# Genetic Code Properties > Genetic code properties in MBBS Biochemistry: triplet codons, degeneracy, wobble, universality with mitochondrial exceptions and mutations. - Canonical URL: https://prepelephant.com/topics/mbbs/biochemistry/genetic-code-properties - Exam / course: MBBS · Subject: Biochemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Genetic Code Properties", PrepElephant, https://prepelephant.com/topics/mbbs/biochemistry/genetic-code-properties ## Direct answer Sixty-four triplet codons read contiguously from a fixed start encode twenty standard amino acids plus stops: 61 sense codons and three terminators (UAA, UAG, UGA), with AUG doubling as the initiation codon for methionine. The code's properties, each a favourite MCQ attribute, are triplet reading, non-overlapping and comma-less progression, degeneracy (most amino acids have several codons, with the redundancy concentrated at the third position), unambiguity (one codon, one meaning), near-universality (mitochondria read AUA as methionine and UGA as tryptophan), colinearity with the polypeptide, and polarity fixed at 5' to 3'. Wobble pairing at the first anticodon position explains degeneracy's pattern, and point mutations — silent, missense, nonsense and frameshift — describe how the code's discipline breaks. ## What you must remember - **Codon arithmetic:** four bases in triplets give 64 possibilities; 20 amino acids plus stops means the code must be degenerate, and three codons (UAA, UAG, UGA) terminate. - **Initiation specifics:** eukaryotes start with AUG (methionine charged by initiator tRNAi), bacteria with N-formylmethionine; Kozak (eukaryotic) and Shine-Dalgarno (bacterial) sequences set the reading frame. - **Degeneracy pattern:** codons differing only at the third base usually code the same amino acid — the basis of silent mutations and of conservative single-nucleotide polymorphisms. - **Wobble rules (Crick):** C pairs G, A pairs U, U pairs A or G, G pairs U or C, and inosine pairs U, C or A at the anticodon's first position — which lets one tRNA read a codon family. - **Mitochondrial exceptions:** AUA codes methionine (not isoleucine), UGA tryptophan (not stop), AGA and AGG become stops, and AUA can serve as an initiator — the point that "universality" carries an asterisk. - **Mutation classes with anchors:** missense (sickle cell: GAG to GTG, glutamate to valine), nonsense (beta-thalassaemia premature stops), silent, and frameshift from indels not divisible by three; nonsense-mediated decay disposes of such transcripts. - **Proof of the code:** Nirenberg's poly-U produced polyphenylalanine; Khorana's repeating copolymers completed the codon assignments — vivas still ask what poly-U encoded. ## How to work through a mutation-prediction case A haematologist sends a beta-globin gene report reading "codon 6 GAG to GTG" and asks what happens to the protein. Work it in the code's own order. Write the mRNA codons: GAG encodes glutamate, GTG encodes valine, so this is a missense mutation placing valine at position 6 — haemoglobin S. Now extend each direction in the same frame: change GAG to GAA (still glutamate, because third-position degeneracy) and nothing clinical happens, a silent polymorphism; change it to a UAG-type stop in frame and translation truncates early, a nonsense mutation producing beta-thalassaemia rather than sickle trait; delete one nucleotide and every codon downstream re-registers — a frameshift, usually the most damaging class because it converts the remaining message into a different language. Then use wobble logic to predict a suppressor phenotype: a tRNA mutated to read a stop codon as sense partially rescues nonsense mutations in bacteria, and aminoglycosides increase stop-codon misreading — the reason gentamicin has been trialled in specific nonsense-cystic fibrosis and Duchenne genotypes. The code is fixed; the readout is pharmacologically negotiable at the margins. ## Where students slip Students equate degeneracy with ambiguity; the code is degenerate (many codons per amino acid) yet unambiguous (each codon specifies exactly one amino acid) — saying "the code is ambiguous" is wrong by definition. Second, "comma-less and non-overlapping" is often recited without understanding: no punctuation exists between codons and each nucleotide belongs to exactly one triplet, which is precisely why single-base deletions are catastrophic while three-base deletions preserve the frame (the cystic fibrosis deltaF508 three-base deletion deletes one amino acid only). Third, wobble occurs in the anticodon's first position pairing with the codon's third position — candidates reverse it. Finally, AUG is the only codon that both starts translation and codes an internal amino acid; treating initiation as a separate codon species is the trap. ## Frequently asked questions ### Why is the genetic code called degenerate but unambiguous? Multiple codons can specify one amino acid (degeneracy), yet each individual codon always specifies exactly one amino acid or stop (unambiguity). ### What is wobble and where does it occur? Flexible base pairing between the first base of the anticodon and the third base of the codon, involving inosine-containing pairings, allowing fewer tRNAs to read all sense codons. ### Which codons terminate translation? UAA, UAG and UGA; in mitochondrial code, UGA instead codes tryptophan while AGA and AGG become terminators. ### What mutation produces sickle cell anaemia at the codon level? A single missense change, GAG to GTG in beta-globin codon 6, substituting valine for glutamate on the haemoglobin surface. ### Why are frameshift mutations usually more severe than missense mutations? Insertions or deletions not divisible by three shift the entire downstream reading frame, garbling every subsequent amino acid and typically introducing a premature stop.