Why does meiosis need two divisions?

The first separates homologous chromosomes and the second separates sister chromatids, giving haploid cells.

Does mitosis change the chromosome number?

No, the daughter cells keep the same diploid number as the parent.

S4 Biology Cell Division, Paper 1: chromosome numbers in mitosis and meiosis

2026-11-02

Tutor Prep Guide · Primary English

Paper 1 tracks the chromosome number from mitosis through meiosis to fertilisation, with every count worked out.

Focus　Mitosis keeps the number, meiosis halves it, and both the cell count and the chromosome count must be stated.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Biology: Cell Division
Level	S4

02　Questions (10 items)

FoundationHow many daughter cells does one mitotic division produce?
FoundationA cell with 2n = 8 divides by mitosis. How many chromosomes per daughter cell?
FoundationIn humans, how many cells does meiosis produce and how many chromosomes does each carry?
FoundationState the main product of mitosis and of meiosis.
IntermediateA cell with 2n = 6 completes meiosis. How many chromosomes per cell?
IntermediateA 20-hour cell cycle spends nine tenths of its time in interphase. How long is that?
IntermediateA cell divides every 30 minutes. How many cells after 2 hours, ignoring death?
ChallengeIn which stage does crossing over occur and why does it matter?
ChallengeHow does fertilisation restore the human chromosome number?
Error redoError: a student says meiosis makes two identical cells. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	Two	Mitosis divides once, so two daughter cells carry the same genes as the parent.
2	8	Chromosomes are copied and shared equally, so the number stays at 8.
3	Four cells with 23 each	Meiosis has two divisions, so 46 chromosomes halve to 23 in each of four cells.
4	Mitosis gives identical diploid cells; meiosis gives haploid gametes	The first keeps the chromosome number and the second halves it while creating variation.
5	3	Meiosis halves the number, so 6 divided by 2 gives 3.
6	18 hours	Twenty times 0.9 gives 18 hours, the growth phase of the cycle.
7	16	Two hours holds four half-hour steps, and 2 to the power 4 is 16.
8	Prophase of the first meiotic division, creating new gene combinations	Homologous chromosomes swap segments, so the four products differ genetically.
9	23 from the sperm plus 23 from the egg gives 46	Two haploid gametes fuse into one diploid zygote.
10	It makes four haploid cells with different gene combinations	Two divisions plus crossing over give four haploid cells with different combinations.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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