RNA synthesis takes place within a ‘transcription bubble’ where the DNA is transiently separated into single strands. The unwinded template strand is used to direct the synthesis of the RNA strand. The RNA chain is synthesized from the 5′ → 3′ direction. That is, new nucleotides are added to the 3′-end of the growing chain. The 3′-OH group of the last nucleotide added to the chain reacts with an incoming nucleotide, 5′-PO4, which loses its β- and γ-phosphate groups as pyrophosphate, while the α-phosphate group forms a phosphodiester linkage with the growing RNA chain (Figure 4.4).
CHEMISTRY OF TRANSCRIPTION