Final answer:
The critical length of the polyA tail is approximately 200 adenine residues. This string of nucleotides is added to the 3' end of pre-mRNA after elongation, playing a key role in protecting the mRNA from degradation and aiding in its export to the cytoplasm.
Step-by-step explanation:
How Long is the PolyA Tail?
The critical length of the polyA tail on eukaryotic pre-mRNA molecules is approximately 200 adenine (A) residues. This polyadenylation process is completed after pre-mRNA elongation. An enzyme called poly-A polymerase adds these adenine nucleotides at the 3' end of the pre-mRNA. The AAUAAA consensus sequence marks where cleavage occurs, and then the poly-A tail is added, which serves to protect the pre-mRNA from degradation and assists in the mRNA's export from the nucleus to the cytoplasm.
The addition of the poly-A tail is an important post-transcriptional modification that also plays a role in the stability and half-life of the mRNA in the cytoplasm. The poly-A tail's length can affect the efficiency of translation initiation and the mRNA's overall translation process. Its gradual shortening during successive rounds of translation can influence the mRNA's longevity within the cell.