To fully understand transcription, students need to appreciate the dynamic nature of this process. Transcription "works" because nucleotides diffuse into position from the cytoplasm, the DNA helix unwinds and then rewinds, and the RNA polymerase travels rapidly along a DNA template strand for hundreds or thousands of bases to produce a very long RNA molecule in its wake. This simple, 40-s animation is presented at three levels of magnification to emphasize the movements of all the players.

At the beginning of the animation, an RNA polymerase (RNAP) is shown binding to the promoter region of a double helix of DNA. The sigma subunit of RNAP helps to locate the promoter, after which it is no longer needed.

Then, as we look "inside" the RNAP, we can see that the double helix begins to unwind. A growing strand of RNA (in red) begins to form along one of the DNA strands. As the strand lengthens, the RNAP continues to travel along the DNA, unwinding it to expose more DNA bases. Behind the RNAP the DNA helix reforms, displacing the RNA strand.

Finally, we zoom in further to the active site of the RNAP molecule. Here, we can see how an RNA nucleotide triphosphate (in this case, an "A," for a ribose triphosphate with an adenine base) diffuses into position and binds to its complement —a "T" on the DNA strand (note that the blue deoxyriboses on the DNA strand have the opposite orientation to the red ribose residues on the the RNA strand). Once hydrogen bonding has positioned the "A" and the "T" together, the RNAP is able to catalyze covalent bonding of the "A" to the 3’-OH on the growing RNA strand. This bond formation is powered by the cleavage of the diphosphate from the RNA nucleotide. The process is quickly repeated as another RNA nucleotide triphosphate, "C," is added to the RNA chain. Moments later, a DNA nucleotide—a "T"—diffuses into position. While this nucleotide can hydrogen bond with the "A" on the DNA template strand, the RNAP discriminates between RNA and DNA nucleotides, and no reaction is catalyzed. Ultimately, the deoxyribonucleotide diffuses away, allowing RNA strand synthesis to continue when another ribonucleotide triphosphate—a "U"—hydrogen bonds with the A on the DNA template strand.