Final answer:
The process that is not a post-transcriptional modification of eukaryotic mRNAs is the removal of exons, as the actual modification involves the removal of introns, while exons are retained in the mature mRNA.
Step-by-step explanation:
The question is asking which of the listed processes is not a post-transcriptional modification that occurs in eukaryotic mRNAs.
The correct answer is the removal of exons. During the processing of eukaryotic pre-mRNA, several modifications occur.
5' capping involves the addition of a 7-methylguanosine cap to the 5' end, which protects the mRNA from degradation and aids in the initiation of translation.
The 3' Poly-A Tail addition occurs at the other end, where a string of adenine nucleotides is added to protect the mRNA and signal its readiness for export from the nucleus.
Pre-mRNA splicing is another post-transcriptional modification where introns are removed and exons are reconnected.
Exons are the sequences that are expressed and remain in the mature mRNA, whereas introns are the non-coding sequences removed during splicing.
Thus, the removal of exons does not occur as a post-transcriptional modification, as it is the introns that are removed.