Biology | Gene Mutation and Cancer Tutor Prep Guide | Two-hit Hypothesis and Cell Cycle Control

2026-11-06

Tutor lesson-prep guides · Secondary Biology

Cancer is not one mutation and done; it is the growth gates failing one after another. The two-hit probability explains why it mostly appears late in life.

Objectives — Students will be able to: (1) separate point mutations (substitution, insertion, deletion) from chromosome changes by their effect on protein; (2) state the opposite roles of oncogenes and tumour suppressor genes in a normal cell; (3) use the two-hit hypothesis to explain why both alleles of a suppressor must fail, and use the gap between 10^-5 and 10^-10 to explain age risk.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Mutation	At what level does a mutation act	A change in DNA base sequence, inherited by daughter cells
Alleles	Where do the two alleles come from	One from each parent
Cell cycle	What are the main stages	G1, S, G2 and M, with DNA copied in S

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Prerequisite check	Ask for the level of mutation, allele origin and the four stages	Oral answers
8-22 min	Types of mutation	Substitution, insertion, deletion; point against chromosome	Classify five examples
22-36 min	Two gene classes	Oncogene as accelerator, suppressor as brake	Draw the pedal diagram
36-46 min	Two-hit hypothesis	Both alleles must fail; compare 10^-5 with 10^-10	Students compute the probability
46-54 min	From mutation to tumour	Cycle escape, apoptosis escape, angiogenesis, metastasis	Order the four steps
54-60 min	Exit check	Three oral questions, wrong answers point back to an act	Work independently

03 Teaching script (read it out)

Scene 1: Act one: take every mutation down to the protein

Point mutations come in three shapes: substitution swaps one base, while insertion and deletion shift the reading frame so every amino acid after the site changes, which is usually the most damaging.
Students must reach the sentence the protein loses its shape and therefore its function. Stopping at the DNA sequence changing is not a full answer.
Chromosome changes are another level entirely: number (aneuploidy) or structure (deletion, duplication, inversion, translocation). Keep the two levels separate in every answer.

Ask the student: does this mutation change every amino acid after it, and why?

Scene 2: Act two: oncogene is the accelerator, suppressor is the brake

An oncogene comes from a mutated proto-oncogene and acts dominantly: one mutated allele is enough for the protein to stay active and keep telling the cell to divide.
A tumour suppressor acts recessively: both alleles must fail before the brake is lost, with p53 and RB as the standard examples.
That asymmetry is the whole two-hit hypothesis: an oncogene needs one hit, a suppressor needs two.

Ask the student: does this class need one hit or two, because it acts dominantly or recessively?

Scene 3: Act three: use probability to explain age

Set the mutation rate for a given gene at 10^-5 per division. Both alleles failing is 10^-5 times 10^-5, which is 10^-10, a hundred thousand times rarer.
So an organ must keep dividing for decades to accumulate two hits, which is why most cancers appear in older patients.
One cell dividing 30 times gives about 1.07 x 10^9 cells, roughly a centimetre across and just palpable; at one doubling every 25 days that takes about 750 days, over two years.

Ask the student: do the two hits multiply or add, and what is the arithmetic?

04 Question types and worked steps

Type	Example	Steps	Answer
Classify the mutation	Five DNA sequence changes	Decide substitution, insertion or deletion, then frame-shift or not	Frame shift must reach every later amino acid
Compare the roles	Oncogene against suppressor	Normal function, effect of mutation, dominance, example	p53 is a suppressor and must not be reversed
Two-hit calculation	Mutation rate 10^-5	Two hits give 10^-10, a factor of 10^5, link to age risk	Never write 2 x 10^-5
Cause and prevention	UV, smoking, HPV	One physical, one chemical, one biological agent, linked to DNA damage	Name the damage mechanism

Sanity check (teach this): Sanity check: 10^-5 times 10^-5 is 10^-10, which is 10^5, a hundred thousand times rarer than one hit, and that order of magnitude is the main explanation for age risk. A 24-hour cycle with an 8-hour S phase puts copying in 33.3% of the cycle, matching when mispairing happens. 2^30 is 1,073,741,824, about 1.07 x 10^9 cells, which is a tumour roughly a centimetre across; at one doubling per 30 days that is 750 days, matching the clinical picture of a tumour present for over two years.

05 Independent practice (3 questions)

Question	Answer	What to watch
Classify five mutations	Substitution, insertion, deletion, inversion, aneuploidy	Can each be taken to the protein level
Draw the pedal diagram	Normal and mutated states of both gene classes	Are dominant and recessive labelled correctly
Compute the two hits	From 10^-5 to 10^-10 with one conclusion sentence	Was multiplication used rather than addition

06 Common mistakes and what to say

Mistake	Student writes	Say this
Thinking an oncogene needs two hits	Writing both copies must be damaged	Oncogenes are dominant, so one allele is enough. The two-hit rule belongs to suppressors.
Filing p53 as an oncogene	p53 mutation makes cells divide	p53 is a suppressor that halts the cycle or triggers apoptosis after DNA damage; losing it is what removes the brake.
Adding the two-hit probabilities	10^-5 + 10^-5 = 2 x 10^-5	Independent events multiply: 10^-5 times 10^-5 is 10^-10, and only that gap explains the age pattern.
Stopping at DNA	Saying the sequence changed and nothing more	Carry it through mRNA, codon, amino acid, protein shape and function. Each missing layer costs a mark.

07 Homework (paper, section and question numbers)

Biology chapter index — Side by side with genetics and the cell cycle
Prep guide master index — Find the cell division chapter
Redo this chapter — Items 1 and 3, one run each

08 Exit check (last 5 minutes)

Question	Answer	Decision
Which class needs two hits	Tumour suppressors, because they act recessively	A wrong answer means act two did not land
With a rate of 10^-5, what is the two-hit chance	10^-10, a hundred thousand times rarer	Answering 2 x 10^-5 means the events were added
What does normal p53 do	Halts the cycle or triggers apoptosis after DNA damage	Prevents cancer is too vague to score

09 Tutor FAQ

Does the two-hit idea cover every cancer?

It describes suppressors, with retinoblastoma as the classic case. Oncogenes need one hit, and most cancers accumulate several mutations.

Should students memorise every carcinogen?

No. One physical (UV, ionising radiation), one chemical (tobacco, nitrosamines) and one biological (HPV, HBV), each linked to DNA damage, is enough.

How deep should metastasis go?

Three points: crossing the basement membrane, entering blood or lymph, and settling in another organ. Molecular names are not required.

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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.