Protein Synthesis

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.17 — RNA as single-stranded, containing uracil instead of thymine (bold B statement — Paper 2 only)
• 3.18 — The stages of protein synthesis: transcription and translation, mRNA, ribosomes, tRNA, codons, anticodons (bold B statement — Paper 2 only)

Specification section 3(b) Inheritance — also taught in DNA, Genes and Chromosomes, Alleles, Genotype and Phenotype, Monohybrid Inheritance and Probability, Pedigrees and Sex Determination.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A gene is a section of DNA that codes for a protein. That leaves an obvious question unanswered: the DNA is locked inside the nucleus, and proteins are assembled by ribosomes out in the cytoplasm. The instructions are in one place and the machinery is in another, and the DNA cannot leave — it is far too large, and letting the master copy wander around the cell would be a poor arrangement in any case.

Think of it like a reference library that does not lend

A rare manuscript stays in the reading room and never leaves the building. If a workshop across town needs the instructions, somebody copies out the one recipe they need, carries the copy across, and the workshop builds from that. The original is never at risk, only the pages actually needed are copied, and several workshops can be given copies of the same recipe at once. That is exactly the arrangement here: DNA is the manuscript, mRNA is the copy, and the ribosome is the workshop.

The copy is made of RNA rather than DNA, and RNA differs in three ways worth learning as a set. It is SINGLE-STRANDED rather than a double helix. It contains the base URACIL (U) in place of thymine, so it pairs A with U rather than A with T. And it is short — a single gene's worth — rather than an entire chromosome. Being single-stranded and short is what lets it leave the nucleus through a pore, which is the whole point of using it.

STAGE ONE — transcription, which happens in the NUCLEUS

1. The two strands of the DNA double helix separate along the length of the gene that is needed.
2. One of the strands acts as a TEMPLATE. Free RNA nucleotides pair up along it by the usual rules, except that adenine on the DNA pairs with uracil rather than thymine.
3. These are joined into a single strand of MESSENGER RNA (mRNA), which is therefore a complementary copy of the gene.
4. The DNA strands rejoin, undamaged. Nothing has been used up.
5. The mRNA leaves the nucleus through a pore in the nuclear membrane and travels to a ribosome in the cytoplasm.

STAGE TWO — translation, which happens at a RIBOSOME

1. The mRNA attaches to a ribosome, which reads it three bases at a time. Each group of three bases on the mRNA is a CODON, and each codon specifies one amino acid.
2. Molecules of TRANSFER RNA (tRNA) are floating in the cytoplasm. Each one carries a particular amino acid at one end, and at the other end has three exposed bases called an ANTICODON.
3. A tRNA whose anticodon is complementary to the codon currently being read binds to it — the pairing rules again, so a codon reading GCU is matched by an anticodon reading CGA.
4. That tRNA has therefore delivered exactly the right amino acid to exactly the right place. The ribosome joins it to the growing chain.
5. The ribosome moves along to the next codon and repeats, and the tRNA is released to collect another amino acid.
6. When the ribosome reaches a stop codon the chain is complete. It folds into its specific three-dimensional shape, and that shape is what makes it a working protein.

The word TRANSLATION is chosen carefully and it is worth taking seriously. Transcription is copying within one language — bases to bases, DNA to RNA, and you could read the product back with the same rules. Translation is a change of language altogether: a sequence of bases becomes a sequence of amino acids, and the two have nothing in common chemically. What connects them is the CODE — the fixed correspondence between each triplet of bases and one amino acid — and the tRNA is the dictionary, because it is the only molecule that touches both languages at once.

That gives the sentence the whole topic rests on: the order of bases in a gene determines the order of amino acids in a protein, and the order of amino acids determines the shape the protein folds into, and the shape determines what the protein does. Change one base and you may change one amino acid; change one amino acid and you may change the shape; change the shape and the protein may no longer work. Every one of those steps is a 'may', and the next lessons on mutation are largely about when it is and when it is not.

A small precision that costs marks: a codon is a triplet on the mRNA, and an anticodon is the matching triplet on the tRNA. They are complementary, not identical, and the amino acid is attached to the tRNA rather than to either triplet. Being able to say which molecule each triplet sits on is the difference between describing the mechanism and gesturing at it.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Transcription

The process, occurring in the nucleus, in which one strand of the DNA of a gene is used as a template to make a complementary strand of messenger RNA.

Learn this definition · Translation

The process, occurring at a ribosome, in which the sequence of codons on the mRNA is used to assemble amino acids in the correct order to make a protein.

Statement 3.17 — DNA against RNA

The four molecules of statement 3.18 — say which is which

mRNA (messenger RNA)

A single-stranded copy of one gene, made in the nucleus, which carries the code out to a ribosome.

Ribosome

The structure in the cytoplasm that reads the mRNA three bases at a time and joins amino acids together.

tRNA (transfer RNA)

Carries one specific amino acid, and has an anticodon that pairs with the matching codon on the mRNA.

Codon

A triplet of three bases ON THE mRNA, coding for one amino acid.

Anticodon

A triplet of three bases ON THE tRNA, complementary to the codon it binds to.

Transcription — the sequence to write

1. The DNA double helix unwinds and the two strands separate along the gene.
2. One strand acts as a template.
3. Free RNA nucleotides pair with the bases of the template strand — adenine pairing with uracil rather than thymine.
4. They are joined together to form a strand of messenger RNA, complementary to the template.
5. The mRNA leaves the nucleus through a nuclear pore and travels to a ribosome.

Translation — the sequence to write

1. The mRNA attaches to a ribosome.
2. The ribosome reads the mRNA in codons — groups of three bases — each coding for one amino acid.
3. A tRNA molecule with the complementary anticodon binds to the codon, bringing with it the specific amino acid for that codon.
4. The ribosome joins that amino acid to the growing chain, and the tRNA is released.
5. The ribosome moves to the next codon and the process repeats until a stop codon is reached.
6. The completed chain of amino acids folds into its specific shape, forming the protein.

Working out a sequence — the routine for the data question

• DNA template strand → mRNA: A becomes U, T becomes A, C becomes G, G becomes C
• mRNA → tRNA anticodon: A becomes U, U becomes A, C becomes G, G becomes C
• Split the mRNA into groups of THREE from the start, never from the end
• Worked example: DNA template T-A-C-G-G-A gives mRNA A-U-G-C-C-U, which is two codons — AUG and CCU
• Their anticodons are UAC and GGA, which is the original DNA template sequence again — a useful check

Codon against anticodon — worth a mark in almost every question on this topic

A. A CODON is a triplet of three bases on the mRNA. It specifies one amino acid.

B. An ANTICODON is a triplet of three bases on the tRNA, complementary to a codon. The amino acid is attached to the tRNA, not to either triplet.

Not this: The DNA travels out of the nucleus to the ribosome, carrying the instructions for the protein.

This: DNA never leaves the nucleus. A complementary copy of the single gene is made as mRNA, and it is that copy which travels — which is the entire reason mRNA exists.

Model answer [4 marks]

Describe the process of transcription. [4]

The DNA double helix unwinds and the two strands separate along the length of the gene. One strand acts as a template, and free RNA nucleotides pair with its bases, with adenine pairing with uracil rather than thymine. These nucleotides are joined to form a strand of messenger RNA that is complementary to the template. The mRNA then leaves the nucleus through a pore and travels to a ribosome.

Model answer [5 marks]

Describe how a protein is assembled at a ribosome. [5]

The mRNA attaches to a ribosome, which reads it in codons of three bases, each coding for one amino acid. A tRNA molecule carrying a specific amino acid has an anticodon complementary to the codon being read, and binds to it. The ribosome joins that amino acid to the growing chain and the tRNA is released. The ribosome then moves to the next codon and repeats the process until a stop codon is reached. The completed chain of amino acids folds into a specific shape to form the protein.

Model answer [4 marks]

Explain why the sequence of bases in a gene determines the function of the protein it codes for. [4]

The sequence of bases determines the sequence of codons on the mRNA. Each codon specifies one amino acid, so the base sequence determines the order in which amino acids are joined together. The order of amino acids determines the shape into which the chain folds. The shape of a protein is what allows it to carry out its function, so a change in the base sequence can change the function.

Mark-losing trap. DNA never leaves the nucleus. mRNA exists precisely because it cannot.

Mark-losing trap. RNA uses URACIL in place of thymine, so adenine pairs with U. Writing A–T in an mRNA answer loses the mark.

Mark-losing trap. Codon = on the mRNA. Anticodon = on the tRNA. Name the molecule each sits on.

Mark-losing trap. Transcription is in the NUCLEUS; translation is at a RIBOSOME. State the location for each.

Mark-losing trap. A protein works because of its SHAPE, and the shape follows from the amino acid order. Say all three links.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Which base does RNA contain in place of thymine?
2. Grade 7 · Explain [2 marks] — Explain why a molecule of mRNA has to be made at all, rather than the DNA itself going to the ribosome.
3. Grade 8 · Describe [5 marks] — Describe how a protein is made at a ribosome, once the mRNA has arrived from the nucleus. Select every statement that belongs in a full-mark answer.
4. Grade 9 · Apply [3 marks] — A template strand of DNA reads T-A-C-G-G-A-C-T-T. Working out the mRNA first, how many amino acids will the resulting section of protein contain?
5. 9+ · Analyse [5 marks] — A cell is treated with a substance that prevents tRNA molecules from binding to the mRNA at the ribosome. Tests show the cell's DNA is undamaged, that mRNA is still being produced in the nucleus in normal amounts, and that it is still reaching the ribosomes. No new protein is made. Select every statement that belongs in a full-mark explanation.

The people behind this science

Two ways into the same idea — the one whose measurements fixed the structure this depends on, and the one who understood what a code actually is. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Rosalind Franklin — the one whose measurements fixed the structure this depends on

None of this process makes sense until you know that DNA is two complementary strands, because the copying at the heart of transcription is only possible if one strand can act as a template for another. Franklin's X-ray work gave the measurements that established the helix, its dimensions and the position of the backbone. She also worked on the structure of viruses afterwards, including tobacco mosaic virus, whose genetic material is RNA — the single-stranded molecule this lesson depends on.

• “Why does it matter that the two strands of DNA are complementary?”
• “What did your measurements tell you that a model alone could not?”
• “How is RNA different from DNA in structure?”
• “What were you working on after DNA?”

Alan Turing — the one who understood what a code actually is

This lesson is about a code in the strict sense — a fixed rule that maps one alphabet onto another, applied mechanically without anything needing to understand the message. Turing spent his life on exactly that idea: that symbol-manipulation according to fixed rules can carry meaning and perform work, whether the machine doing it is made of valves or of protein. He also worked, at the end of his life, on how chemical reactions could generate biological patterns, which is the same question one level up.

• “What makes a set of symbols into a code rather than just marks?”
• “Can something follow a code without understanding it?”
• “Why does the length of a message matter more than the size of its alphabet?”
• “How could chemistry alone produce a pattern that looks designed?”

Carry on through the course

• Previous lesson: DNA, Genes and Chromosomes
• Next lesson: Alleles, Genotype and Phenotype
• Sexual and Asexual Reproduction — Two ways to make a new organism — one produces copies, the other produces something that has never existed before. Why any species would want both.
• Flowers, Pollination and Fertilisation — Two completely different solutions to one problem — a plant that cannot move has to get its male gamete to another plant's female gamete.
• Seeds and Germination — What a seed needs to start growing, what it lives on before it has a single leaf — and why light is not on the list.
• The Human Reproductive Systems — Two systems built around the same problem — making gametes and bringing them together — and the hormones that switch them on.
• The Menstrual Cycle — Four hormones, two organs and one repeating question — should the uterus lining be built up, held, or let go?
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification