Question
List and explain the properties of the genetic code. What do we mean by degeneracy, universality, and the non-overlapping nature of the code? How many codons code for amino acids and how many are stop codons?
(NEET 2023, similar pattern)
Solution — Step by Step
Each codon consists of 3 consecutive nucleotides (triplet) on mRNA that specifies one amino acid.
With 4 bases (A, U, G, C) taken 3 at a time: possible codons.
Out of 64: 61 code for amino acids and 3 are stop codons (UAA, UAG, UGA).
AUG is the start codon — it codes for methionine and signals the ribosome to begin translation.
Degeneracy means that more than one codon can code for the same amino acid. Since 61 codons code for only 20 amino acids, most amino acids have multiple codons.
Examples:
- Leucine: 6 codons (UUA, UUG, CUU, CUC, CUA, CUG)
- Methionine: only 1 codon (AUG)
- Tryptophan: only 1 codon (UGG)
The degeneracy is mostly in the third position (wobble position) of the codon. This reduces the impact of point mutations — a change in the third base often codes for the same amino acid (silent mutation).
The genetic code is nearly universal — the same codons code for the same amino acids in almost all organisms, from bacteria to humans.
This is powerful evidence that all life on Earth shares a common ancestor.
Exceptions: mitochondrial DNA and a few organisms (Mycoplasma, some protists) have slightly modified codes. For NEET, mention “nearly universal” or “universal with minor exceptions.”
Non-overlapping: Each nucleotide belongs to only one codon. The sequence AUGCCA is read as AUG-CCA (two codons), NOT as AUG-UGC-GCC (overlapping triplets).
Commaless: There are no punctuation marks or spacers between codons. The reading frame is set by the start codon (AUG), and codons are read continuously in groups of three without gaps.
Unambiguous: Each codon specifies only ONE amino acid (though one amino acid can have multiple codons — that’s degeneracy, not ambiguity).
Why This Works
The triplet nature is a mathematical necessity. A singlet code (1 base per amino acid) gives only 4 codes — not enough for 20 amino acids. A doublet code gives — still not enough. A triplet code gives 64 — more than enough, which is why the code is degenerate.
The non-overlapping, commaless nature means that a single insertion or deletion mutation (frameshift) disrupts the entire reading frame downstream. This explains why frameshift mutations are usually more harmful than substitution mutations.
Alternative Method — Codon Chart Quick Facts
For NEET, memorise these facts:
- Start codon: AUG (methionine)
- Stop codons: UAA (ochre), UAG (amber), UGA (opal) — no amino acids
- Most codons: Leucine, Serine, Arginine (6 codons each)
- Fewest codons: Methionine, Tryptophan (1 codon each)
- Total: 64 codons = 61 sense + 3 nonsense (stop)
Mnemonic for stop codons: U Are Away (UAA), U Are Gone (UAG), U Go Away (UGA).
Common Mistake
Students confuse degeneracy with ambiguity. Degeneracy means many codons → one amino acid (multiple codons for the same amino acid). Ambiguity would mean one codon → many amino acids (one codon coding for different amino acids) — this does NOT happen. The code is degenerate but NOT ambiguous. NEET tests this distinction directly.