucleotides (small "T" shapes) one by one to the RNA chain at the polymerisation site. The polymerase rewinds the two DNA strands behind this site to displace the newly formed RNA. A short region of DNA/RNA helix is therefore formed only transiently, and the RNA transcript is a single-stranded complementary copy of one of the two DNA strands. The incoming nucleotides are in the form of ribonucleoside triphosphates (ATP, UTP, CTP, and GTP), whose hydrolysis provides the energy for the polymerisation reaction.