Genetic code
The genetic code is the set of rules that translates nucleotide triplets in mRNA into amino acids in a protein. Its 64 codons specify 20 amino acids plus start and stop signals, and it is read three bases at a time without overlap.
Each codon is three nucleotides long. With four bases, three positions yield 4³ = 64 codons: 61 specify amino acids and 3 are stop codons — UAA, UAG, and UGA. AUG serves double duty as the start codon and as the codon for methionine. Transfer RNAs carry complementary anticodons and deliver the matching amino acid to the ribosome, so the code is implemented physically by tRNA charging.
Four properties define the code. It is degenerate (redundant): most amino acids are specified by more than one codon, with synonymous codons usually differing at the third position, the basis of the wobble hypothesis. It is unambiguous: each codon specifies only one amino acid. It is non-overlapping and commaless: bases are read in consecutive non-shared triplets from a fixed reading frame with no punctuation between codons. And it is nearly universal, shared across nearly all organisms, with limited exceptions including human mitochondria.
Degeneracy shapes how mutations behave. A silent mutation changes a base without changing the amino acid, often at the wobble position. A missense mutation substitutes a different amino acid — conservative if the new residue has similar properties, non-conservative if it does not, as in sickle cell disease, where hydrophilic glutamate is replaced by hydrophobic valine. A nonsense mutation creates a premature stop codon. An insertion or deletion that is not a multiple of three causes a frameshift, garbling every codon downstream and usually producing a truncated, nonfunctional protein.
The code links directly to the central dogma: DNA is transcribed to mRNA, and the ribosome translates that mRNA codon by codon into a polypeptide, with elongation continuing until a release factor recognizes a stop codon.
MCAT questions on the transmission of genetic information from gene to protein and USMLE Step 1 questions in cell and molecular biology both lean on this material — expect to classify mutation types, apply degeneracy and wobble, and reason about the consequences of a shifted reading frame.
Key takeaways
- Codons are non-overlapping nucleotide triplets; 61 of the 64 encode amino acids and 3 are stop codons.
- AUG is both the start codon and the codon for methionine.
- The code is degenerate, unambiguous, non-overlapping, and nearly universal, with mitochondria among the exceptions.
- Degeneracy at the third base (wobble) allows silent mutations that leave the protein unchanged.
- Insertions or deletions not divisible by three cause frameshifts that corrupt all downstream codons.