Process of transcription
In prokaryotes
Transcription takes place in three steps
- Initiation
- RNA polymerase binds to promoter and initiates transcription.
- Initiation factor or sigma (σ) recognizes the promoter of the DNA.
- Elongation
- RNA polymerase facilitates opening of the helix and continues elongation.
- RNA polymerase uses nucleoside triphosphates as substrate and polymerizes in a template depended fashion following the rule of complementarity.
- Only a short stretch of RNA remains bound to the enzyme.
- Termination
- Once the polymerases reaches the terminator region RNA polymerase binds with the termination-factor (ρ) to terminate transcription.
- The nascent RNA falls off with the RNA polymerase which results in termination of transcription.
- The transcription and translation can be coupled in bacteria as the mRNA does not require any processing to become active, and also transcription and translation take place in the same compartment
In eukaryotes
- There are two additional complexities in eukaryotes.
- The first complexity is that there are at least three RNA polymerases in the nucleus.
- The RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S)
- The RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs (small nuclear RNAs).
- The RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA).
- The second complexity is that the primary transcripts contain both the exons and the introns and are non-functional.
- Primary transcripts are subjected to a process called splicing where the introns are removed and exons are joined in a defined order.
- hnRNA undergo two additional processing called as capping and tailing.
- In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of hnRNA.
- In tailing, adenylate residues (200-300) are added at 3'-end in a template independent manner and the fully processed hnRNA is called mRNA
- mRNA is transported out of the nucleus for translation.
Significance of complexities
- The split-gene arrangements represent probably an ancient feature of the genome.
- The presence of introns is reminiscent of antiquity, and the process of splicing represents the dominance of RNA-world.
Transcription
