Post-transcriptional Processing of RNAs

Post-transcriptional processing is the modification of a primary RNA transcript after transcription and before translation or final functional use. In eukaryotes, the first transcript formed is often called 'hnRNA' or 'primary transcript', and it is not fully ready for protein synthesis.

hnRNA stands for heterogeneous nuclear RNA, which is the freshly synthesised RNA formed in the nucleus of eukaryotic cells before maturation. It contains both coding and non-coding regions and therefore needs further processing.

In prokaryotes, the newly formed mRNA is mature enough to carry out translation immediately. This is because the coding region is smooth and there is no school-level description of introns and exons interrupting the sequence.

1. Transcription begins and DNA forms mRNA.
2. The mRNA has a continuous coding region and is already sufficiently mature.
3. No further major processing is required before translation.
4. Since there is no nuclear membrane, transcription and translation occur in the same region.
5. The mRNA can immediately associate with ribosomes and take part in protein synthesis.

A fully processed mRNA of prokaryotes

Mechanism of mRNA processing in eukaryotes

A fully processed (matured) mRNA of eukaryotes

In eukaryotes, the newly formed mRNA is not mature and cannot carry out translation immediately. It needs extra stability and must be processed in the nucleus before moving to the cytoplasm.

• The eukaryotic primary transcript contains many exons and introns, so it is heterogeneous and biologically inactive. Therefore, it requires processing and stabilisation to become mature mRNA.
• In eukaryotic cells, mRNA processing occurs in a biological sequence while transcription is going on and after it nears completion.

Step 1: Formation of Primary Transcript

• DNA is transcribed to form hnRNA or primary transcript.
• This transcript contains both exons and introns and is not ready for translation.

Step 2: 5′ Capping

• A cap of methylated guanine (G-cap) is added at the 5′ end of the growing RNA transcript.
• This is the earliest major processing event and helps provide stability to the RNA.

Step 3: Splicing

• During splicing, introns are removed and exons are joined together.
• This shortens the unusual length of hnRNA and makes it more effective and stable.
• Splicing is carried out by spliceosomes, which recognise the beginning and end of introns, cut them out, and rejoin the exon segments.

Step 4: 3′ Tailing / Polyadenylation

• At the 3′ end, about 200–300 adenylate residues are added.
• This forms the poly A tail, and the process is called tailing or polyadenylation.

Step 5: Formation of Mature mRNA

• After capping, splicing, and tailing are completed, the transcript becomes a fully processed functional mRNA.
• The mature mRNA has a defined 5′ G-cap and a defined 3′ poly A tail.

Step 6: Transport and Translation

The mature mRNA is transported from the nucleus to the cytoplasm, where it can participate in translation.

• Primary transcripts or hnRNA in eukaryotes are biologically inactive and contain both exons and introns, so they need post-transcriptional processing.
• In prokaryotes, the newly formed mRNA has a continuous coding region and is mature enough to undergo translation immediately in the same region where it is transcribed.
• Eukaryotic genes are split genes; their coding exons are interrupted by non-coding introns, making the initial hnRNA heterogeneous and unstable.
• Eukaryotic hnRNA is processed by 5′ capping, splicing (removal of introns and joining of exons by spliceosomes), and 3′ tailing (polyadenylation) to form stable mature mRNA with a 5′ G-cap and a 3′ poly A tail.
• Mature eukaryotic mRNA, after these processing steps, is transported from the nucleus to the cytoplasm, where it participates in translation and protein synthesis.