The three post-transcriptional modifications are splicing, capping and tailing.
Transcription is the formation of RNA from DNA. The genetic information present in the DNA is copied to RNA, which further codes for proteins. In transcription, only a segment of DNA is copied unlike replication, where total DNA gets duplicated.
Only one strand of DNA acts as a template and copied during transcription. Transcription is catalysed by a DNA-dependent RNA polymerase in the 5′ to 3′ direction.
In prokaryotes, only one type of RNA polymerase catalyses the transcription of all types of RNA. Prokaryotic mRNA does not require to be processed to be functional and synthesise a protein. Transcription and translation, both occur in the cytosol and can occur simultaneously in bacteria. Translation can even start before the mRNA is fully transcribed.
In eukaryotes, the primary RNA transcript undergoes structural and chemical changes after transcription to become a mature and functional RNA, which is then transported out of the nucleus to perform various functions. These modifications are called post-transcriptional modifications.
RNA polymerases II produces hnRNA (heterogeneous nuclear RNA), which is the precursor of mRNA. hnRNA is non-functional and undergoes post-transcriptional modifications to produce mature mRNA.
The three main post-transcriptional modifications are:
- Splicing -In eukaryotes, the structural gene has coding and non-coding regions. Introns are removed and coding regions or exons join together to form mature mRNA.
- Capping – 5′-end of hnRNA is capped by methyl guanosine triphosphate.
- Tailing – 3′-end of hnRNA undergoes tailing by addition of 200-300 adenylate residues to form poly-A tail.
The mature mRNA produced after the processing is transported to the cytoplasm for protein synthesis. Post-transcriptional modifications are an important point at which gene expression is regulated.
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