Key Idea: A gene is a stretch of DNA holding the instructions for one protein. But DNA stays locked in the nucleus, while proteins are built outside it, at a ribosome. So protein synthesis happens in two stages: first transcription copies the gene into a portable mRNA message; then translation reads that message to build the polypeptide. This topic (D1.2) shows up on Paper 1A (codon-table reading, 'which events happen in translation?') and on Paper 2 ('describe how mRNA is produced', labelling a protein-synthesis diagram, the role of RNA in translation).
The whole pathway: a gene (DNA) is transcribed into mRNA in the nucleus, the mRNA is read in three-base codons, and translation builds a polypeptide at the ribosome.
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Transcription = DNA gene → mRNA (in the nucleus). Translation = mRNA → polypeptide (at the ribosome). Keep the two names and their locations straight — examiners love to swap them.
📝 Transcription — making mRNA (4.2.1)
In transcription, the enzyme RNA polymerase unwinds the DNA gene, reads one strand (the template) and joins free RNA nucleotides into a complementary strand of mRNA. The mRNA is then released and leaves the nucleus to carry the gene's code to a ribosome.
A gene is a stretch of double-stranded DNA. In transcription, RNA polymerase reads ONE strand (the template) and copies it into a complementary mRNA — A on the template gives U, not T.
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The steps of transcription, in order
- Bind & unwind — RNA polymerase binds the gene and separates the two DNA strands.
- Read the template — one strand is read as the template.
- Pair the bases — free RNA nucleotides pair by complementary base pairing (A→U, T→A, C→G, G→C).
- Join — RNA polymerase links the nucleotides into a single mRNA strand.
- Release — the mRNA leaves the nucleus and the DNA zips back up.
| Base on the DNA template | Complementary RNA base added to mRNA |
|---|---|
| A (adenine) | U (uracil) — NOT thymine |
| T (thymine) | A (adenine) |
| C (cytosine) | G (guanine) |
| G (guanine) | C (cytosine) |
RNA has no thymine. Wherever the DNA template has an A, the mRNA gets a U (uracil). This is the single most-tested detail of transcription — check every base you copy.
| Feature | RNA polymerase | DNA polymerase |
|---|---|---|
| Process it carries out | Transcription (makes mRNA) | DNA replication (copies DNA) |
| Product made | A single mRNA molecule | Two new DNA molecules |
| Base used opposite A | Uracil (U) | Thymine (T) |
| When it acts | When a gene is expressed | Before a cell divides |
DNA is too large and too precious to leave the nucleus, so the cell sends out a portable copy instead. mRNA carries the gene's base sequence to a ribosome, where it is translated — without it, DNA's instructions could never reach the protein-building machinery.
🔢 The genetic code & codon tables (4.2.2)
Once mRNA exists, it is read in non-overlapping groups of three bases called codons. Each codon codes for one amino acid (or a start/stop signal), and a codon table lists those rules. With 4 bases read 3 at a time, there are 4 × 4 × 4 = 64 possible codons — more than enough for the 20 amino acids plus start and stop.
| Property of the genetic code | What it means | Why it matters |
|---|---|---|
| Triplet | read in non-overlapping groups of THREE bases (codons), one codon per amino acid | 64 codons easily cover 20 amino acids plus start/stop |
| Universal | almost every organism uses the SAME codons for the same amino acids | a gene from one species can be read by another — the basis of genetic engineering |
| Degenerate (redundant) | most amino acids are coded by MORE THAN ONE codon | a base change in the third position is often SILENT — same amino acid |
Universal: The **same codons** mean the same amino acids in (almost) **all organisms**. Compares the code **between species**. Lets a human gene be read correctly inside a bacterium. Evidence of a **common ancestor**.
Degenerate (redundant): **Several codons** can code for the **same amino acid**. Compares **codons within one code**. Most amino acids have **2, 4 or even 6** codons. Allows **silent mutations** (same amino acid).
Key Idea: Because the code is degenerate, a base substitution does not always change the protein. If the new codon still codes the same amino acid (e.g. UUU → UUC, both phenylalanine), the amino acid sequence is unchanged — a silent (neutral) mutation.
Universal compares the code between species (same code everywhere). Degenerate compares codons within one code (many codons → one amino acid).
🧬 Translation — building the polypeptide (4.2.3)
In translation, a ribosome reads the mRNA three bases at a time. For each codon, a tRNA whose anticodon matches brings the correct amino acid, and the ribosome joins it to the chain with a peptide bond. The polypeptide grows codon by codon, in the order set by the mRNA, until a stop codon ends the process and the chain is released.
Step 2 — translation (bottom arrow). At the ribosome the mRNA is read codon by codon; each tRNA's anticodon matches a codon and delivers the right amino acid into the growing polypeptide.
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The translation cycle
- Read — the ribosome reads the next mRNA codon (three bases).
- Match — a tRNA whose anticodon is complementary to that codon arrives, carrying its amino acid.
- Join — the ribosome joins the amino acid to the chain with a peptide bond.
- Move on — the ribosome shifts one codon; the empty tRNA leaves and the next one arrives.
- Stop — at a stop codon no tRNA matches, so translation ends and the polypeptide is released.
| Molecule / structure | Its job in translation | Key feature |
|---|---|---|
| mRNA | carries the coded message; read in 3-base codons | sequence of codons = order of amino acids |
| Ribosome | reads the codons and joins the amino acids | two subunits; catalyses peptide bonds |
| tRNA | brings the correct amino acid for each codon | anticodon base-pairs with the codon |
| Polypeptide | the product — amino acids in the coded order | folds up into a functional protein |
Key Idea: Because the ribosome reads three bases per amino acid, the coding length of the mRNA is simply number of mRNA bases = number of amino acids × 3. So a 100-amino-acid polypeptide needs at least 300 bases of coding sequence (add 3 more for the stop codon). This quick link is a Paper-1A favourite.
| Feature | Transcription (4.2.1) | Translation (4.2.3) |
|---|---|---|
| What is made | mRNA (a copy of the gene) | a polypeptide (chain of amino acids) |
| Where it happens | in the nucleus | at a ribosome (in the cytoplasm) |
| What is read | a DNA template strand | the mRNA, in 3-base codons |
| The machinery | RNA polymerase | tRNA anticodons + the ribosome |
| Bond formed | between RNA nucleotides | peptide bonds between amino acids |
✍️ Worked examples
IB-style question — describe transcription
Describe how a molecule of mRNA is produced from a gene in the nucleus. [4]
How to score all four marks:
Enzyme + unwinding. RNA polymerase binds the gene and unwinds/separates the two DNA strands, breaking the hydrogen bonds between the bases.
Template strand. One strand acts as the template that is read and copied.
Complementary base pairing. Free RNA nucleotides pair with the template bases — adenine pairs with uracil (not thymine), cytosine with guanine.
Join & release. RNA polymerase joins the RNA nucleotides into a single mRNA strand, which then leaves the nucleus. (1 mark per distinct point, max 4.)
RNA polymerase unwinds and separates the DNA strands; one strand is the template; free RNA nucleotides pair by complementary base pairing (A→U); they are joined into a single mRNA strand, which leaves the nucleus.
Step 1 — transcription (top arrow). RNA polymerase copies the DNA template strand into a complementary mRNA molecule, which then leaves the nucleus.
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IB-style question — read a codon table
Using a codon table where UCU = serine, UAU = tyrosine and CAU = histidine, deduce the order of amino acids coded by the mRNA UCU–UAU–CAU. [3]
How to read it codon by codon:
Split into codons. The sequence is already in threes: UCU, UAU, CAU. Read it left to right (5′→3′), one codon at a time.
Look up each codon. UCU → serine (Ser); UAU → tyrosine (Tyr); CAU → histidine (His).
Write them in order. The segment is Ser – Tyr – His. (1 mark per correctly matched codon, kept in order.)
Serine – Tyrosine – Histidine (Ser–Tyr–His), read one non-overlapping codon at a time, kept in order.
IB-style question — translation calculation
A polypeptide is made of 60 amino acids. Calculate the minimum number of mRNA bases needed to code for it, and name the molecule that brings each amino acid to the ribosome. [2]
Model answer:
Use the codon rule. Each amino acid = one codon = 3 bases, so multiply: 60 × 3 = 180 bases (183 if the stop codon is counted).
Name the carrier. A tRNA (transfer RNA) brings each amino acid; its anticodon base-pairs with the matching codon. (Mark 1: 180 bases. Mark 2: tRNA.)
60 × 3 = 180 mRNA bases (183 with the stop codon); a tRNA carries each amino acid to the ribosome.
✅ Quick self-check
Tap each card to check yourself.
What happens in transcription, and where? RNA polymerase copies one DNA template strand of a gene into a complementary mRNA molecule, in the nucleus. A on the template gives U (uracil) in the mRNA, never T.
RNA polymerase vs DNA polymerase? RNA polymerase carries out transcription (makes mRNA, pairs A with U); DNA polymerase carries out DNA replication (makes two new DNA molecules, pairs A with T).
What is a codon, and how much does it code for? A codon is three consecutive mRNA bases. Each codon codes for exactly one amino acid (or a start/stop signal); the mRNA is read in non-overlapping triplets.
Universal vs degenerate? Universal = the same codons mean the same amino acids in nearly all organisms (between species). Degenerate = several codons code for the same amino acid (within the code), which allows silent mutations.
What carries out translation, and how? A ribosome reads mRNA codons; a tRNA whose anticodon matches each codon brings the correct amino acid; the ribosome joins them with peptide bonds, building the polypeptide until a stop codon.
How many mRNA bases code for a polypeptide? Three bases per amino acid, so number of mRNA bases = number of amino acids × 3 (add 3 more for the stop codon).
Visual recap of the whole topic: DNA → (transcription) → mRNA → (translation) → polypeptide. Transcription happens in the nucleus; translation happens at the ribosome.
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Exam Tips
- Two stages: transcription makes mRNA in the nucleus (RNA polymerase); translation makes a polypeptide at the ribosome (tRNA + ribosome). Don't swap them.
- In mRNA, a template adenine pairs with uracil (U), NEVER thymine — the single most common transcription slip.
- Match the enzymes: RNA polymerase → transcription; DNA polymerase → DNA replication. A transcription inhibitor blocks RNA polymerase, so no mRNA is made.
- To read mRNA from a codon table, split into non-overlapping threes, look up each codon, and keep the amino acids in order.
- Universal = same code between species; degenerate = many codons for one amino acid within the code — and degeneracy is why a substitution can be a silent mutation.
- mRNA bases = amino acids × 3 (multiply, never divide); add 3 for the stop codon only if the question asks for the full coding region.
- For 'role of RNA in translation', score all three: mRNA carries the codons, tRNA brings amino acids via its anticodon, the ribosome reads codons and forms peptide bonds — then mention the stop codon.
Key Idea: Protein synthesis = transcription then translation. Transcription (nucleus): RNA polymerase copies one DNA template strand into mRNA by complementary base pairing (A→U). The mRNA leaves the nucleus. The genetic code: mRNA is read in triplet codons; the code is universal (same codons everywhere) and degenerate (many codons → one amino acid, allowing silent mutations). Translation (ribosome): tRNA anticodons match codons to deliver amino acids, which the ribosome joins by peptide bonds into a polypeptide, until a stop codon ends it. Coding length = amino acids × 3 bases.