Elongation:

After transcription initiation the issue is released from the transcriptional complex  to leave the core enzyme α2 ββ that continues elongation of the RNA transcript. Therefore the core enzyme holds the catalytic site for polymerization possibly within the subunit. In the first nucleotide of the RNA transcript is usually pppA or pppG. RNA polymerase then synthesizes RNA in the 5' → 3' direction, by using the four ribonucleoside 5 -triphosphates that are ATP, CTP, GTP, and UTP as precursors. 3' -OH  at  the end  of the growing RNA chain attacks the phosphate set of the incoming ribonucleoside 5 -triphosphate to form a 3 5 phosphodiester  bond that is shown in the figure. The complex of RNA polymerase DNA sample and new RNA transcript is known a ternary complex example for three components and the region of unwound DNA which is undergoing transcription is known the transcription  bubble in the figure.  The RNA transcript forms a transient RNA-DNA hybrid helix with its template strand but then peels away from the DNA as transcription proceeds. DNA is the unwound ahead of the transcription bubble and after the transcription complex has passed the DNA will rewinds.

Figure:  Transcription by RNA polymerase.

In every step the incoming ribonucleotide selected is that which can base pair with the next base of the DNA template strand. In the diagram the incoming nucleotide is rUTP to base pair with the A residue of the sample of DNA. The 3′5′ phosphodiester bond is created, extending the RNA chain through one nucleotide and pyrophosphate is released. Whole the RNA molecule grows in a 5′ to 3′ direction.

Figure: A transcription bubble. The DNA double helix is unwound and RNA polymerase then synthesizes an RNA copy of the DNA template strand. The nascent RNA transiently forms an RNA-DNA hybrid helix but then peels away from the DNA which is subsequently rewound into a helix once more.