Conceptually, the process of transcription is straightforward. As a region of the DNA duplex unwinds, one strand is used as a template for the synthesis of an RNA transcript that is complementary in sequence to the template according to the base-pairing rules, except that the transcript contains U (uracil) where the template has an A (Fig. 3.14). The transcript is produced by polymerization of ribonucleoside triphosphates. The enzyme that carries out the polymerization is known as RNA polymerase, which acts by adding successive nucleotides to the 3′ end of the growing transcript (Fig. 3.14). Only the template strand of DNA is transcribed. Its partner, called the nontemplate strand, is not transcribed.

It is important to keep in mind the direction of growth of the RNA transcript and the direction that the DNA template is read. All nucleic acids are synthesized by addition of nucleotides to the 3′ end. That is, they grow in a 5′-to-3′ direction, also described simply as the 3′ direction. Just like two strands of DNA in a double helix, the DNA template and the RNA strand transcribed from it are antiparallel, meaning the DNA template runs in the opposite direction from the RNA (Fig. 3.14).

Quick Check 2 A segment of one strand of a double-stranded DNA molecule has the sequence 5′-ACTTTCAGCGAT-3′. What is the sequence of an RNA molecule synthesized from this DNA template?