The 17-bp transcriptional complex has an 8-bp DNA-RNA hybrid, that is, 8 base-pairs involve the RNA transcript bound to the DNA template strand. As transcription progresses, ribonucleotides are added to the 3′ end of the RNA transcript and the RNAP complex moves along the DNA. The characteristic elongation rates in prokaryotes and eukaryotes are about 10–100 nts/sec.
Aspartyl (asp) residues in the RNAP will hold on to Mg ions, which will, in turn, coordinate the phosphates of the ribonucleotides.  The first Mg will hold on to the α-phosphate of the NTP to be added.  This allows the nucleophilic attack of the 3′-OH from the RNA transcript, adding another NTP to the chain.  The second Mg will hold on to the pyrophosphate of the NTP.  The overall reaction equation is:
:(NMP) + NTP → (NMP) + PP