Fig. 12.14.

RNA transcription by T7 DNA-dependent RNA polymerase. (a) Initiation of RNA synthesis. The T7 RNA polymerase binds at the promoter site, separates the dsDNA genome, and initiates RNA transcription. The polymerase synthesizes short three- to eight-nucleotide long RNA oligomers de novo that are released from the polymerase–promoter complex. Unlike the DNA replication seen in Fig. 12.11, only a small portion of dsDNA is separated to form a DNA–RNA hybrid (transcription bubble). Two DNA strands reanneal around the transcription bubble as the polymerase moves along the template. (b) RNA elongation. Once the RNA chain reaches about nine nucleotides, the polymerase elongates the RNA transcript with high processivity. (c) Termination. Once the polymerase encounters a termination signal, the polymerase and the RNA transcript are released from the nucleic acid