Question

Why are there 64 different codons for 20 different amino acids?

Answer 1

The existence of 64 codons for only 20 amino acids is due to the mathematical necessity of the genetic code and a biological property called degeneracy.

1. Mathematical Necessity: Since there are 4 different nitrogenous bases (A, U, G, C) and the code is a triplet, the total number of possible combinations is 4³ = 64. A doublet code (4² = 16) would not have been enough to provide unique instructions for all 20 amino acids.
2. Redundancy (Degeneracy): Because 64 is much larger than 20, most amino acids are specified by more than one codon. For example, the amino acid Leucine is coded by six different codons.
3. Biological Advantage: This redundancy acts as a buffer against mutations. If a single base is accidentally changed (especially in the third position of a codon), it often still codes for the same amino acid, preventing the protein from becoming non- functional.
4. Special Signals: Out of the 64 codons, 61 code for amino acids, while the remaining 3 (UAA, UAG, and UGA) serve as “Stop” signals to end protein synthesis.