IB Biology SLTopic 4 — Making ProteinsPaper 1 & 2Core skill~8 min read
The Genetic Code
Four bases have to spell out twenty different amino acids. Reading them one at a time would only give four options, and two at a time only sixteen. Three at a time gives sixty-four — more than enough. That is the whole reason the genetic code works in threes.
📘 What you need to know
The code is a triplet code: three bases code for one amino acid.
Four bases read three at a time gives 4 × 4 × 4 = 64 possible codons, but only 20 amino acids are commonly used.
The code is degenerate — most amino acids have more than one codon that codes for them.
The code is universal — the same codons mean the same amino acids in almost every living thing.
The code is non-overlapping — each base belongs to one codon only and is read once.
AUG is the start codon (and codes for methionine). UAA, UAG and UGA are stop codons and code for no amino acid at all.
Why three bases?
Work through the maths and it becomes obvious. With four bases to play with:
Read one base at a time and you get 4 combinations. Nowhere near 20.
Read two at a time and you get 4 × 4 = 16. Still not enough.
Read three at a time and you get 4 × 4 × 4 = 64. Comfortably enough.
Sixty-four labels for twenty things means plenty of spares. Those spares turn out to be useful.
Degenerate: more codons than amino acids
Because there are 64 codons and only 20 amino acids, most amino acids have several codons that all mean the same thing. Alanine, for example, is coded by GCU, GCC, GCA and GCG. Notice that only the third base changes.
This is not a design flaw. It protects the organism:
If a mutation changes the third base of a codon, very often the amino acid stays exactly the same.
The polypeptide is unchanged, so the protein still works.
In other words, the degenerate nature of the code limits the effect of mutations.
If a question asks how a base substitution can have no effect on the protein, the answer is one word plus one sentence: degenerate — the new codon codes for the same amino acid.
Universal: everything uses the same dictionary
AUG means methionine in a human, in a mushroom and in a bacterium. Almost every organism on Earth uses the same code, with only a few rare exceptions.
That has a big practical consequence. It means a gene taken from one species will still make the right protein inside another species — which is exactly why genetic engineering works. Put the human insulin gene into a bacterium, and the bacterium reads it correctly and produces human insulin.
Universal also hints at something bigger: if every living thing uses the same dictionary, it is very strong evidence that all life shares a common ancestor.
Non-overlapping: each base is used once
The bases are read in blocks of three, one after another, and no base is ever shared between two codons. Once the start codon has set the reading frame, everything after it is read in fixed blocks.
The start codon does two jobs at once: it puts methionine at the front, and it decides where every codon after it begins.
Reading a codon table
You will be given a table of mRNA codons in the exam — you never have to memorise it. Codon tables are written for mRNA, so get your sequence into mRNA form before you look anything up.
Here are some of the codons used on this page, so you can follow the worked examples:
mRNA codon(s)
Amino acid
AUG
Methionine (Met) — also the start codon
GCU, GCC, GCA, GCG
Alanine (Ala)
CCU, CCC, CCA, CCG
Proline (Pro)
GAA, GAG
Glutamic acid (Glu)
GUU, GUC, GUA, GUG
Valine (Val)
UAU, UAC
Tyrosine (Tyr)
AAA, AAG
Lysine (Lys)
UUU, UUC
Phenylalanine (Phe)
UAA, UAG, UGA
No amino acid — stop codons
🧩 Turning a DNA sequence into amino acids
Check which strand you have. Coding strand or template strand? The whole answer depends on this.
Get to the template strand if you were given the coding strand, by pairing the bases (A with T, C with G).
Transcribe the template into mRNA, remembering U instead of T.
Split the mRNA into groups of three and write them out with gaps.
Look up each codon in the table and write the amino acids in order.
Worked examples
WORKED EXAMPLE
From DNA coding strand to amino acid sequence
The coding strand of a gene reads ATG CCA GAG TAC TGA. Use the table above to work out the amino acid sequence it codes for.
Step 1: Write the template strandTAC GGT CTC ATG ACTStep 2: Transcribe it into mRNAAUG CCA GAG UAC UGACheck: same as the coding strand with U for T. It matches, so the working is sound.Step 3: Look up each codonAUG = Met, CCA = Pro, GAG = Glu, UAC = Tyr, UGA = stopMet − Pro − Glu − TyrFour amino acids, not five. The stop codon does not add one.
WORKED EXAMPLE
A change with no effect
A mutation changes an mRNA codon from GCU to GCC. Explain the effect on the polypeptide.
Step 1: Look up both codonsGCU = alanine and GCC = alanineStep 2: Compare
Same amino acid, so the sequence of the polypeptide is unchanged.
No effect on the proteinThis works because the code is degenerate, and because it was the third base that changed.
WORKED EXAMPLE
Explain a claim about the code
A student says that because the genetic code is universal, a human gene can be expressed in a bacterium. Explain why the student is right.
The reasoning
Universal means the same codons code for the same amino acids in nearly all organisms.
So what follows
A bacterium reading a human gene builds the same order of amino acids as a human cell would.
And the result
The bacterium makes the human protein, which is exactly how insulin is made industrially.
Same code, so same proteinAdd “with a few rare exceptions” if you want to sound precise. Examiners like it.
💡 Exam tip
Codon tables are always for mRNA. Convert your DNA to mRNA before you look anything up, every single time.
Write the sequence in threes with spaces as soon as you get it. Miscounting bases is the biggest cause of lost marks here.
Learn the three key words — degenerate, universal, non-overlapping — and one sentence to explain each.
AUG is worth remembering: start codon and methionine.
If a question asks “how many amino acids”, count the codons and then take off the stop codon.
Watch for tables written for the template strand instead. Read the heading of any data you are given.
⚠ Common mix-up
Thinking degenerate means “damaged”. It just means several codons share the same meaning.
Thinking 64 codons means 64 amino acids. There are only 20, plus three stop signals.
Looking up DNA triplets in a codon table. The table is for mRNA, so every letter T will send you to the wrong row.
Believing overlapping reading is possible. Each base is read once and belongs to one codon.
Saying stop codons code for a “stop amino acid”. They code for no amino acid at all.
Confusing universal with identical. Organisms share the code, not the genes themselves.
Up next: Protein Structure & Mutations — what happens to the finished polypeptide, and what one wrong base can do to it.
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