Which of the following codon do not specify arginine ?

Question

Accepted Answer

The correct option is 4:

Understanding which codons specify which amino acids is a fundamental concept based on the universal genetic code. The question asks to identify the codon among the given choices that does *not* code for the amino acid Arginine. The correct option is AGU because, according to the standard genetic code table, the codon AGU specifies the amino acid **Serine (Ser)**. The genetic code is a triplet code, meaning three consecutive nitrogenous bases (a codon) specify one amino acid. AGU forms a specific triplet that calls for a Serine transfer RNA (tRNA) during translation, directing its incorporation into a growing polypeptide chain. Therefore, since AGU codes for Serine and not Arginine, it is the one that does not specify Arginine, making it the correct answer to this exclusionary question.

The incorrect option is 1:
The codon **CGC** is incorrect because it is one of the six different codons that all specify the amino acid Arginine (Arg). Arginine is known as a highly degenerate amino acid, meaning that multiple codons (CGU, CGC, CGA, CGG, AGA, AGG) can code for it. Since CGC is unequivocally a codon for Arginine, it must be excluded as the answer to a question asking which codon does *not* specify this amino acid. The fact that various codons result in the same amino acid provides redundancy in the genetic code, a crucial factor in minimizing the impact of point mutations.

The incorrect option is 2:
The codon **AGA** is also incorrect because it is another of the six codons that specify the amino acid Arginine. The presence of both AGA and the C-prefixed codons (like CGC and CGG) for Arginine demonstrates the unique distribution of codons across the genetic code table. A key point to remember is that in the standard code, AGA and AGG are also involved in coding for Arginine, alongside the four CGN codons. Therefore, the AGA triplet directs the ribosome to add Arginine during protein synthesis, confirming that it specifies the amino acid in question.

The incorrect option is 3:
Similarly, the codon **CGG** is incorrect for the same reason as CGC: it is a known codon for the amino acid Arginine. In fact, the group of codons beginning with CG- (CGU, CGC, CGA, CGG) collectively form the main family of codons responsible for coding Arginine. The CGG sequence, like its relatives, signals for the incorporation of Arginine into the growing protein. Since the goal is to find the codon that *does not* specify Arginine, CGG must be eliminated as a possibility, as it performs this function precisely.