**Core Concept**
The poly-A tail is a sequence of adenine (A) bases added to the 3' end of eukaryotic messenger RNA (mRNA) during post-transcriptional modification. This structure plays a critical role in mRNA stability, nuclear export, and translational efficiency.

**Why the Correct Answer is Right**
In eukaryotic mRNA, a poly-A tail—composed of 150–250 adenine nucleotides—is added to the 3' end after RNA polymerase II transcribes the gene. This process occurs during mRNA processing in the nucleus and is catalyzed by the poly-A polymerase enzyme. The poly-A tail protects the mRNA from degradation by exonucleases and enhances its export from the nucleus to the cytoplasm, where translation occurs. Prokaryotic mRNAs lack this feature entirely due to the absence of nuclear envelope and complex RNA processing.

**Why Each Wrong Option is Incorrect**
Option A: The 5' end of prokaryotic mRNA contains a Shine-Dalgarno sequence for ribosome binding, not a poly-A tail.
Option B: Prokaryotic mRNA does not have a poly-A tail at the 3' end; it is typically degraded rapidly without such a structure.
Option C: The 5' end of eukaryotic mRNA contains the cap structure (m7GpppG), not a poly-A tail.

**Clinical Pearl / High-Yield Fact**
The poly-A tail is essential for mRNA stability and is often used in molecular diagnostics; its shortening in diseases like cancer correlates with mRNA instability and poor prognosis.

✓ Correct Answer: D. 3' end of a eukaryotic mRNA