What you need to know
- 2.13(i)transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus. and translationMaking a polypeptide on a ribosome, using the sequence of codons on mRNA. tRNAs bring the amino acids.: RNA polymeraseThe enzyme that joins RNA nucleotides into mRNA during transcription, using one DNA strand as a template., messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome., tRNATransfer RNA: a folded RNA that carries a specific amino acid and has an anticodon that pairs with a codon on mRNA., ribosomeA small organelle made of rRNA and protein, where translation takes place., start and stop codonA sequence of three bases on mRNA that codes for one amino acid or a stop signal..
- 2.13(ii)Template (antisense) DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code., mRNA codons and tRNA anticodons.
The main sequence follows a eukaryotic cell. Detailed RNA processing and ribosomal subunit names are outside this lesson’s required scope.
By the end, you should be able to…
- Trace information from template DNA to a polypeptideA chain of many amino acids joined by peptide bonds..
- Assign a precise role to RNA polymerase, mRNA, tRNA and ribosomes.
- Use complementary pairing without translating a DNA template directly.
Build your understanding
Transcription makes an RNA copy
In a eukaryoticDescribes cells with a nucleus and membrane-bound organelles, such as animal and plant cells. cell, transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus. occurs in the nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores.. A region of DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. unwinds and the strands separate. One strand is used as the template, also called the template strandThe DNA strand that RNA polymerase reads during transcription. The mRNA is complementary to it.. Free RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. nucleotideThe monomer of nucleic acids: a pentose sugar, a phosphate group and a nitrogenous base. pair with exposed template bases: RNA A with DNA T, RNA U with DNA A, and C with G.
RNA polymeraseThe enzyme that joins RNA nucleotides into mRNA during transcription, using one DNA strand as a template. catalyses formation of the RNA sugar–phosphate backbone. It reads the template 3′ → 5′ while building RNA 5′ → 3′. The RNA separates from the DNA and the DNA strands rejoin. messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. carries the information to a ribosome in the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles., including ribosomeA small organelle made of rRNA and protein, where translation takes place. attached to rough endoplasmic reticulumA network of flattened membrane sacs covered in ribosomes. It makes and transports proteins..
Keep template and coding strands separate
For template DNA 3′–TAC GGA AAA ACT–5′, the mRNA is 5′–AUG CCU UUU UGA–3′. The corresponding coding DNA strand is 5′–ATG CCT TTT TGA–3′. The coding strandThe DNA strand that is not transcribed. Its sequence matches the mRNA, with T in place of U. matches the mRNA base sequence except that DNA has T where RNA has U.
The coding strand is not the strand being copied in this example. Always identify the template and its direction before generating mRNA. codonA sequence of three bases on mRNA that codes for one amino acid or a stop signal. are read on mRNA, not on the tRNATransfer RNA: a folded RNA that carries a specific amino acid and has an anticodon that pairs with a codon on mRNA. or the DNA template.
Translation assembles a polypeptide
A ribosome binds to mRNA and starts translationMaking a polypeptide on a ribosome, using the sequence of codons on mRNA. tRNAs bring the amino acids. at a start codon, usually AUG in the standard examples used here. This establishes the reading frame. A tRNA carries a particular amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group. and has an anticodonThe three bases on a tRNA that pair with a complementary codon on mRNA. complementary to an mRNA codon.
For mRNA codon 5′–AUG–3′, the paired anticodon is 3′–UAC–5′. Pairing positions the correct amino acid at the ribosome. As successive tRNAs bind, the ribosome catalyses peptide-bond formation between amino acids and moves along the mRNA. Empty tRNAs leave and can be loaded with amino acid again.
Stop and release
At a stop codon, translation ends and the polypeptideA chain of many amino acids joined by peptide bonds. is released. A stop codon does not specify an amino acid and is recognised by a release factor rather than an ordinary amino-acid-carrying tRNA. The chain then folds; its amino-acid sequence influences its structure and function.
Using AUG = methionine, CCU = proline, UUU = phenylalanine and UGA = stop, the example mRNA produces methionine–proline–phenylalanine before release. DNA remains available for further transcription; it is not consumed to make the protein.
Diagram A
NOT: RNA polymerase is not DNA polymerase. The coding strand is not the template.
- Template 3′–TAC GGA AAA ACT–5′
- mRNA 5′–AUG CCU UUU UGA–3′
- Coding DNA matches the mRNA, with T where RNA has U
- Codons are read on mRNA, not on the DNA template
Two traps this diagram blocks
- Do not copy the template letters unchanged into mRNA. Transcribe the complementary bases and replace thymine with uracil.
- Do not say the coding strand is the strand being copied.
From the diagram alone
Question
Give the mRNA transcribed from the template 3′–TAC GGA AAA ACT–5′, and name the enzyme. [3]
Show a model answer
The mRNA is 5′–AUG CCU UUU UGA–3′. RNA polymerase catalyses its synthesis. Adenine in DNA is paired with uracil in RNA, and the mRNA is built in the 5′ to 3′ direction.
Diagram B
NOT: the ribosome does not translate DNA, and a stop codon is not an amino acid.
- mRNA carries the message; tRNA brings the amino acid
- Example: AUG CCU UUU, then UGA stop
- Using the lesson key: methionine, proline, phenylalanine
- DNA remains available; it is not consumed to make the protein
Two traps this diagram blocks
- Do not say DNA polymerase or RNA polymerase makes the peptide bond. The ribosome does.
- Do not count the stop codon as one more amino acid.
From the diagram alone
Question
Explain the roles of mRNA and tRNA in translation. [3]
Show a model answer
mRNA carries the codon sequence that the ribosome reads, starting at AUG. Each tRNA carries one amino acid and pairs its anticodon with a complementary codon, which places the correct amino acid in the chain. The ribosome joins the amino acids by peptide bonds.
Make your reasoning visible
Name the right enzyme
DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. polymerase synthesises DNA in replication. RNA polymeraseThe enzyme that joins RNA nucleotides into mRNA during transcription, using one DNA strand as a template. synthesises RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. in transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus.. ribosomeA small organelle made of rRNA and protein, where translation takes place. catalyse peptide-bond formation in translationMaking a polypeptide on a ribosome, using the sequence of codons on mRNA. tRNAs bring the amino acids..
Follow the whole chain
For an explanation linking DNA to function, show base sequence → messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. codonA sequence of three bases on mRNA that codes for one amino acid or a stop signal. → amino-acid sequence → folding and function.
Label codon and anticodon
Write the molecule and direction. Complementary sequences run in opposite directions; anticodonThe three bases on a tRNA that pair with a complementary codon on mRNA. UAC paired with AUG is written 3′ → 5′ here.
Key explanations
- Free RNA nucleotideThe monomer of nucleic acids: a pentose sugar, a phosphate group and a nitrogenous base. pair with the template: RNA adenine with DNA thymine, RNA uracil with DNA adenine, and cytosine with guanine.
- The mRNA leaves the DNA, and the DNA strands rejoin. The gene can be transcribed again.
- The coding strandThe DNA strand that is not transcribed. Its sequence matches the mRNA, with T in place of U. has the same base sequence as the mRNA except that DNA has thymine where RNA has uracil.
- Empty tRNATransfer RNA: a folded RNA that carries a specific amino acid and has an anticodon that pairs with a codon on mRNA. leaves and can be reused. mRNA is the message; tRNA is the adaptor.
Avoid this misconception
The claim: A ribosome translates the DNA template directly.
The correction: It reads mRNA codons. tRNA anticodons pair with those codons and deliver amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group..
How a gene in DNA becomes a chain of amino acids
DNA holds instructions in its order of bases. A cell first copies one gene into a messenger molecule (mRNA), then a ribosome reads that message three bases at a time and joins amino acids in the matching order.
What the model shows
A short piece of DNA being copied into mRNA, then the mRNA being read at a ribosome. tRNA molecules bring amino acids into the ribosome’s A site, move to the P and E sites, and leave. A codon table sits underneath.
What to look for
Which DNA strand is copied and in which direction. How each three-base codon matches one tRNA anticodon. How many tRNAs are in the ribosome at once, and when the first one leaves.
What it tells us
mRNA is made by complementary base pairing with the template strand. The ribosome reads mRNA codons, not DNA, starting at AUG and stopping at a stop codon, joining amino acids with peptide bonds one codon at a time.
Be careful what you claim
The genes are tiny and the steps are slowed down. Steps that happen in real cells, such as mRNA processing in the nucleus and the helper proteins at the ribosome, are left out. What happens when a base changes is in the next lesson (2C 1).
Try it in the model: press Play and follow the lesson example: template 3′–TAC GGA AAA ACT–5′ should give methionine–proline–phenylalanine, then stop. Use Step during translation to watch the third tRNA arrive before the first one leaves.
From gene to protein
Transcription of a template strand into mRNA, then translation at a ribosome with its E, P and A sites, one tRNA and one codon at a time. Mutations have their own model in 2C 1.
What you see
- The two DNA strands (coding strand above, template below) with their 5′ and 3′ ends, RNA polymerase and a growing mRNA.
- Then the mRNA at a ribosome with three sites, E, P and A, from the 5′ side. tRNAs with anticodons enter the A site, move to P and then E, and leave.
- The polypeptide hangs from the tRNA in the P site and grows one amino acid per cycle. A codon table sits under the controls.
How to use it
- Play, Pause or Step. Step stops after each event: a tRNA entering, a tRNA leaving, a peptide bond, the ribosome moving.
- Choose a gene: the lesson example or a longer one. ½× to 4× change the speed; even at 4× one codon takes about 1.7 seconds.
- The codon table highlights the codon being read.
What it shows
- Transcription: the template strand is read 3′→5′ and mRNA is built 5′→3′ by complementary pairing.
- Translation: codons on mRNA are read in threes from AUG; tRNA anticodons pair with them in the A site; peptide bonds join the amino acids; a stop codon ends the chain.
- Two tRNAs at a time, briefly three: an empty tRNA leaves the E site only after the next tRNA enters the A site.
Limits of the model
- A eukaryotic cell would process the mRNA (splicing) before it leaves the nucleus; that is not shown, as the lesson boundary says.
- Initiation and release need extra proteins (initiation and release factors) that are not drawn. The ribosome’s two subunits are drawn as simple shapes.
- Genes here are tiny and the steps are slowed right down. Real ribosomes add several amino acids a second, and many ribosomes can read one mRNA at once.
Guided tour · step 1 of 7
- In Run, just below these steps, press Play.
- Watch RNA polymerase move along the DNA in the simulator, building mRNA.
RNA polymerase reads the template strand 3′→5′ and joins RNA nucleotides into mRNA 5′→3′. U pairs with A; there is no T in RNA.
Run
Questions
The genetic code (mRNA codons)
| 1st | U | C | A | G |
|---|---|---|---|---|
| U | UUU PheUUC PheUUA LeuUUG Leu | UCU SerUCC SerUCA SerUCG Ser | UAU TyrUAC TyrUAA StopUAG Stop | UGU CysUGC CysUGA StopUGG Trp |
| C | CUU LeuCUC LeuCUA LeuCUG Leu | CCU ProCCC ProCCA ProCCG Pro | CAU HisCAC HisCAA GlnCAG Gln | CGU ArgCGC ArgCGA ArgCGG Arg |
| A | AUU IleAUC IleAUA IleAUG Met | ACU ThrACC ThrACA ThrACG Thr | AAU AsnAAC AsnAAA LysAAG Lys | AGU SerAGC SerAGA ArgAGG Arg |
| G | GUU ValGUC ValGUA ValGUG Val | GCU AlaGCC AlaGCA AlaGCG Ala | GAU AspGAC AspGAA GluGAG Glu | GGU GlyGGC GlyGGA GlyGGG Gly |
AUG (methionine) is the start codon. UAA, UAG and UGA are stop codons.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Transcribe template DNA 3′–TAC GGA AAA ACT–5′. Use the codon key in the notes to translate it.
Show suggested answer
mRNA is 5′–AUG CCU UUU UGA–3′. The polypeptide is methionine–proline–phenylalanine; UGA signals termination.
Practice question
Retrieval and explanation · suggested answer
Explain the different roles of mRNA and tRNA.
Show suggested answer
mRNA carries the codon sequence read by the ribosome. tRNA carries a specific amino acid and pairs its anticodon with a complementary mRNA codon.
Practice question
Retrieval and explanation · suggested answer
A drug prevents peptide bonds forming at a ribosome. Which stage is directly affected, and could mRNA still exist?
Show suggested answer
Translation is directly affected. mRNA made by transcription could still be present; failure to make protein does not by itself show failure of transcription.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A cell contains normal amounts of a particular mRNA but little of its protein. Suggest two explanations involving translation or the protein after translation. The question supplies that proteins can be broken down.
Show a hint
The amount of mRNA is not a direct count of completed, stable protein molecules.
Explore the reasoning
Translation might be inhibited or less efficient, producing fewer polypeptides per mRNA. Alternatively, protein may be produced but broken down rapidly. Measuring mRNA alone cannot distinguish these explanations.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the DNA strand that is transcribed, and give its alternative name.
Transcription and translation
Practice question
Retrieval and explanation · suggested answer
Name the DNA strand that is transcribed, and give its alternative name.
Lesson: 2B 6: DNA and Protein Synthesis
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Seeing the protein-making machinery
2B · DNA and Protein SynthesisThe 2009 Nobel Prize in Chemistry recognised studies of the structure and function of the ribosome. Structural biology connects the arrangement of molecules with the reactions they perform.…Specification 2.13(i) · 2.13(ii)Read & use the scienceClose article
Why this matters
The 2009 Nobel Prize in Chemistry recognised studies of the structure and function of the ribosome. Structural biology connects the arrangement of molecules with the reactions they perform. Researchers mapped the positions of components and investigated how molecules, including some antibiotics, interact with the machinery. A structural map helps explain why a molecule can interfere with translation, but it is not by itself proof of a safe and effective medicine. Biological activity and selectivity still need testing. The Unit 1 connection is that reading mRNA, positioning tRNAs and forming peptide bonds depend on a working molecular arrangement, not simply on having the correct genetic message available.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.13(i) understand the process of protein synthesis (transcription and translation), including the role of RNA polymerase, translation, messenger RNA, transfer RNA, ribosomes and the role of start and stop codons
- 2.13(ii) understand the roles of the DNA template (antisense) strand in transcription, codons on messenger RNA and anticodons on transfer RNA
Use the science
Why could a change in a ribosome’s shape interfere with translation even if the mRNA sequence is unchanged?
Explore a possible response
It could disrupt binding or positioning of mRNA and tRNAs, or peptide-bond formation. An intact message still needs working translation machinery.
Explore the source
- The ribosome translates the DNA code into lifeNobel Prize, Chemistry 2009
A medicine written as a temporary message
2B · DNA and Protein SynthesisAn mRNA vaccine supplies instructions that cells can translate into a protein; it does not supply a replacement human genome. Research recognised by the 2023 Nobel Prize showed…Specification 2.9(i) · 2.13(i) · 2.13(ii)Read & use the scienceClose article
Why this matters
An mRNA vaccine supplies instructions that cells can translate into a protein; it does not supply a replacement human genome. Research recognised by the 2023 Nobel Prize showed how modified RNA bases could reduce unwanted inflammatory responses and improve protein production. The underlying connection is translation: ribosomes read codons and transfer RNAs deliver amino acids in a specified order. Producing a useful message also involves its stability and how cells respond to it, not just choosing a protein sequence. This example shows how research into apparently fundamental RNA chemistry helped overcome practical barriers to a medical technology.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.9(i) know the basic structure of mononucleotides (deoxyribose or ribose linked to a phosphate and a base, including thymine, uracil, adenine, cytosine or guanine) and the structures of DNA and RNA (polynucleotides composed of mononucleotides linked by condensation reactions to form phosphodiester bonds)
- 2.13(i) understand the process of protein synthesis (transcription and translation), including the role of RNA polymerase, translation, messenger RNA, transfer RNA, ribosomes and the role of start and stop codons
- 2.13(ii) understand the roles of the DNA template (antisense) strand in transcription, codons on messenger RNA and anticodons on transfer RNA
Use the science
Why would a premature stop codon in the supplied mRNA be expected to change the protein made, even if the cell’s own DNA were unchanged?
Explore a possible response
Ribosomes terminate translation at the stop signal, producing a shorter polypeptide from that message. Altering a supplied RNA template does not require changing genomic DNA.
Explore the source
- The Nobel Prize in Physiology or Medicine 2023: Advanced informationNobel Committee for Physiology or Medicine