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The following points highlight the four main stages involved in synthesis of RNA. The stages are: 1. Binding of RNA Polymerase to DNA 2. Initiation of Transcription 3. Elongation of RNA chain 4. Termination of Transcription.
Stage # 1. Binding of RNA polymerase to DNA:
RNA polymerase binds to the “promoter site” on the DNA molecule. The promoter site is composed of specific base sequences which is generally located upstream (on the 3′ site of the DNA strand being transcribed) of the start site, but in some cases it may be situated downstream. The start site is the point from which transcription is begun by RNA polymerase.
The promoters have a sequence of 6-8 base pairs, popularly known as TATA Box or Pribnow box the consensus sequence is “TATAAT”. Such sequences have been identified about 20-30 bp upstream from start sites of transcription in both prokaryotes and eukaryotes. Another sequence “TTGACA” also occurs about 35 bases upstream and is called the recognition region.
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In eukaryotes, RNA polymerase II binds directly to the DNA promoter sequence located upstream from transcription initiation site (Fig. 3.23). However, the promoter sequence for transcription of 5SrRNA lies in the middle of the 5S gene. A particular protein (non-histone protein) called “5S transcription factor” binds to the promoter sequence that is situated downstream of the transcription start site.
The RNA polymerase III interacts with this specific protein and begins the transcription of the 5S rRNA genes from the start site.
Stage # 2. Initiation of RNA Synthesis:
Transcription begins about 20-30 bases downstream from the TATA box. A deletion of the region between the TATA box and the transcription initiation site results into a shift of transcription initiation about 30 bases downstream form the TATA box. RNA synthesis always starts with a purine, i.e., ATP or GTP is the first nucleotide triphosphate of the RNA.
Stage # 3. Elongation of the RNA Chain:
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After the selection of the first base (A or G), RNA synthesis, complementary to the DNA strand being transcribed, continues in 5’—>3′ direction. During this process, DNA double helix progressively unwinds. When the RNA polymerase moves out of the promoter region making this site free, another RNA polymerase molecule may attach to this region and initiates transcription.
Stage # 4. Termination of RNA Synthesis:
There are certain termination signals in the DNA; these are called terminators. In E. coli, termination of transcription may or may not depend on the “rho (p)” factor. In both the cases, a hairpin structure is formed by pairing within the RNA chain being synthesized. This pairing occurs due to the presence of short complementary sequences in the form of palindromes, e.g., 
near the termination site- When transcribed, this palindromic sequence will yield the following sequence in the RNA transcript: GGUACC. This sequence could fold back on itself to form a hairpin structure as follows 
hairpin structure in the RNA transcript causes the RNA polymerase to slow down in transcription.
It is suggested that “rho (p)” factor binds to the RNA transcript at the 5′ end and moves along it; it interacts with the RNA polymerase when it catches up with the latter. Following this the RNA molecule is released and the transcription is terminated. In case of “rho (p)” independent termination, there is a poly (A) sequence on the DNA which is transcribed as a poly (U) sequence in the RNA transcript.
This run of Us acts as transcription termination signal.
