Question

Describe post-transcriptional processing of RNA in eukaryotes.

Answer 1

In the eukaryotic nucleus, there are at least three distinct RNA polymerases, besides those found in organelles. RNA polymerase I transcribes rRNAs (28s, 18s, and 5.8s), while RNA polymerase III is responsible for the transcription of tRNA, 5s rRNA, and snRNAs. The precursor of mRNA, called heterogeneous nuclear RNA (hnRNA), is transcribed by RNA polymerase II.

Since eukaryotic primary transcripts include both exons and introns, they must undergo a process called splicing, where the introns are removed, and the exons are linked in a specific sequence. Additionally, hnRNA undergoes capping and tailing. During capping, an unusual nucleotide, methyl guanosine triphosphate, is added to the 5’-end of the hnRNA. In tailing, approximately 200-300 adenylate residues are attached to the 3’-end. Once fully processed, the hnRNA becomes mature mRNA and is transported out of the nucleus for translation.