Genetic Code

The genetic code is composed of codons, each a sequence of three consecutive nucleotide bases that specifies a particular amino acid or a stop signal. Because there are four possible bases (A, U, G, C) at each of the three positions of a codon, the total number of unique combinations equals 4 multiplied by 4 multiplied by 4, which is 4 cubed, giving 64 codons. The option 20 represents the number of standard amino acids coded for, not the number of codons; because the code is degenerate, multiple codons can specify the same amino acid. The value 16 would result from a doublet (two-base) code (4 squared), which cannot uniquely encode 20 amino acids and was ruled out historically. The number 4 simply counts the bases themselves, not their triplet combinations. As explained in NCERT Class 12, Chapter 6 (Molecular Basis of Inheritance), the triplet nature of the code was deduced because a doublet code (16 codons) is insufficient for 20 amino acids. As a numerical check, 4^3 = 64, of which 61 codons specify amino acids and 3 act as stop codons, consistent with the standard genetic code table.