6.2.5—Post-transcriptional modification

In a eukaryotic cell, the first RNA made from a gene is a primary transcript. It contains both the gene’s coding exons and non-coding introns, so it must be processed before it becomes mature mRNA ready to leave the nucleus.

1. Transcribe the whole gene region: Transcription produces a primary RNA transcript containing the exon and intron sequences that were present in the transcribed gene region.
2. Identify the sections: Exons are the coding sequences retained for the message; introns are non-coding sequences that are not to be translated into the polypeptide.
3. Remove introns: The intron sections are cut out of the primary transcript. This processing changes the RNA molecule, not the DNA gene itself.
4. Join exons: The remaining exon sections are joined to form one continuous mature mRNA molecule. This joining step is splicing.
5. Export the message: The mature mRNA leaves the nucleus through a nuclear pore and becomes available for the later translation stage. Translation is the next use of the processed message, not part of this card’s processing sequence.

Processing converts a mixed primary transcript into a continuous message: primary transcript (exons + introns) → introns removed → exons joined → mature mRNA → export from the nucleus. Without intron removal and exon joining, the RNA would not present the intended continuous coding sequence for later use.

Introns are removed from the RNA transcript, not deleted from the DNA template, and mature mRNA is not a protein. The primary transcript and mature mRNA are different RNA forms; the detailed codon/anticodon and peptide-bond process belongs to card 4578, not this post-transcriptional processing card.