Codon Structure & Stop Signals — NEET Biology

Correct answer: A — S1, S3 and S5 are not correct

Let's evaluate each statement about the genetic code: S1. The genetic code is a doublet code, where two nucleotides form a codon, coding for twenty amino acids. This statement is NOT correct. The genetic code is a *triplet* code, meaning three nucleotides form a codon. A doublet code (4^2 = 16 combinations) would not be sufficient to code for 20 amino acids. S2. Degeneracy of the genetic code implies that one amino acid can be specified by multiple codons, with the third base of the codon often being less specific. This statement is correct. This accurately describes degeneracy, and the 'wobble' hypothesis (though not explicitly named in NCERT) explains the less specific nature of the third base. S3. The genetic code is universally applicable from bacteria to humans, coding for the same amino acids, without any exceptions. This statement is NOT correct. The genetic code is *nearly* universal, but not absolutely. Some exceptions have been found in mitochondrial codons and in some protozoans. S4. The codon is read in mRNA in a non-overlapping and contiguous manner, ensuring that each nucleotide is part of only one codon and there are no gaps. This statement is correct. This accurately describes the non-overlapping and contiguous nature of the genetic code. S5. AUG always functions as an initiator codon and solely codes for Methionine, never for any other amino acid. This statement is NOT correct. While AUG has a dual function (initiator and codes for Methionine), it does not *solely* code for Methionine *never* for any other amino acid. It *always* codes for Methionine *when it functions as a regular codon for Methionine*, but its specific role as *initiator* is also distinct. S6. UAA, UAG, and UGA are referred to as nonsense codons because they do not code for any specific amino acids, leading to termination of translation. This statement is correct. These three codons are indeed the stop/terminator codons and do not specify an amino acid. Therefore, statements S1, S3, and S5 are NOT correct. The tested concept is a comprehensive understanding of the salient features of the genetic code, including its triplet nature, degeneracy, universality, contiguous reading, and the specific roles of start and stop codons.