**Question:** The following are "stop codons" EXCEPT

A. UAA
B. UAG
C. UGA
D. UAA

**Correct Answer: D. UAA**

**Core Concept:**

Stop codons are specific triplet nucleotide sequences in the genetic code that signal the end of translation and termination of protein synthesis in cells. In the standard genetic code, there are three stop codons: UAA, UAG, and UGA. These codons are recognized by specific release factors during translation, resulting in the release of the newly synthesized polypeptide chain and termination of protein synthesis.

**Why the Correct Answer is Right:**

UAA (Uridine Antisense Amino Acid) is a stop codon that, when encountered during translation, triggers the release of the polypeptide chain and terminates protein synthesis. This is due to the binding of release factor RF2 to UAA codon, leading to the hydrolysis of the peptide bond and release of the amino acid chain.

**Why Each Wrong Option is Incorrect:**

A. UAG (Uridine Antisense Amino Acid) is also a stop codon, similar to UAA, which means that it is recognized by release factor RF2 and terminates protein synthesis.
B. UGA (Uridine Antisense Glycine) is also a stop codon, recognized by release factor RF1, which leads to the release of methionine in prokaryotic cells. In eukaryotic cells, UGA is a codon for tryptophan. However, it does not contribute to the incorrect answer because all stop codons, regardless of their specific amino acid assignment, ultimately lead to termination of protein synthesis.
C. UAA (Uridine Antisense Amino Acid) is also a stop codon, used to signal the release of the polypeptide chain and terminate protein synthesis.

**Clinical Pearl:**

Understanding the distinction between amino acids and stop codons is crucial for understanding gene expression and protein synthesis processes in cells. While stop codons trigger the termination of protein synthesis, amino acids represent the building blocks that form the primary structure of the polypeptide chain. In eukaryotic cells, UGA is used for tryptophan synthesis, but it is not relevant to the incorrect answer because all stop codons, regardless of their amino acid assignment, lead to protein synthesis termination.

**Why Option D is Right:**

Option D, UAA (Uridine Antisense Amino Acid), is the correct answer because it is a stop codon, which signals the termination of protein synthesis. In prokaryotic cells, UAA is used to release the polypeptide chain, while in eukaryotic cells, it is used for tyrosine synthesis. However, this does not make UAA wrong as all stop codons eventually lead to protein synthesis termination.