Simulation · Biology · Class 12
Reading the genetic code
From the lesson The genetic code and mutations in Molecular Basis of Inheritance. Change the values and watch what happens.
The idea behind it
NCERT § "Genetic Code"
- 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.