**Question:** Poly (A) tail translates into

A. 5' cap
B. Introns
C. Stop codons
D. Uracil

**Correct Answer:** C. Stop codons

**Core Concept:**
Poly (A) tail is a long chain of adenine nucleotides (A) at the 3' end of a messenger RNA (mRNA) molecule. In eukaryotic cells, this tail plays a crucial role in mRNA stability, localization, and translation efficiency.

**Why the Correct Answer is Right:**
The correct answer, "Stop codons," refers to the specific mRNA codons that signal the ribosome to terminate protein synthesis. These codons (UAA, UAG, and UGA) are located in the stop codons region of the mRNA sequence, and the presence of a poly(A) tail facilitates the recognition of these stop codons by ribosomes during translation.

**Why Each Wrong Option is Incorrect:**
A. 5' cap is incorrect because the 5' cap is a structure added to the mRNA molecule at the 5' end, not the 3' end where the poly(A) tail is located.
B. Introns are non-coding regions within a gene that are spliced out during mRNA processing, not translated by ribosomes.
D. Uracil is a nucleobase found in DNA, not mRNA, and is not related to the function of the poly(A) tail.

**Clinical Pearl:**
Understanding the role of poly(A) tail in mRNA processing is crucial for proper translation of proteins in eukaryotic cells. Adequate poly(A) tail ensures efficient translation by facilitating ribosome recognition of stop codons and promoting mRNA stability and localization. This concept is particularly important for medical students studying gene expression, protein synthesis, and cellular processes involved in protein synthesis.