Genetic Code
மரபணு குறியீடு
A single point mutation in the beta-globin gene changes one amino acid, causing Sickle Cell Anemia. The genetic code is not a suggestion; it is a rigid biological instruction set. If you misread the triplet, the protein fails. Precision is survival.
The idea
The genetic code is a triplet sequence of nitrogenous bases on mRNA that specifies a particular amino acid. It is universal, meaning UUU codes for Phenylalanine in both bacteria and humans. It is degenerate, as one amino acid can be encoded by multiple codons, reducing the impact of mutations. The code is non-overlapping and comma-less, ensuring the reading frame remains consistent during translation.
Why it matters
It provides a universal language for translating the four-letter nucleotide alphabet into the twenty-letter amino acid alphabet of proteins.
NEET lens
NEET focuses on the properties: triplet, degenerate, and universal. Memorize the stop codons (UAA, UAG, UGA) and the initiator codon (AUG). Expect match-the-following questions on amino acid-codon assignments. Do not confuse 'degenerate' with 'ambiguous'; a codon codes for only one amino acid, but an amino acid can have multiple codons.
Clinical significance
When the genetic code is misread due to frameshift mutations, the entire downstream protein sequence is altered, leading to non-functional proteins. This is the molecular basis for many genetic disorders, including Cystic Fibrosis and Duchenne Muscular Dystrophy. Loss of translational fidelity results in cellular toxicity and metabolic collapse.
Introduction
The genetic code is the relationship between the sequence of bases in mRNA and the sequence of amino acids in a protein. It was worked out by George Gamow, Har Gobind Khorana, Marshall Nirenberg and others.
Core concept
Features: (1) triplet — 3 bases (codon) for one amino acid; 64 codons; 61 code for amino acids, 3 are stop codons (UAA, UAG, UGA). (2) Unambiguous and specific — one codon codes for only one amino acid. (3) Degenerate — some amino acids are coded by more than one codon. (4) Read contiguously, with no punctuation. (5) Nearly universal — e.g. UUU codes for phenylalanine in bacteria and humans (exceptions in mitochondria and some protozoans). (6) AUG is dual function — codes for methionine and is the initiator codon.
Go deeper
Universality is what lets bacteria produce human insulin by recombinant DNA technology: the bacterial ribosome reads the human gene's codons the same way. Khorana synthesised RNA molecules with defined base combinations; Nirenberg's cell-free system helped decode codons.
From Class 10
Just as three letters make a word, three bases make a codon that 'spells' one amino acid.