Final answer:
A poly-A sequence is added at the 3' end of mRNA. It is added by the enzyme poly-A polymerase following transcription and is crucial for mRNA stability and function.
Step-by-step explanation:
A poly-A sequence is added at the 3' end of mRNA. This crucial process in the maturation of eukaryotic pre-mRNA occurs after the elongation phase of transcription. Once the newly synthesized pre-mRNA is complete, it undergoes several modifications before becoming a fully functional mRNA. An enzyme known as poly-A polymerase initiates the polyadenylation process by recognizing a sequence near the 3' end of the mRNA. It then adds approximately 200 adenine (A) residues to form what is known as the poly-A tail. This polyadenylation provides protection against enzymatic degradation and also plays a role in the export of mRNA to the cytoplasm and in subsequent translation into protein.
The 5' untranslated region (UTR) of mRNA receives a different modification called 5' capping, where a 7-methylguanosine cap is added to protect the mRNA and aid in the initiation of translation by ribosomes. However, the specific addition referred to in the question is the 3' poly-A tail, not the 5' cap. The addition of the poly-A tail is an indicator that the transcription process is complete for that mRNA molecule and it is ready for subsequent processing steps before it can be translated into a protein.