Which of the following is not true about the Genetic code?

Question

Accepted Answer

The correct option is 4: It is universal

The statement "It is universal" is the property that is *not* entirely true about the genetic code, which is why it is the correct choice for this question. The genetic code, which is the dictionary that translates the sequence of nucleotide triplets (codons) into the sequence of amino acids in a polypeptide chain, is often cited as being universal. This means that a specific codon, such as UUU, should code for the same amino acid (Phenylalanine) in all organisms, from bacteria to humans. While the code is *nearly* universal, serving as a powerful piece of evidence for the common ancestry of life, there are significant and well-documented exceptions. These exceptions are primarily found in the mitochondrial DNA of many organisms, as well as in the nuclear DNA of certain protozoans (like ciliates) and yeasts. For instance, the codon UGA, which typically acts as a 'Stop' codon in the standard code, is known to code for Tryptophan in mammalian mitochondria. Similarly, the codon AUA, which normally codes for Isoleucine, codes for Methionine in mitochondria. Because these non-standard assignments exist, the property of absolute universality is violated, making this statement the false one among the choices.

The incorrect option is 1: It is unambiguous

The property of being unambiguous is a fundamental characteristic of the genetic code, and therefore this statement is absolutely true. Unambiguity means that any single codon specifies *one and only one* amino acid. For example, the codon AUG will always code for the amino acid Methionine (and also serve as the initiation signal); it will never code for, say, Valine or Serine. If the code were ambiguous, a single change in the nucleotide sequence of the mRNA could lead to multiple different amino acid sequences being produced, resulting in chaotic protein synthesis. This fixed, one-to-one relationship between a codon and its specified amino acid ensures the accuracy and fidelity of translation.

The incorrect option is 2: It is degenerate

The statement that the genetic code is degenerate is correct and reflects an essential feature of the code. Degeneracy refers to the fact that most amino acids are specified by more than one codon. Since there are 64 possible triplet codons (4^3 = 64) but only 20 common amino acids to code for, there is an inherent redundancy. Only Methionine (AUG) and Tryptophan (UGG) are coded by a single codon. All other amino acids are coded by two, three, four, or six different codons (synonymous codons). This redundancy is particularly important because it provides a buffer against spontaneous mutations, as a mutation in the third base of a codon often still results in the same amino acid being incorporated, minimizing potential damage to the resulting protein.

The incorrect option is 3: It is read in contiguous fashion

The code being read in a contiguous fashion means it is read continuously without any pauses, gaps, or overlapping. The genetic code is read as non-overlapping triplets (codons) starting from a fixed initiation point, typically AUG. Once translation begins, the ribosome moves along the mRNA molecule reading three nucleotides at a time (e.g., AUG-CUC-GUA...) without skipping any bases. If the code were overlapping, a single base would be part of two or three different codons, and if it were punctuated, there would be non-coding nucleotides between the codons. The contiguous, non-overlapping nature of the code is key to establishing the correct reading frame, and maintaining this frame is critical for the production of the correct functional protein.