Gene Expression Lesson Plan: Transcription and Translation

This gene expression lesson plan uses a free simulation to make the step from DNA to protein concrete. Students write the mRNA for a short gene by hand, watch RNA polymerase build it one base at a time, then follow the ribosome codon by codon to a stop. Then they change single bases and see which mutations matter. You get learning goals, setup, predictions, a step-by-step sequence with the sequences students should see, a five-question set for the class link, a regulation extension, and ideas for differentiation. Every codon and amino acid below was checked against the simulation's own model.

Lesson at a glance

• Level: grades 9–12 (ages 14–18).
• Time: one 60-minute period, or two 40-minute periods with the extension.
• Prior knowledge: DNA structure and base pairing (A–T, G–C); that proteins are chains of amino acids.
• Format: pairs on laptops or tablets, or the whole class with one projector. The sequence rows are easier to read on a wide screen.
• Simulation: Transcription, translation and point mutations. Extension: Gene expression – transcription, translation and regulation.

Learning goals

By the end of the lesson, students can:

1. Write the mRNA made from a template strand, using U instead of T.
2. Use the genetic code table to translate mRNA from the start codon to a stop codon.
3. Explain the roles of RNA polymerase, the ribosome and tRNA.
4. Classify a point mutation as silent, missense, nonsense or frameshift, and explain its effect on the protein.
5. (Extension) Describe how transcription factors switch a gene on or off.

What the simulation does

The default gene is an 18-base template strand, read 3'→5': TAC AAA GGC TTA CCG ACT. The screen shows four rows: the template strand, the complementary strand, the mRNA (5'→3') and the polypeptide. The full genetic code table sits underneath, and the codon being read lights up.

With Auto-run on (the default), the simulation moves one step every 0.45 seconds: 18 transcription steps, then 6 translation steps, about 11 seconds in all. Turn Auto-run off and use Next step to go at your own pace. The readout narrates each step:

• "Ready: the gene has 18 nucleotides (6 codons)"
• "Transcription: nucleotide 1/18 — template T → mRNA A"
• "Translation: codon 1/6 — AUG → Met (start)"

During translation, the picture shows the ribosome on the mRNA and the tRNA anticodon, for example "anticodon 3'-UAC-5'" for AUG. The result is Met–Phe–Pro–Asn–Gly, then the stop codon UGA: "complete: 5 amino acids".

Mutations: clicking any template base substitutes it. Each click moves to the next base, in the order A, T, G, C, skipping the original. An extra row, "Original (no mutation)", appears for comparison, changed amino acids are highlighted, and a line below names the mutation type. Insertions and deletions are set in the starting-values panel below the simulation: Mutation, Mutation position on the template strand and Nucleotide substituted / inserted.

Two model details are worth knowing:

• Translation starts at the first AUG anywhere in the mRNA. If a mutation destroys the start codon, the ribosome finds the next AUG, even out of frame.
• At the stop codon, the picture still shows an anticodon box. In a cell, no tRNA reads a stop codon; a release factor ends translation. A good correction for strong students to spot.

Materials and setup before class

Materials: one device per pair (or a projector), a printed codon table for the hand-written step, and the record tables below.

Setup (10 minutes, once):

1. Open the simulation. The defaults are the gene above, no mutation and Auto-run on.
2. Turn off Auto-run step by step in the starting values, so the gene waits at "Ready" instead of playing as soon as the page opens. Then click Share and create a link for each class, for example "Biology · Period 5". The link pins these starting values.
3. Create a second link named "Nonsense gene" and set Gene template strand to TACAAAGGCATACCGACT (step 6 uses it).
4. Optional: on the Questions tab, enter the question set below and attach it to the first link.
5. Post the links, or open the first one in present mode and show the QR code.

Lesson sequence

1. Hook and predictions (7 minutes)

Ask: "Your cells all hold the same DNA. Yet a muscle cell and a skin cell make different proteins. Where does the protein actually come from?" Collect a few answers. Then students commit to two predictions, on paper or on the link:

• P1. "Which mRNA codon is made from the template triplet TAC? TAC / ATG / AUG / UAC"
• P2. "One base in a gene is replaced by another. Does the protein always change?"

Expect ATG (forgetting uracil) or UAC (copying instead of pairing) in P1, and "yes, always" in P2. Don't correct anyone yet.

2. Transcription by hand, then by machine (10 minutes)

Pairs write the mRNA for the default template before they press anything. Then they step through transcription with Next step, checking each base:

Template (3'→5')	TAC	AAA	GGC	TTA	CCG	ACT
mRNA (5'→3')	AUG	UUU	CCG	AAU	GGC	UGA

Ask: "The mRNA matches one of the two DNA strands almost exactly. Which one, and what's the difference?" It matches the complementary strand, with U in place of T. Check P1 now.

3. Translation and the code table (8 minutes)

Keep stepping. For each codon, students find the amino acid in the code table and write the anticodon the tRNA must carry:

Codon	AUG	UUU	CCG	AAU	GGC	UGA
Amino acid	Met (start)	Phe	Pro	Asn	Gly	stop
Anticodon (3'→5')	UAC	AAA	GGC	UUA	CCG	none

Ask: "Six codons, but only five amino acids. Why?" The last codon is a stop signal and codes for no amino acid.

4. Substitutions: a class survey (15 minutes)

Each pair gets three template positions (1–18). For each position, they click the base once, twice and three times, and record the polypeptide and the mutation type each time. Collect the results on the board. Some examples students will find:

Change	mRNA codon	Polypeptide	Type shown
Position 5: A → T	UUU → UAU	Met–Tyr–Pro–Asn–Gly	missense
Position 5: A → G	UUU → UCU	Met–Ser–Pro–Asn–Gly	missense
Position 9: C → A	CCG → CCU	Met–Phe–Pro–Asn–Gly	silent
Position 1: T → A	AUG → UUG	Met–Ala (from a later AUG)	start codon lost

Across all 54 possible substitutions in this gene, the simulation gives 36 missense (8 of them turn the stop codon into an amino acid), 9 silent and 9 start-codon changes. None is nonsense: no single change turns an earlier codon into UAA, UAG or UGA. Eight of the nine silent ones sit at positions 6, 9, 12 and 15, the third base of a codon. Ask: "Why the third base?" The code is degenerate: many amino acids have several codons that differ only in the last base. Check P2.

5. Frameshifts (7 minutes)

In the starting-values panel, pairs set Mutation to Insertion of one nucleotide pair, position 5, base G. Then they try Deletion of one nucleotide pair at position 5:

Mutation	mRNA codons read	Polypeptide
None	AUG UUU CCG AAU GGC UGA	Met–Phe–Pro–Asn–Gly
Insert G after position 5	AUG UUC UCC GAA UGG CUG	Met–Phe–Ser–Glu–Trp–Leu
Delete position 5	AUG UUC CGA AUG GCU	Met–Phe–Arg–Met–Ala

Every codon after the change is read in a new frame, and in both cases the ribosome never meets a stop codon before the gene ends. Ask: "Which do you expect to be more harmful, a substitution or an insertion? Why?"

6. A nonsense mutation (5 minutes)

Open the "Nonsense gene" link. One codon differs from the default, so the protein is Met–Phe–Pro–Tyr–Gly. Pairs click position 12 three times:

• Once (A → T): codon UAU becomes UAA, a stop. The chain is just Met–Phe–Pro: a nonsense mutation.
• Twice (A → G): UAC, still Tyr. Silent.
• Three times (A → C): UAG, another stop. Nonsense again.

One base, three outcomes.

7. Exit check (5 minutes)

Use questions 3–5 of the set below. Then open View answers and show the Prediction and After columns for P1 and P2 side by side. The formative assessment guide explains how to use those columns to plan the next lesson.

Question set for this lesson

Enter these on the simulation's Questions tab. Suggested Instructions for students: "Use Next step to go one base at a time. Press Reset before each new test. Click a template base to change it."

1. Multiple choice · Before, as a prediction · Ask again after the simulation

• Question: "Which mRNA codon is made from the template triplet TAC?"
• Options: TAC / ATG / AUG (correct) / UAC
• Explanation: "RNA polymerase pairs each template base with its partner: T with A, A with U, C with G. RNA uses uracil (U) instead of thymine (T). So TAC gives AUG, the start codon for methionine."

2. Multiple choice · Before, as a prediction · Ask again after the simulation

• Question: "One base in a gene is replaced by another. Does the protein always change?"
• Options: Yes, always / No, sometimes the protein stays the same (correct) / No, a single base never matters / Only if the new base is A
• Explanation: "The genetic code is degenerate: several codons code for the same amino acid. In the default gene, changing CCG to CCU still gives proline, a silent mutation. Other single changes swap one amino acid (missense) or create a stop (nonsense)."

3. Number · After the simulation

• Question: "Run the default gene to the end. How many amino acids does the finished polypeptide have?"
• Answer: 5, tolerance ± 0
• Explanation: "The mRNA has six codons: AUG UUU CCG AAU GGC UGA. The first five code for Met, Phe, Pro, Asn and Gly. UGA is a stop codon, which ends the chain without adding an amino acid."

4. Multiple choice · After the simulation

• Question: "Click position 9 of the template strand once, so C becomes A. What type of mutation is this?"
• Options: silent (correct) / missense / nonsense / frameshift
• Explanation: "The codon changes from CCG to CCU. Both code for proline, so the polypeptide stays Met–Phe–Pro–Asn–Gly. A change that leaves the protein unchanged is a silent mutation."

5. Short answer · After the simulation

• Question: "Why does inserting one base near the start of a gene usually change the protein more than substituting one base?"
• Accepted answers (optional): leave empty and read the answers yourself.
• Model answer: "The ribosome reads the mRNA in groups of three with no gaps. An extra base shifts the reading frame, so every codon after it changes, and a stop codon can be lost or appear early. A substitution changes at most one codon."

Questions 1 and 2 are the predictions, asked again after the simulation. For tips on wording questions like these, see writing good questions for virtual labs.

Extension: switching genes on and off

The first simulation shows how a gene is read. The second shows how a cell decides whether to read it at all. On its Expression screen, a stretch of DNA carries three genes. Students drag molecules from a toolbox and follow hints at each step:

1. Drag a positive transcription factor onto the matching dashed outline in the gene's regulatory region.
2. Drag RNA polymerase onto the promoter. The mRNA grows and leaves the DNA.
3. Drag a ribosome onto the mRNA's 5' end. The polypeptide grows and folds into a protein.
4. Drag the protein into the collection box. Collecting one protein from each of the three genes completes the screen.

Each gene has its own rule. Gene 1 needs factor A. Gene 2 needs factor B, and its negative factor B⁻ blocks RNA polymerase. Gene 3 needs both C1 and C2. Ask: "Why does gene 3 need two factors?" It lets the cell switch the gene on only when two conditions are met. Students also find that one mRNA can be translated many times, and that the degrading enzyme removes it.

For older students, the Many Cells screen runs the same gene in up to 90 cells. Protein levels differ from cell to cell because molecular events are random, but the readout gives the expected mean. With the default settings it shows E[P] = 182 proteins. Doubling the protein degradation rate from 0.05 to 0.10 per minute halves it to 90.9. The cell-by-cell numbers change on every run, so ask for the pattern, not exact values.

Differentiation

Support:

• Give a half-filled mRNA row and a large-print codon table.
• Use only steps 2–4 and the substitutions at positions 5 and 9.
• Give sentence starters: "The codon changed from … to …, so the amino acid…"

Stretch:

• Explain why changing position 1 gives Met–Ala instead of a shorter version of the original protein.
• Change the stop codon (position 16, A → T gives AGA instead of UGA). The simulation calls this missense, but the chain now runs to the end of the gene. Biologists call it a stop-loss mutation. Why might it be harmful?
• Design a template strand of 12 bases that codes for Met–Trp–Lys and stop. Type it into Gene template strand to check.

English learners: the simulation is available in six languages. Create a second link in the student's language so the labels are familiar while the discussion stays in English.

Standards alignment

This lesson matches NGSS HS-LS1-1: construct an explanation based on evidence for how the structure of DNA determines the structure of proteins. The mutation activities support HS-LS3-2, on inheritable genetic variation arising from mutations. It fits the protein synthesis topic of GCSE Biology and the gene expression units of AP and IB Biology.

For more biology activities, see interactive biology lesson ideas. For classroom routines, see how to use interactive simulations in the classroom. If your students work best in a guided cycle, the 5E lesson plan guide shows how to wrap this activity in Engage, Explore and Explain phases.

FAQ

Why does the template start with TAC instead of ATG?

The simulation shows the template strand, read 3'→5'. TAC pairs with AUG on the mRNA. The complementary strand, ATG, has the same sequence as the mRNA, with T in place of U.

Can I use my own gene?

Yes. Type any sequence of A, T, G and C, from 6 to 60 bases, into Gene template strand in the starting values, and pin it on a link. Start it with TAC so translation begins at the first codon.

Why is there no nonsense mutation in the default gene?

None of the five codons before UGA is one base away from a stop codon, so no single substitution can end the chain early. Use the second link from step 6, where position 12 creates UAA or UAG.