HKDSE Biology｜Protein Synthesis Lesson-Prep Guide (for tutors)｜Transcription, translation, codons, mutation and polypeptides

2026-11-03

Tutor lesson-prep guides · Secondary Biology

Students can say that DNA makes protein, yet cannot say who does it, where, or how many bases buy one amino acid, and cannot explain why a single missing base is worse than a substitution. This lesson uses three numbers and one formula to make transcription and translation checkable.

Objectives — Students can: (1) derive 64 codons from four bases read three at a time, identify 3 stop signals and 61 coding codons for 20 amino acids, and explain why two bases per codon would fail; (2) state the location, template and product of transcription (nucleus, DNA strand, mRNA) and translation (ribosome, mRNA, polypeptide); (3) calculate that a 150-amino-acid polypeptide needs a 453-base coding region including the stop codon and forms 149 peptide bonds, then use this to explain why a point substitution and a frameshift differ in severity.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
DNA structure	Which base pairs with adenine in DNA?	Thymine, or uracil in RNA
Base pairing	Which mRNA base pairs with DNA template A?	Uracil
Organelles	Which organelle assembles proteins?	The ribosome, free or on rough ER
Enzymes and energy	Which enzyme carries out transcription?	RNA polymerase, with ATP

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0–6 min	Three numbers	64, 61, 3	Numbers before process
6–16 min	Why three bases	Two bases give only 16 combinations	Not enough for 20 amino acids
16–30 min	Transcription	DNA to mRNA in the nucleus	Template and product direction
30–44 min	Translation	mRNA to polypeptide at the ribosome	tRNA anticodon pairing
44–54 min	Calculation drill	150 amino acids	453 bases, 149 bonds
54–60 min	Mutation and exit check	Substitution versus deletion	Frameshift changes everything

03 Teaching script (read it out)

Scene 1: Derive 64 first, then split the three numbers

Have students compute it: 4 bases, three at a time, repeats allowed, so 4 times 4 times 4 equals 64. Then ask whether two would do: 4 times 4 is 16, fewer than 20 amino acids, so three are required. Split 64 into 3 stop signals and 61 coding codons, and note that 20 amino acids share 61 codons, which is why most amino acids have several codons. That degeneracy is exactly what explains why many substitutions are silent.

Ask: why are two bases per codon not enough? (Only 16 combinations, fewer than 20 amino acids)

Scene 2: Ask the same three questions twice

For both processes ask: where, what template, what product. Transcription: nucleus in eukaryotes, one DNA strand, mRNA. Translation: ribosome, mRNA, polypeptide. The third follow-up is who does the matching: the tRNA anticodon pairs with the mRNA codon, and the amino acids it carries are joined by peptide bonds. Insist that RNA uses U where DNA uses T; this is the most common lost mark in long answers.

Ask for three sentences on transcription: location, template, product

Scene 3: Close with peptide bonds and use them as a check

150 amino acids in one chain give 150 minus one, that is 149 peptide bonds. Each amino acid occupies one codon, plus one stop codon, so the coding region needs at least 453 equals 453 bases. That single question checks two habits: subtract one and multiply by three. Then mutation: a substitution alters one codon and may be silent; a deletion shifts every later codon, changing the whole downstream sequence, which is usually far more serious.

Ask for an example of why a deletion is worse than a substitution

04 Question types and worked steps

Type	Example	Steps	Answer
Codon arithmetic	Four bases read three at a time	Compute 4 times 4 times 4, subtract 3 stops, map to 20 amino acids	64, 61, 3
Transcription	DNA template 3′-TAC GG-5′	Pair complements, use U for A, write the mRNA	5′-AUG CC-3′
Translation and bonds	A polypeptide of 20 amino acids	Bonds equal n minus 1, bases equal (n plus 1) times 3	19 bonds, 63 bases
Mutation analysis	Base five is deleted	Find the shift point, rewrite later codons, state the effect	Frameshift, whole segment changes

Sanity check (teach this): Sanity checks: 64 is four cubed, never four times three; 61 plus 3 must equal 64; 20 amino acids are fewer than 61 codons, so redundancy is certain; peptide bonds are always one fewer than amino acids, so 20 amino acids give 19; bases equal (amino acids plus one) times three because a stop codon is needed; in eukaryotes transcription is nuclear and translation is cytoplasmic, never both at the ribosome; RNA uses U and DNA uses T, and mixing them is a conceptual error; the human gene count of about 20000 is an approximation, not a precise figure to argue over.

05 Independent practice (3 questions)

Question	Answer	What to watch
Calculate: 30 amino acids, how many bonds and coding bases?	29 bonds, 93 bases	(30 plus 1) times 3 is 93
Write: the mRNA codon transcribed from 3′-TAC-5′	5′-AUG-3′	A pairs with U, T with A, C with G
Judge: is a substitution at base three always harmful?	Not necessarily, it may be silent	Degeneracy must be named for the mark

06 Common mistakes and what to say

Mistake	Student writes	Say this
Computing 64 as 12	A student writes 4 times 3 equals 12	Tutor line: “A codon is three bases and repeats are allowed, so it is four times four times four, not four times three.”
Confusing where each process happens	A student says both happen at the ribosome	Tutor line: “Transcription in the nucleus, translation at the ribosome. One makes mRNA, one makes protein. Different place, different product.”
Counting peptide bonds as equal to amino acids	A student gives 20 bonds for 20 amino acids	Tutor line: “A chain has two free ends, so it is always one fewer: n minus one.”
Calling a frameshift a one-amino-acid change	A student says a deleted base changes one position	Tutor line: “A deletion is not one swapped letter, it shifts the whole frame, so every codon after it is rewritten.”

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — Copy five key terms from the protein synthesis chapter and define each in one sentence
Genetics and Mendel (S5 chapter) — Redo practice question 2 and add the genotype to phenotype link
Immunity and vaccination (S6 chapter) — Use this chapter to explain why an antibody counts as a protein

08 Exit check (last 5 minutes)

Question	Answer	Decision
How many codons come from four bases read three at a time?	64, of which 3 are stop signals	Wrong: 12 means the power was missed
Where do transcription and translation happen?	Nucleus and ribosome respectively	Wrong: both at the ribosome means location was missed
How many peptide bonds join 20 amino acids?	19	Wrong: 20 means n minus one was missed

09 Tutor FAQ

Which year group?

S6 Biology, genetics and molecular biology; S5 can extend into it after DNA structure.

How many figures must be memorised?

Three core numbers: 64, 61 and 3; bonds and bases always come from the formulas.

Is there practice attached?

The homework row links three site chapters, and the practice section carries answers and checks.

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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.