Chapter 3. Nucleic Acids and Transcription

The RNA polymerase complex is a molecular machine that opens, transcribes, and closes duplex DNA.

Transcription does not take place spontaneously. It requires template DNA, a supply of ribonucleoside triphosphates, and RNA polymerase, a large multiprotein complex in which transcription occurs. To illustrate RNA polymerase in action, we consider here bacterial RNA polymerase because of its relative simplicity.

The transcription bubble forms and transcription takes place inside the polymerase (Fig. 3.19). RNA polymerase contains structural features that separate the DNA strands, allow an RNA–DNA duplex to form, elongate the transcript nucleotide by nucleotide, release the finished transcript, and restore the original DNA double helix. Fig. 3.19 shows how structure and function come together in the bacterial RNA polymerase.

FIG. 3.19 The RNA polymerase complex in prokaryotes. This molecular machine has channels for DNA input and output, nucleotide input, and RNA output, and features that disrupt the DNA double helix, stabilize the RNA–DNA duplex, and allow the DNA double helix to re-form.

RNA polymerase is a remarkable molecular machine capable of adding thousands of nucleotides to a transcript before dissociating from the template. It is also very accurate, with only about 1 incorrect nucleotide incorporated per 10,000 nucleotides.