Lesson 8 of 13 · 11 min
The genetic code and mutations
NCERT § "Genetic Code"
Four DNA bases have to specify twenty amino acids. Two bases per 'word' gives only 4² = 16 combinations — not enough. What is the smallest word length that works?
The lesson in notes
In short
George Gamow reasoned that 4 bases coding 20 amino acids need a code of three bases, since 4³ = 64 combinations covers 20.
Har Gobind Khorana's chemical method for making RNA of defined base combinations (homopolymers and copolymers) and Marshall Nirenberg's cell-free protein-synthesis system helped crack the code. Severo Ochoa's enzyme (polynucleotide phosphorylase) polymerised RNA of defined sequence without a template.
The code is a triplet: of 64 codons, 61 code for amino acids and 3 (UAA, UAG, UGA) are stop codons that code for none.
Some amino acids have more than one codon, so the code is degenerate.
Codons are read continuously in one direction without punctuation (comma-less).
The code is nearly universal, for example UUU codes for phenylalanine from bacteria to humans. Exceptions occur in mitochondrial codons and in some protozoans.
AUG has two jobs: it codes for methionine and it acts as the start (initiator) codon.
A single base change can change one amino acid, as in sickle-cell anaemia. Inserting or deleting one or two bases shifts the reading frame from that point (frame-shift mutation); inserting or deleting three bases or a multiple of three adds or removes whole codons and keeps the downstream reading frame unchanged.