During eukaryotic RNA processing, what must be added to the 5' end of the pre-mRNA?

During eukaryotic RNA processing, the addition of a 5' cap to the pre-mRNA is an essential step that occurs shortly after transcription begins. This 5' cap, which consists of a modified guanine nucleotide, serves several important functions. It protects the mRNA from degradation by exonucleases, facilitates the export of mRNA from the nucleus to the cytoplasm, and is crucial for the initiation of translation.

Additionally, the cap structure helps ribosomes recognize the mRNA for protein synthesis, ensuring that translation commences properly. The presence of the 5' cap is vital for the stability and functionality of the mRNA molecule in eukaryotic cells.

The other options, while relevant to RNA processing, do not pertain to the addition of modifications at the 5' end specifically. A poly(A) tail, for instance, is added to the 3' end, and introns are non-coding sequences that are removed from the pre-mRNA during splicing. The 3' UTR is located at the opposite end of the mRNA molecule and plays a role in regulation and stability, but does not pertain to the modification at the 5' end.