DSE Biology | Cell Division Tutor Prep Guide | Mitosis, Meiosis and Chromosome Numbers

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students recite that mitosis makes two identical cells but cannot say why meiosis is needed, and they cannot place the moment the chromosome number halves. They have not separated DNA replication from the number of divisions. This lesson puts both into one table of numbers.

Objectives — Students can: (1) sequence the stages of mitosis and state the chromosome number at each; (2) separate what meiosis I and meiosis II each halve; (3) use 2n and n to count daughter cells and DNA content; (4) explain how crossing over and independent assortment create variation.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Chromosome structure	DNA plus protein	sister chromatids
Cell cycle	interphase and division	DNA replicates in S phase
Alleles	a pair of alleles on homologous chromosomes	Aa sits on two homologues

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Draw the number table	2n to 4n back to 2n	human 2n = 46
8-22 min	Mitosis	four stages in order	two cells with 46
22-36 min	Meiosis	two divisions, one replication	four cells with 23
36-46 min	Comparison	purpose, site, cell count	growth versus gametes
46-54 min	Variation	crossing over, assortment	2^23 = 8,388,608
54-60 min	Exit check	collect and mark	independent

03 Teaching script (read it out)

Scene 1: First move: write the number table before naming any stage

DNA replicates in S phase: the chromosome count stays at 46 but each chromosome now has two sister chromatids, so DNA goes from 2C to 4C. This is the cell students get wrong, because they read replication as doubling the chromosome number.
Mitosis: one division, two daughter cells with 46 chromosomes each, DNA back to 2C. Meiosis: two divisions, four daughter cells with 23 chromosomes each, DNA at 1C. Ask: after replication, has the chromosome number changed? (No, each one simply has a sister chromatid.)

Task: fill a four-row table of stage, chromosome count, DNA content and cell count.

Scene 2: Mitosis and meiosis: separate them by purpose, not by stage names

Mitosis serves growth, repair and asexual reproduction, so the daughters must be genetically identical: sister chromatids separate and homologues never pair.
Meiosis serves gamete production, so the number must halve for fertilisation to restore the diploid count: meiosis I separates homologous chromosomes, taking 2n to n, and meiosis II separates sister chromatids, taking DNA from 2C to 1C. Reminder: when you say halved, name meiosis I, because students assume each division halves it. Ask: what does meiosis II halve? (Sister chromatids, not homologues.)

Scene 3: Variation: make students derive 2^23 rather than quote it

With 23 homologous pairs, independent assortment gives 2^23 = 8,388,608 combinations before crossing over is counted. Crossing over happens in prophase I, when non-sister chromatids of homologous chromosomes exchange segments, reshuffling alleles along the same chromosome.
Keep crossing over apart from mutation: crossing over only rearranges existing alleles, while mutation creates new ones.

Task: an organism has four pairs, how many combinations? (Sixteen.) Task: write three differences between mitosis and meiosis.

04 Question types and worked steps

Type	Example	Steps	Answer
Ordering	stage diagrams to sequence	find interphase and S phase	interphase to telophase
Counting	chromosome number at a stage	replication adds no number	92 at anaphase
Comparison	mitosis against meiosis	purpose, cell count, number	two versus four daughters
Explanation	source of variation	crossing over plus assortment	2^23 = 8,388,608

Sanity check (teach this): Chromosome counts are even apart from gametes, so 45 needs a check on ploidy. Mitosis always gives two daughter cells and meiosis four; an answer of three means a stage was counted twice. DNA content doubles only in S phase and halves only at division. Combination counts use powers of two: 8,388,608 comes from 2^23, not 23 squared. For 2n = 46, n is 23.

05 Independent practice (3 questions)

Question	Answer	What to watch
Chromosome number in a human gamete	23	46 divided by 2
Daughter cells from one meiosis	4	two divisions
Combinations from 23 pairs	8,388,608	two to the power 23

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating replication as doubling the count	answers 92 chromosomes after S phase	Replication adds a sister chromatid, the count is still 46.
Halving at both meiotic divisions	answers 11.5 chromosomes	The second division halves DNA, not the chromosome count.
Calling crossing over mutation	says crossing over makes new genes	Crossing over swaps segments; only mutation makes new alleles.
Squaring instead of exponentiating	answers 23 squared	Each pair has two choices, so it is two to the power 23.

07 Homework (paper, section and question numbers)

Secondary biology chapter index — questions 2 and 5
Prep guide directory — other chapters in this subject
DSE biology practice papers — genetics set, questions 3-7

08 Exit check (last 5 minutes)

Question	Answer	Decision
Chromosome number in a human gamete	23	three correct means move to genetics
What meiosis I separates	homologous chromosomes	one wrong means reteach next lesson
Combinations from 23 pairs	8,388,608	two wrong means restart act one

09 Tutor FAQ

Why does anaphase of mitosis show 92 chromosomes?

Once sister chromatids separate each counts as a chromosome, so the number doubles briefly until the cell divides.

Does crossing over happen before meiosis I?

Yes, during prophase I, when non-sister chromatids of homologous chromosomes exchange segments.

Do plants undergo meiosis?

Yes, in pollen mother cells and embryo sac mother cells, by much the same process.

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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.