Final answer:
Eukaryotic mRNA's typically have a poly(A) tail, which consists of a string of adenine nucleotides added at the 3' end by the enzyme poly(A) polymerase, providing several protective and regulatory benefits.
Step-by-step explanation:
Most eukaryotic mRNA's have a string of adenine nucleotides at the 3' end. This modification is known as a 3' poly(A) tail, which is added post-transcriptionally to the pre-mRNA. The enzyme responsible for this addition is poly(A) polymerase, which recognizes a specific sequence near the 3' end on the pre-mRNA and proceeds to add approximately 20-25 adenylate residues. This tail, along with the 5' cap, plays a crucial role in protecting the mRNA from degradation, aiding in its export from the nucleus, and ensuring efficient translation.