In simple terms
A friendly intro before the formal notes — no formulas yet.
From DNA Blueprint to Protein Machine
Your DNA holds the instructions for building every protein in your cells, but the DNA itself stays in the nucleus. Protein synthesis copies one instruction (a gene) into a portable message (mRNA), then reads that message to assemble amino acids into a polypeptide in the correct order.
Picture your DNA as a priceless reference book of recipes locked in a library (the nucleus). To cook a dish (a protein) you cannot remove the book, so a librarian (RNA polymerase) photocopies just one recipe - that is transcription, and the photocopy is the mRNA. The copy is carried to the kitchen (a ribosome in the cytoplasm), where the chef reads it three words at a time (codons). Kitchen assistants (tRNA) each fetch one exact ingredient (an amino acid) whose label (the anticodon) matches the recipe word, and the chef links the ingredients in order until the recipe says stop.
- 1
In the nucleus, RNA polymerase unwinds a gene and builds a complementary mRNA strand from one DNA strand - the template strand - pairing A with U, T with A, C with G and G with C.
- 2
The finished mRNA leaves the nucleus and travels to a ribosome in the cytoplasm.
- 3
The ribosome reads the mRNA in non-overlapping groups of three bases (codons), starting at the start codon.
- 4
For each codon a tRNA with the complementary anticodon delivers a specific amino acid; the amino acids are joined by peptide bonds in order until a stop codon ends the chain.
Explore the concept
Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.
Step 1
In the nucleus, RNA polymerase unwinds a gene and builds a complementary mRNA strand from one DNA strand - the template strand - pairing A with U, T with A, C with G and G with C.
Full topic notes
Formal explanation with the rigour you need for the exam.
The gene to polypeptide pathway
Genetic information flows in one main direction: from DNA, to RNA, to protein. A gene is a length of DNA whose base sequence specifies the amino acid sequence of one polypeptide. Turning that sequence into a protein takes two steps. First, transcription copies the gene into a messenger RNA (mRNA) molecule. Second, translation uses the mRNA as a set of instructions to join amino acids in the correct order. Think of it as reading a master blueprint (DNA), making a working copy (mRNA), then using the copy to build the final product (the polypeptide).
Transcription - copying the gene into mRNA
Transcription takes place in the nucleus. The enzyme RNA polymerase binds near the start of a gene, unwinds the DNA double helix, and uses just one of the two strands - the template strand - as a guide to build a complementary mRNA molecule. Free RNA nucleotides are added by complementary base pairing: a template cytosine (C) pairs with guanine (G) and vice versa, a template thymine (T) pairs with adenine (A), and a template adenine (A) pairs with uracil (U). That last rule is the key difference from DNA replication: RNA contains uracil instead of thymine, so wherever the template reads A, the new mRNA carries U, never T. RNA polymerase adds nucleotides to the growing mRNA in the 5' to 3' direction; when it reaches the end of the gene the completed mRNA is released, leaves the nucleus, and travels to a ribosome in the cytoplasm.
Location: nucleus (of a eukaryotic cell).
Enzyme: RNA polymerase.
Template: one DNA strand - the template strand - is read; the mRNA is complementary to it.
Base pairing: C-G and G-C, template T-A, and template A pairs with U (uracil replaces thymine in RNA).
Product: a single-stranded mRNA molecule that carries the coded message out to a ribosome.
The genetic code
The mRNA message is read as a genetic code with four defining properties. It is a triplet code: the bases are read in groups of three called codons, and each codon specifies one amino acid (or a stop signal). It is non-overlapping: each base belongs to only one codon, so once the reading frame is set at the start codon the triplets do not share bases. It is degenerate (also called redundant): there are 64 possible codons but only about 20 amino acids, so most amino acids are specified by more than one codon, which offers some protection against mutations in the third base. And it is near-universal: the same codons specify the same amino acids in almost every organism, which is powerful evidence for common ancestry and the reason genes can be transferred between species and still be read correctly.
Triplet: codons are read in threes; each codon = one amino acid or a stop signal.
Non-overlapping: each base is part of only one codon; the start codon sets the reading frame.
Degenerate / redundant: 64 codons for about 20 amino acids, so most amino acids have more than one codon.
Near-universal: the same codons mean the same amino acids in almost all life - evidence for common ancestry.
Start and stop: AUG starts translation (and codes for methionine); UAA, UAG and UGA are stop codons and code for no amino acid.
Translation - reading the message to build the polypeptide
Translation happens at a ribosome in the cytoplasm. The ribosome binds the mRNA and reads it from the 5' end in successive codons, beginning at the start codon (AUG). For each codon, a transfer RNA (tRNA) carrying a specific amino acid arrives; each tRNA has an anticodon - three bases complementary to the codon - and only the tRNA whose anticodon pairs with the exposed codon can bind. This anticodon-codon pairing is what matches each codon to the right amino acid. As successive tRNAs bind, the ribosome catalyses the formation of a peptide bond between neighbouring amino acids, linking them into a growing polypeptide chain. The ribosome then moves one codon along, the used tRNA is released to collect another amino acid, and the cycle repeats. When the ribosome reaches a stop codon, no tRNA pairs with it, translation ends, and the finished polypeptide is released to fold into a functional protein.
The direction and the outcome are the point of the whole pathway. Because the codons are read in order, in a fixed frame, in the 5' to 3' direction, the sequence of bases in the gene determines the sequence of codons in the mRNA, which determines the sequence of amino acids in the polypeptide. Change the base sequence of the gene and you can change the polypeptide.
Location: cytoplasm, at a ribosome.
Reading: mRNA is read 5' to 3', one codon at a time, starting at the AUG start codon.
tRNA: each carries a specific amino acid and has an anticodon that pairs with the mRNA codon.
Bond formed: the ribosome joins successive amino acids with peptide bonds.
End: a stop codon (UAA, UAG or UGA) terminates translation; the polypeptide is released.
Going further (HL detail)
At Higher Level the picture is filled in with more molecular detail. Transcription proceeds only in the 5' to 3' direction as RNA polymerase reads the template 3' to 5', and in eukaryotes the initial (pre-)mRNA is modified before it is translated. The genetic code, though degenerate, is not ambiguous — each codon specifies only one amino acid, so translation is unambiguous even though most amino acids have several codons. During translation the ribosome has distinct binding sites for the tRNAs, and it moves along the mRNA one codon at a time (translocation) as peptide bonds form between the amino acids held in adjacent sites. You are not expected to reproduce all of this at SL, but recognising that 'degenerate but unambiguous' and 'read in one direction, one codon at a time' underlie the simple account here will make the mechanism easier to reason about.
Common mistakes examiners penalise
Confusing transcription and translation - transcription makes mRNA from DNA in the nucleus (RNA polymerase); translation makes a polypeptide from mRNA at a ribosome. Naming the wrong stage, enzyme or location loses marks.
Writing T instead of U in RNA - RNA contains uracil, so a template adenine pairs with U, never T. Putting thymine in an mRNA sequence is a guaranteed lost mark.
Swapping codon and anticodon - the codon is on mRNA, the anticodon is on tRNA. Saying the ribosome reads anticodons, or that tRNA carries codons, is penalised.
Saying the code is degenerate means one codon codes for several amino acids - it is the reverse: several codons can code for the SAME amino acid. Each codon still specifies only one amino acid.
Reading the mRNA in the wrong direction or the wrong frame - codons are read 5' to 3' in non-overlapping triplets from the start codon; shifting the frame changes every amino acid.
Forgetting that a stop codon adds no amino acid - it terminates the chain; do not translate UAA/UAG/UGA into an amino acid.
Vague phrases like 'DNA turns into protein' - name the intermediate (mRNA), the mechanism (complementary base pairing, codon-anticodon pairing) and the bond (peptide bond).
Model answer - marked the way our engine marks it
Explain questions in D1.2 are marked analytically: each distinct valid point is worth one mark. The engine credits correct, named steps rather than loose phrasing, applies error-carried-forward (ECF) so a slip early on does not automatically cost the marks that follow, and accepts equivalent wording where it means the same biology. Study how each mark below is tied to a specific idea.
Where this leads
The gene → polypeptide pathway underpins much of the rest of biology. The universality of the code is why human genes can be expressed in bacteria - the basis of producing medicines such as insulin - and why the code is used as evidence for common ancestry. Mutations make sense only against this pathway: a change in a gene's bases can change a codon, and the degeneracy of the code explains why some changes alter the polypeptide while others do not. Master the habit of tracing information from base sequence, to codon, to amino acid, and you have a template that explains gene expression, mutation and genetic engineering alike.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Part of the template strand of a gene reads 3'-TAC GGT CAT-5'.
(a) Deduce the base sequence of the mRNA transcribed from this template. (b) Write the base sequence of the coding (non-template) strand of this gene. [4]
- 1
(a) mRNA sequence. Transcription uses complementary base pairing, and in RNA uracil (U) replaces thymine (T). Pair each template base and read the mRNA 5' to 3':
- template T → A
- template A → U
- template C → G
- template G → C
A short mRNA molecule has the sequence 5'-AUG GCA CGU UAA-3'. Using the codon table below, determine the amino acid sequence of the polypeptide produced, and explain what happens at the final codon. [4]
Codon table: AUG = Met (start), GCA = Ala, CGU = Arg, UAA = Stop.
- 1
Step 1 - set the reading frame at the start codon. The ribosome reads the mRNA 5' to 3' in non-overlapping triplets, beginning at AUG.
Explain how the base sequence of a gene determines the sequence of amino acids in a polypeptide. [4]
- 1
Model answer. During transcription the base sequence of the gene is copied into a complementary mRNA molecule by complementary base pairing (with uracil pairing to the template adenine). The mRNA carries this information as a sequence of codons, each codon being three consecutive bases. Each codon specifies one particular amino acid. During translation at the ribosome, tRNA molecules with anticodons complementary to the codons pair with them in order, bringing the correct amino acids in the sequence set by the mRNA; these amino acids are then joined by peptide bonds. Because the codons are read in a fixed, non-overlapping order, the base sequence of the gene therefore fixes the order of amino acids in the polypeptide.
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Try to recall each definition before you reveal it.
Quick check
Answer in your head first — then tap to check. No pressure.
Revision flashcards
Flip the card. Test yourself before the exam.
Transcription
Synthesis of a messenger RNA (mRNA) molecule from a DNA template strand by RNA polymerase, using complementary base pairing. Occurs in the nucleus of eukaryotic cells.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
Location: nucleus (of a eukaryotic cell).
- ✓
Enzyme: RNA polymerase.
- ✓
Template: one DNA strand - the template strand - is read; the mRNA is complementary to it.
- ✓
Base pairing: C-G and G-C, template T-A, and template A pairs with U (uracil replaces thymine in RNA).
- ✓
Product: a single-stranded mRNA molecule that carries the coded message out to a ribosome.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Get a Paper 2 answer marked: explain how a gene's base sequence determines a polypeptide, and translate an mRNA sequence with full working
Get a Paper 2 answer marked: explain how a gene's base sequence determines a polypeptide, and translate an mRNA sequence with full working
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Get a Paper 2 answer marked: explain how a gene's base sequence determines a polypeptide, and translate an mRNA sequence with full working on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.