Question
State the properties of the genetic code. Which of the following is not a property of the genetic code?
(a) It is triplet (b) It is overlapping (c) It is degenerate (d) It is universal
(NEET 2023 — this type appears almost every year in some form)
Solution — Step by Step
The genetic code is the set of rules by which information encoded in mRNA (as codons — triplets of nucleotides) is translated into amino acids. There are 4³ = 64 possible codons for 20 amino acids.
The six universally accepted properties of the genetic code are:
| Property | What it means |
|---|---|
| Triplet | Each codon = 3 nucleotides |
| Degenerate | Multiple codons → same amino acid |
| Non-overlapping | Each nucleotide read only once |
| Commaless | No “pause” signal between codons |
| Universal | Same code in bacteria, plants, humans |
| Unambiguous | One codon → only one amino acid |
Option (b) says the code is overlapping — but this is exactly what the genetic code is not. In a non-overlapping code, reading frame moves exactly 3 bases after each codon. No nucleotide is shared between adjacent codons. This was experimentally confirmed by Crick and Brenner’s frameshift mutations.
The correct answer is (b) It is overlapping — this is the property the genetic code does NOT have. The genetic code is non-overlapping.
Why This Works
The non-overlapping nature is not arbitrary — it’s mechanistically enforced. The ribosome moves exactly one codon (3 nucleotides) at a time along the mRNA. If the code were overlapping, a single mutation would alter multiple adjacent amino acids simultaneously, making proteins catastrophically unstable during evolution.
Degeneracy (multiple codons per amino acid) acts as a buffer against point mutations. Most synonymous codons differ only in the 3rd position — called the “wobble position.” So a mutation at position 3 often doesn’t change the amino acid at all. This is why degeneracy and non-overlapping work together to give proteins mutational stability.
Universality is what makes recombinant DNA technology possible. The human insulin gene inserted into E. coli produces human insulin — because both organisms read the same codons the same way. There are minor exceptions (mitochondrial code), but these prove the rule rather than break it.
Alternative Method
If you blank out on the property list, use the 64-codon logic as a memory anchor.
We have 4 bases, 3 positions → 4³ = 64 codons. But only 20 amino acids exist. So:
- 64 > 20 → the code must be degenerate (extra codons left over)
- 3 of 64 are stop codons (UAA, UAG, UGA), leaving 61 for amino acids
Now ask: if the code were overlapping, a 9-nucleotide sequence would encode only 7 amino acids instead of 3. Crick showed this creates mathematical contradictions with observed protein sequences — ruling out an overlapping code entirely.
In NEET, “unambiguous” and “non-overlapping” are the two properties most often tested as negatives. Memorise: one codon → one amino acid (unambiguous), one nucleotide → one reading (non-overlapping).
Common Mistake
Students confuse degenerate with ambiguous. Degenerate means multiple codons can code for the SAME amino acid (UUU and UUC both → Phenylalanine). Ambiguous would mean one codon codes for MULTIPLE amino acids — which does NOT happen. The code is degenerate but never ambiguous. Mixing these two in an MCQ explanation will cost you the mark even if you circle the right option.
A secondary error: students write that AUG is only the start codon. AUG codes for Methionine in the middle of a protein too — it’s context (position + Kozak sequence in eukaryotes, Shine-Dalgarno in prokaryotes) that makes it a start codon, not the codon itself inherently.