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Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for DNA; therefore, transcription has a lower copying fidelity than DNA Replication .

Like DNA Replication , transcription proceeds in the 5' → 3' direction (ie the old polymer is read in the 3' → 5' direction and the new, complementary fragments are generated in the 5' → 3' direction). Transcription is divided into 3 stages: ''initiation'', ''elongation'' and ''termination''.


PROKARYOTIC TRANSCRIPTION




Initiation

The following steps occur, in order, for transcription initiation:
  • RNA polymerase ( RNAP ) recognizes and specifically binds to the Promoter region on DNA. At this stage, the DNA is double-stranded ("closed"). This RNAP/wound-DNA structure is referred to as the ''closed complex''.

  • The DNA is unwound and becomes single-stranded ("open") in the vicinity of the initiation site (defined as +1). This RNAP/unwound-DNA structure is called the ''open complex''.

  • The RNA polymerase transcribes the DNA, but produces about 10 abortive (short, non-productive) transcripts which are unable to leave the RNA polymerase because the exit channel is blocked by the σ-factor .

  • The σ-factor eventually dissociates from the Holoenzyme , and elongation proceeds.


Promoters can differ in "strength"; that is, how actively they promote transcription of their adjacent DNA sequence. Promoter strength is in many (but not all) cases, a matter of how tightly RNA polymerase and its associated accessory proteins bind to their respective DNA sequences. The more similar the sequences are to a Consensus Sequence , the stronger the binding is.

Most transcripts originate using adenosine-5'-triphosphate ( ATP ) and, to a lesser extent, guanosine-5'-triphosphate ( GTP ) ( Purine nucleoside triphosphates) at the +1 site. Uridine-5'-triphosphate ( UTP ) and cytidine-5'-triphosphate (CTP) ( Pyrimidine nucleoside triphosphates) are disfavoured at the initiation site.


Termination

Two termination mechanisms are well known:
  • Intrinsic termination (also called '''Rho-independent termination''') involves terminator sequences within the RNA that signal the RNA polymerase to stop. The terminator sequence is usually a Palindromic sequence that forms a stem-loop Hairpin structure that leads to the dissociation of the RNAP from the DNA template.

  • Rho-dependent termination uses a Termination Factor called ρ Factor to stop RNA synthesis at specific sites. This protein binds and runs along the mRNA towards the RNAP. When ρ-factor reaches the RNAP, it causes RNAP to dissociate from the DNA, terminating transcription.


Other termination mechanisms include where RNAP comes across a region with repetitious thymidine residues in the DNA template.
or where a GC-rich inverted repeat followed by 4 A residues. the inverted repeat forms a stable stem loop structure in the Rna, which causes the RNA to dissociate from the DNA template.