During post-transcriptional modification in eukaryotes, what is added to the mRNA?

During post-transcriptional modification in eukaryotes, the addition of a 5' cap is a crucial step that enhances the stability and translation of mRNA. The 5' cap consists of a modified guanine nucleotide that is added to the 5' end of the newly synthesized mRNA transcript. This modification plays several important roles: it protects the RNA from degradation by exonucleases, facilitates the binding of ribosomes for the initiation of translation, and is involved in the nuclear export of the mRNA.

The choice of the 5' cap aligns with the known mechanisms of eukaryotic mRNA maturation, as it is an essential feature that distinguishes eukaryotic mRNA from prokaryotic mRNA, where a similar structure is not present. In contrast, the addition of a poly-A tail at the 3' end is also part of post-transcriptional modification, but it does not address the 5' end of the mRNA, which is specifically what the question focuses on. The Shine-Dalgarno sequence is a ribosomal binding site found in prokaryotic mRNA, and introns are non-coding regions that are removed from the mRNA during splicing, rather than