中文（繁體）
中文（简体）
English
DSE Biology | DNA and Protein Synthesis Lesson-Prep Guide (Tutor Edition) | Genetic Code, Mutation and Base Ratios
Students can chant DNA to RNA to protein but divide by four the moment you ask how many amino acids 300 bases give. This lesson turns the whole flow into three checkable steps.
Objectives — Students can (1) use complementary base pairing to work out all four base percentages from one; (2) say where transcription and translation happen and what each uses as a template; (3) convert gene length into amino acid count and approximate protein mass, and rank three mutations by impact.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Percentages | A is 30%; how much is left for the other three bases? | 70% |
| Base pairing | What pairs with A, and what pairs with G? | A-T, G-C |
| Division | What is 300 divided by 3? | 100 |
02 Lesson flow (60 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-8 min | Structure recap | Double helix, four bases, hydrogen bonds | Draw A-T and G-C pairs |
| 8-22 min | Base ratios | A equals T, G equals C, subtract from 100% | G = 20% |
| 22-36 min | Transcription | In the nucleus, mRNA off the template strand | Write a complementary mRNA |
| 36-50 min | Translation | At the ribosome, three bases per amino acid | 100 amino acids |
| 50-56 min | Mutation | Substitution, insertion, deletion | Rank the impact |
| 56-60 min | Exit check | Collect and mark three questions | Work independently |
03 Teaching script (read it out)
Scene 1: Base ratios: one subtraction is enough
In double-stranded DNA adenine always equals thymine and guanine always equals cytosine. With A at 30%, T is also 30%, so 40% is split evenly between G and C: G = C = 20%. On a single strand the rule does not hold; you can only say that this strand A matches T on the opposite strand.
Ask them to compute: if G is 22%, what is A? (44% is G plus C, so A is 28%.)
Scene 2: Transcription and translation: two locations, two templates
Transcription happens in the nucleus: mRNA is built off the template strand (A pairs with U, T with A, G with C, C with G). Translation happens at the ribosome in the cytoplasm: every three bases form a codon, each codon names one amino acid, and tRNA carries it in. So a coding region of 300 bases gives 300 / 3 = 100 codons and at most 100 amino acids, since a stop codon carries none.
Ask them to explain: why divide by three and not four (four bases, but the code is read in triplets).
Scene 3: Mutation: substitution versus frameshift
A substitution changes one codon: it may be silent, may swap one amino acid, or may create a stop codon. An insertion or deletion shifts every later codon, so the damage is usually far greater. A protein of 100 amino acids at about 110 Da each has a mass near 11,000 Da. For cell division: 10 rounds take one cell to 2 to the power 10, which is 1,024.
Remind them: when a question asks which mutation matters most, the answer is always a frameshift.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Calculation | A is 30%, find G | A=T, G=C, total 100% | 20% |
| Calculation | Amino acids from 300 bases | bases / 3 | 100 |
| Process | Write the complementary mRNA | A to U, T to A, G to C, C to G | Base by base |
| Judgement | Which mutation hurts most | Does the reading frame shift? | Insertion or deletion |
Sanity check (teach this): A must equal T and G must equal C, and the four must add to 100%; if they do not, you subtracted once only. The amino acid count must be a whole number: 300 / 3 = 100 exactly, but 301 bases gives 100 amino acids and one leftover base that codes for nothing.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| C is 18% of double-stranded DNA; find A | 32% | G is 18%, split the rest |
| Maximum amino acids from 300 bases | 100 | divide by 3 |
| An inserted base causes which mutation? | Frameshift | check the reading frame |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Treating one strand as both | A is 30% so T is 30% on the same strand | On one strand there is no partner; look across. |
| Dividing by four | Answering 75 amino acids | A codon is three bases, so divide by 3. |
| Swapping the two locations | Translation happens in the nucleus | Transcription in the nucleus, translation in the cytoplasm. |
| Forgetting the stop codon | Exactly 100 amino acids, always | Say at most; the stop codon carries no amino acid. |
07 Homework (paper, section and question numbers)
- DSE Biology practice paper (genetics and molecular biology) — Section B DNA and protein synthesis
- Secondary Biology chapter index — All chapters
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| G is 22% of DNA; find A | 28% | Three correct means move on to genetic engineering |
| Maximum amino acids from 300 bases | 100 | One wrong means re-teach the codon |
| Result of deleting one base | Frameshift mutation | Two wrong means restart scene one |
09 Tutor FAQ
Must students memorise the codon table?
DSE usually supplies it; the skill is reading triplets, not recalling 64 entries.
Where did the T go in mRNA?
RNA uses uracil instead of thymine, so DNA adenine pairs with mRNA uracil.
Is a silent mutation still a mutation?
Yes, the DNA sequence changed; the protein is unchanged so the phenotype usually is too.
Find a tutor — freeTutor Registration →
A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.
