Transcription in prokaryotes
In prokaryotic cells there is a single RNA polymerase that catalyzes the synthesis of all RNAs.
The following stages are distinguished in the synthesis of mRNA:
The RNA polymerase changes configuration to recognize and bind to a specific region of DNA called the promoter.
In the promoter there are consensus sequences (TTGACA or TATAAT), located before the starting point, which indicate where the RNA synthesis should comment and which DNA strand has to be transcribed.
Then, the RNA polymerase configuration opens and unwinds one turn of the helix of the DNA molecule, creating a transcription bubble, and allowing the synthesis of RNA from the strand that it uses as a template.
When the RNA polymerase has already been attached, the σ factor is released and can be used by other enzymes to recognize where to attach to another DNA strand.
After the RNA polymerase has unwound one turn of the DNA helix, it will move along the DNA template strand in the 3'→ 5' sense, while adding ribonucleotides to the new mRNA strand in the 5'→3' sense, both being antiparallel chains.
As the RNA polymerase moves along the DNA strand, it recovers its initial double helix configuration.
RNA polymerase selects the ribonucleotide triphosphate whose nitrogenous base is complementary to the deoxyribonucleotide of the template DNA strand, and joins it by means of an ester bond, releasing a pyrophosphate group (PPi). C, G, A and U of RNA pair with G, C, T and A of DNA.
An example of a transcription would be:
DNA sequence: 3 '... ATGCGCTAG ... 5'
mRNA sequence: 5 '... U ACGCGA U C ... 3'
The RNA polymerase continues to add nucleotides until it reaches a region of DNA called termination signal. This area is characterized by having a palindromic sequence (sequences that are read in the same way from left to right as from right to left) with many nucleotides with G and C, followed by several with T. This allows the end of RNA and a loop is generated that causes its separation from DNA. At that point, the RNA polymerase breaks apart and the double helix re-forms in the DNA.
The maturation process is not necessary to synthesize all types of RNA. In prokaryotes, the newly formed mRNA can already be used to synthesize proteins in the translation process. In contrast, the RNA that has been transcribed (primary transcribed RNA) that will give rise to the tRNA and rRNA have to undergo a maturation process before being functional. Fragments are cut and joined until the final RNA is obtained.
Transcription errors also occur (one per 104-105 nucleotides linked) rather than during DNA replication, but are not as important because they are not transmitted to offspring.