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In this article we will discuss about the process of transcription in prokaryotes and eukaryotes.
Transcription in Prokaryotes:
DNA dependent RNA polymerase is the single enzyme that catalyses the transcription of all types of bacterial RNA.
(i) Initiation:
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RNA polymerase binds to promoter and initiates transcription. It associates transiently with ‘initiation factor’ (σ) and uses nucleoside triphosphates as substrate, obeying the rule of complementarity and polymerises in a template depended fashion. It also facilitates the opening of the helix and continues elongation.
(ii) Elongation:
Chain elongation proceeds in the 5′ → 3′ direction, and the transcription bubble travels with the RNA polymerase. The RNA polymerase after imitation of RNA loses the σ factor but continues the polymerisation of ribonucleotides to form RNA.
(iii) Termination:
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When the RNA polymerase reaches the terminator region of DNA, the RNA polymerase is separated from DNA-RNA hybrid, it associates transiently with the termination factor (ρ). The nascent RNA separate and the RNA polymerase fall off resulting in termination of transcription.
In prokaryotes, mRNA does not require any processing, so both transcription and translation takes place in cytosol (as there is no separation of nucleus and cytosol in bacteria), Therefore, translation can start much before the mRNA is fully transcribed, i.e., transcription and translation can be coupled.
Transcription in Eukaryotes:
The process of transcription in eukaryotes is similar to that in prokaryotes. Structural genes are monocistronic in eukaryotes.
Two additional complexities are present in eukaryotes:
(i) In the nucleus,, there are at least three RNA polymerases.
(a) RNA Polymerase I Transcribes rRNAs (28 S, 18 S, 5.8 S).
(b) RNA Polymerase II Transcribes precursor of mRNA, which is called heterogeneous nuclear RNA (hnRNA).
(c) RNA Polymerase III Transcribes tRNA, 5 srRNA and (small nuclear RNAs) snRNAs.
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(ii) The primary transcript contains both the exons and introns and are non-functional. Therefore, they undergo a process called splicing to remove the introns and to join the exons in the proper order. The process of splicing represents the dominance of RNA world.
