The Genetic Code — Triplet, Degenerate, Non-Overlapping, Universal

The genetic code is the set of rules by which information encoded in the sequence of DNA bases is translated into sequences of amino acids in proteins. It is one of the most elegant and universal features of biology. This lesson covers the OCR A-Level Biology A specification point 2.1.3 (f) — the nature of the genetic code, including the key descriptors: triplet, degenerate, non-overlapping and (nearly) universal.

1. Why a Triplet Code?

There are four bases in DNA (A, T, C, G) and 20 amino acids commonly found in proteins. The minimum number of bases required to specify one amino acid — assuming a fixed number per amino acid — can be worked out using powers of 4:

Bases per codon	Possible combinations	Enough for 20 amino acids?
1	4¹ = 4	No — only 4 amino acids
2	4² = 16	No — only 16 amino acids
3	4³ = 64	Yes — with room to spare
4	4⁴ = 256	Yes, but wastefully so

A triplet code — three bases per amino acid — is therefore the smallest workable unit that can specify all 20 amino acids. Evolution has converged on this solution: every cell in every organism on Earth uses a three-base codon.

Key Definition — Codon: A triplet of bases in mRNA (or a gene) that codes for a single amino acid (or for starting/stopping translation).

2. The Four Properties of the Genetic Code

The OCR specification requires you to understand four specific descriptors of the genetic code:

2.1 Triplet

Each amino acid is specified by a sequence of three consecutive bases — a codon on mRNA (or a triplet on the coding strand of DNA). There are 64 possible codons (4 × 4 × 4).

mRNA:   5' — A U G — G C A — U U U — U G G — U A A — 3'
            Met   Ala   Phe   Trp   STOP

2.2 Degenerate

With 64 codons but only 20 amino acids, there are more codons than amino acids. This means:

Most amino acids are coded by more than one codon.
For example, leucine is coded by six different codons (CUU, CUC, CUA, CUG, UUA, UUG).
Only two amino acids (methionine and tryptophan) are coded by just one codon each.
Three codons (UAA, UAG, UGA) do not code for an amino acid but act as stop codons, signalling the end of translation.
AUG acts as the start codon and also codes for methionine.

Key Definition — Degenerate code: A code in which more than one codon can specify the same amino acid.

Biological importance of degeneracy: Because several codons code for the same amino acid, some point mutations (changes in a single base) are "silent" — they produce a different codon that still codes for the same amino acid. The degenerate code therefore provides some resistance to the harmful effects of mutations.

2.3 Non-Overlapping

The bases of the genetic code are read in sequence, one codon after another, without any overlap between codons. Each base belongs to only one codon; each codon is read separately.

Correct reading (non-overlapping):
   AUG UUC GCA UGA
   Met Phe Ala STOP

Hypothetical overlapping code (NOT used):
   AUG
    UGU
     GUU
      UUC

Consequence: the reading frame matters enormously. If a base is inserted or deleted (a frameshift mutation), every codon downstream of the change is read in a different frame and the amino acid sequence becomes entirely wrong. This is much more damaging than a single-base substitution.

Exam Tip: Questions often ask about the effects of different types of mutation. A substitution changes only one codon (and may be silent if the code is degenerate). An insertion or deletion causes a frameshift, changing every subsequent codon — nearly always catastrophic.

2.4 Universal

The same codons specify the same amino acids in (almost) all organisms — bacteria, archaea, plants, fungi and animals.

AUG codes for methionine in humans, in E. coli, in Arabidopsis thaliana and in yeast.
UAA is a stop codon in all of these.

Biological significance:

Lesson 5: The Genetic Code — Triplet, Degenerate, Non-Overlapping, Universal