NEET 2025 Zoology Question 163 Solution

After the discovery of the double helix structure of DNA, deciphering how a sequence of 4 nucleotide bases (A, U, G, C) directs the synthesis of proteins containing 20 distinct amino acids emerged as the central challenge:

• • If a codon were a singlet ($4^1 = 4$), only 4 amino acids could be specified.
• • If a codon were a doublet ($4^2 = 16$), only 16 amino acids could be specified, which is insufficient for 20 amino acids.

The physical and mathematical argument:

• • George Gamow (a theoretical physicist and cosmologist) proposed that the genetic code must be a triplet of nucleotides.
• • A triplet code generates $4^3 = 64$ possible codons, providing ample capacity to specify all 20 amino acids with degeneracy.
• • Subsequent biochemical experiments by Har Gobind Khorana, Marshall Nirenberg, and Severo Ochoa experimentally verified Gamow's triplet hypothesis.

Contributions of the other scientists listed:

• • Francis Crick: Proposed the Central Dogma, adaptor hypothesis (tRNA), and comma-less reading frame.
• • Jacques Monod (with François Jacob): Elucidated the operon model (*lac* operon) of transcriptional gene regulation.
• • Franklin Stahl (with Matthew Meselson): Proved the semi-conservative replication of DNA using $^{15}\text{N}$ isotope labeling.

George Gamow is the scientist who proposed that the genetic code must be made up of three nucleotides.