Four bases have to describe twenty amino acids. Read them one at a time and you can only name four things; read them in pairs and you get sixteen — still not enough. Read them in threes and you get sixty-four, which is more than enough. That is the genetic code, and the leftovers turn out to be surprisingly useful.
📘 What you need to know
The code is a triplet code: three bases code for one amino acid.
There are 43 = 64 possible codons, but only 20 amino acids are used in proteins.
The code is degenerate: most amino acids have more than one codon.
The code is non-overlapping: each base is read once, as part of one codon only.
The code is universal: the same codons mean the same amino acids in almost every organism.
AUG is the start codon and also codes for methionine. UAA, UAG and UGA are stop codons and code for no amino acid at all.
Degeneracy is why some mutations have no effect on the protein.
Why three bases?
Work through the maths and the reason becomes obvious. With four bases to choose from:
reading them one at a time gives 4 messages — nowhere near 20;
reading them in twos gives 4 × 4 = 16 — still short;
reading them in threes gives 4 × 4 × 4 = 64 — comfortably enough.
Three is the smallest block that works. Anything bigger would waste bases and make the DNA longer than it needs to be.
64 codons for 20 jobs leaves plenty of spares. Nature spends them on duplicates rather than on extra amino acids.
Degenerate — and why that is a good thing
Because there are 64 codons and only 20 amino acids to name, most amino acids end up with several codons of their own. Leucine, serine and arginine have six each; glycine, alanine, valine, proline and threonine have four each. Only methionine and tryptophan have just one.
Look at where the duplicates differ and you will notice a pattern: it is usually the third base that changes. GGU, GGC, GGA and GGG all mean glycine. So if a mutation swaps that third base, the protein comes out exactly the same.
The consequence. Degeneracy acts as a safety net. A change in the DNA does not automatically mean a change in the protein — a lot of mutations are quietly absorbed by the spare codons.
Students often describe degeneracy as “the code being wasteful”. It is the opposite. Those spare codons are the reason your cells can survive constant copying errors without falling apart.
Non-overlapping: each base is used once
Once the ribosome has found the start codon, the mRNA is chopped mentally into blocks of three and read straight through. Base 4 is never borrowed to help read the first codon, and base 3 is never re-read as part of the second.
This means the whole message depends on where you start. That fixed starting point is called the reading frame, and it is set by the AUG start codon.
The blocks never share bases. That is what “non-overlapping” means, and it is why adding or deleting a single base is so damaging.
Universal: the same code in almost everything
A bacterium, an oak tree and you all read AUG as methionine and GGU as glycine. The code has barely changed since very early life, which is strong evidence that all living things share a common ancestor.
It also has a very practical result: genetic engineering works. If you put a human insulin gene into a bacterium, the bacterium reads it correctly and makes human insulin, because it is using the same codon dictionary.
“Universal” is not quite absolute — a handful of organisms and mitochondria read one or two codons differently. IB accepts “almost universal, with a few rare exceptions”, and that phrasing is safer than claiming it is identical everywhere.
The codon table
You are not expected to memorise this. You will be given a table in the exam and asked to use it, so what matters is reading it quickly and accurately. Read the codon in the mRNA, never in the DNA.
Amino acid
Short name
mRNA codons
Phenylalanine
Phe
UUU, UUC
Leucine
Leu
UUA, UUG, CUU, CUC, CUA, CUG
Isoleucine
Ile
AUU, AUC, AUA
Methionine (start)
Met
AUG
Valine
Val
GUU, GUC, GUA, GUG
Serine
Ser
UCU, UCC, UCA, UCG, AGU, AGC
Proline
Pro
CCU, CCC, CCA, CCG
Threonine
Thr
ACU, ACC, ACA, ACG
Alanine
Ala
GCU, GCC, GCA, GCG
Tyrosine
Tyr
UAU, UAC
Histidine
His
CAU, CAC
Glutamine
Gln
CAA, CAG
Asparagine
Asn
AAU, AAC
Lysine
Lys
AAA, AAG
Aspartic acid
Asp
GAU, GAC
Glutamic acid
Glu
GAA, GAG
Cysteine
Cys
UGU, UGC
Tryptophan
Trp
UGG
Arginine
Arg
CGU, CGC, CGA, CGG, AGA, AGG
Glycine
Gly
GGU, GGC, GGA, GGG
Stop signal
Stop
UAA, UAG, UGA
Worked examples
WORKED EXAMPLE
Deducing an amino acid sequence
The coding strand of a short gene reads ATG GCA AAG GGT TAG. Use the codon table to work out the polypeptide it codes for.
Step 1: Write the template strandATG GCA AAG GGT TAG → TAC CGT TTC CCA ATCStep 2: Transcribe the template into mRNATAC CGT TTC CCA ATC → AUG GCA AAG GGU UAGStep 3: Look each codon up in the tableAUG = Met GCA = Ala AAG = Lys GGU = Gly UAG = StopMet – Ala – Lys – Gly (then stop)The stop codon is not an amino acid, so do not write it in the chain.
WORKED EXAMPLE
Degeneracy in action
A mutation changes the mRNA codon GGU to GGC. State the effect on the polypeptide and explain your answer.
Step 1: Look up both codonsGGU = glycine and GGC = glycineStep 2: Compare
The amino acid is the same, so the polypeptide is unchanged.
Step 3: Give the reason
The genetic code is degenerate — glycine has four codons, and only the third base has altered.
No effect on the polypeptideThird-base changes are the ones most likely to be harmless.
WORKED EXAMPLE
Counting amino acids
The coding sequence of an mRNA molecule is 900 bases long and ends in a stop codon. How many amino acids are in the polypeptide it produces?
Step 1: Turn bases into codons900 ÷ 3 = 300 codonsStep 2: Take off the stop codon
One of those 300 codons codes for nothing.
300 − 1 = 299299 amino acidsAlways check whether the question includes the stop codon — it changes the answer by one.
💡 Exam tip
Always convert to mRNA first. Codon tables are written in RNA. Reading a DNA triplet straight off the table is the classic way to lose every mark in the question.
Show the template step. Even if you use the shortcut, writing the template strand earns method marks and catches your own errors.
Learn the meaning of the three key words: degenerate (more than one codon per amino acid), non-overlapping (each base read once), universal (same code in nearly all organisms).
Know what universality is evidence for — a shared common ancestor — and what it allows — genetic engineering between species.
Remember there are three stop codons and one start codon.
Keep your bases in groups of three from the very first line of working. Lose your place and every codon after it is wrong.
⚠ Common mix-up
Using the DNA sequence in the codon table. The table needs mRNA codons. Convert first, every time.
Thinking degenerate means “faulty”. In everyday English it sounds negative; in biology it just means there is more than one codon per amino acid.
Counting the stop codon as an amino acid. It codes for nothing.
Saying 20 codons. There are 64 codons and 20 amino acids. Read the question wording carefully.
Confusing “universal” with “identical”. A few rare exceptions exist, so write “almost universal”.
Forgetting that AUG has two jobs. It starts translation and codes for methionine, which is why so many polypeptides begin with Met.
Up next: Protein Structure & Mutations — if one base in the code is changed, what actually happens to the protein? Sometimes nothing at all, and sometimes it changes a person’s whole life.
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