Consider the following statements and select the incorrect one in reference to the genetic code :

Question

Accepted Answer

The correct option is 4: Each amino acid is coded by a separate codon.

This statement is the **incorrect** one in reference to the genetic code, and is therefore the correct choice for the MCQ. The idea that each of the 20 amino acids is coded by a separate, unique codon is false. A crucial property of the genetic code is its **degeneracy** or **redundancy**. This means that multiple codons can and do code for the same amino acid. For instance, the amino acid Serine is specified by six different codons (UCA, UCU, UCC, UCG, AGU, and AGC). This redundancy is a vital evolutionary adaptation, as it minimizes the detrimental effects of point mutations; a change in a single nucleotide may still result in the original amino acid being incorporated, maintaining the protein's function.

The incorrect option is 1: There are 64 codons.

This statement is fundamentally **correct**. The genetic code is based on a triplet code, meaning three consecutive nitrogenous bases (A, U, G, or C) form one codon that specifies an amino acid or a termination signal. Since there are four bases and the code is a triplet, the total number of possible combinations is calculated as 4 raised to the power of 3 ($4^3$), which equals 64. These 64 codons constitute the entire vocabulary of the genetic message on the messenger RNA (mRNA). Because this statement is a fact about the code, it cannot be the incorrect one the question asks for.

The incorrect option is 2: The codon AUG has dual functions.

This statement is also **correct** and describes a unique feature of the AUG codon. Its dual functions are, firstly, to code for the amino acid Methionine (Met) at any location within the polypeptide chain. Secondly, and most critically, when AUG appears at the start of the mRNA, it functions as the universal **Start Codon** (or Initiation Codon), signaling the ribosome to begin the process of translation. All polypeptide synthesis begins with a Methionine, making AUG essential for both initiating the process and contributing to the growing amino acid chain.

The incorrect option is 3: Three codons do not code for any amino acid.

This statement is **correct**. Out of the 64 total codons, 61 of them code for the 20 amino acids. The remaining three codons—UAA, UAG, and UGA—are specifically designated as **Stop Codons** (or Nonsense Codons). When the translating ribosome encounters one of these three codons, the process of translation is halted. They do not have a corresponding transfer RNA (tRNA) with an amino acid attached, but instead, they are recognized by release factors, which cleave the newly synthesized polypeptide from the ribosome.