Stop codon
**Core Concept**
A stop codon is a sequence of three nucleotides (codon) in messenger RNA (mRNA) that signals the termination of protein synthesis. The three stop codons are UAA, UAG, and UGA, which are recognized by release factors that lead to the disassembly of the ribosome and the release of the completed protein.
**Why the Correct Answer is Right**
During translation, the ribosome reads the sequence of codons on the mRNA in a 5' to 3' direction. When a stop codon is encountered, the ribosome recognizes it and recruits a release factor that hydrolyzes the peptide bond between the last amino acid and the growing polypeptide chain. This leads to the release of the completed protein and the disassembly of the ribosome.
**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because it is not a stop codon; it is a start codon that initiates translation.
* **Option B:** Incorrect because it is a sense codon that codes for a specific amino acid, not a stop signal.
* **Option C:** Incorrect because it is a nonsense mutation that can lead to premature termination of translation, but it is not a normal stop codon.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that stop codons are recognized by release factors, which are distinct from elongation factors that promote protein synthesis. Understanding the role of stop codons is crucial for diagnosing and managing genetic disorders caused by premature stop codons.
**Correct Answer: C. UAG**