Every gene has a single role: to encode RNA via transcription. Transcription is the synthesis of a single strand of RNA – a transcript – whose nucleotide sequence is complementary to a portion of a gene. As molecular processes go, transcription is highly regulated, moderately accurate, and, once started, fast-paced. Transcription is performed by a small troop of large proteins, chief among which is RNA polymerase. RNA polymerase uses the template strand of DNA to synthesize a complementary strand of RNA in a 5′ to 3′ direction.
Transcription is a play in four acts: (1) binding of RNA polymerase to a gene's promoter, (2) initiation of the RNA chain, (3) elongation of the RNA chain, and (4) termination of transcription.
In many cases, RNA synthesis does not end with transcription, because some transcripts are processed into mature RNAs by enzymatic modifications. The next chapter describes posttranscriptional processing.
The key chemical reaction in RNA polymerization is an esterification, in which the α phosphate of ribonucleotide triphosphate (NTP) is added to the 3′ oxygen of the growing polynucleotide chain, yielding a lengthened chain plus diphosphate. The substrates for the reaction are the four NTPs: ATP, GTP, CTP, and UTP. Synthesis of an RNA molecule begins at its 5′ end, proceeds by adding one nucleotide at a time, and stops at its 3′-OH end. RNA polymerase catalyzes the polymerization reaction (Figure 7.1).