Final answer:
The initial step in mRNA degradation is the addition of a 5' cap to the pre-mRNA, which protects it from degradation, assists in its nuclear export, and facilitates protein synthesis initiation.
Step-by-step explanation:
The first step in the degradation of most mRNAs begins while the mRNA is still a primary transcript, known as pre-mRNA. This step involves the addition of a special nucleotide cap, known as the 5' cap, to the 5' end of the growing transcript. The 5' cap serves multiple functions: it protects the mRNA from degradation, it is a signal for exporting the mRNA to the cytoplasm, and it is recognized by translational machinery to initiate protein synthesis.
After the completion of transcription, another important modification occurs at the 3' end, which is the addition of a poly-A tail. This series of adenine nucleotides further enhances the stability of mRNA and contributes to the regulation of its export and translation. Together, the 5' capping and 3' polyadenylation are crucial modifications that precede mRNA degradation and are essential for mRNA's proper function and longevity in eukaryotic cells.