Cell Cycle and Cell Division ICSE Class 10: Interphase, Mitosis and Meiosis

What Cell Cycle and Cell Division is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Biology. Cell Cycle and Cell Division is in the 1. Basic Biology (i) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2027

Key concepts

The cell cycle: interphase and M phase

Interphase is the long period between two divisions. The cell is very active, not 'resting':

• G1 (first gap): the cell grows, makes proteins and RNA, and forms new organelles.
• S (synthesis): DNA replicates; each chromosome now has two sister chromatids joined at the centromere. The chromosome number does not change.
• G2 (second gap): the cell keeps growing and makes the proteins needed for division (such as those of the spindle).

M phase (mitotic phase): karyokinesis (division of the nucleus: mitosis) followed by cytokinesis (division of the cytoplasm).

Worked example

In which phase of the cell cycle is the amount of DNA doubled? Does the chromosome number change?

Try it first, then show the solutionSolution

1. DNA is copied in the S phase of interphase.
2. Each chromosome then has two chromatids held at one centromere, so it still counts as one chromosome.

Answer: S phase; the chromosome number stays the same.

Mitosis: prophase and metaphase

Prophase:

• Chromatin threads coil and condense into visible chromosomes, each of two sister chromatids joined at a centromere.
• The nucleolus and nuclear membrane disappear.
• In animal cells, the two centrioles move to opposite poles and spindle fibres form between them (with asters). Plant cells form a spindle without centrioles.

Metaphase:

• Chromosomes line up on the equatorial plate (middle of the cell).
• Spindle fibres attach to each chromosome at its centromere.
• Chromosomes are most clearly visible and countable at this stage.

Worked example

At which stage of mitosis are chromosomes best counted, and why?

Try it first, then show the solutionSolution

1. At metaphase, chromosomes are fully condensed (thickest and shortest).
2. They lie in one plane at the equator, separate from each other.

Answer: Metaphase.

Mitosis: anaphase, telophase and cytokinesis

Anaphase:

• Each centromere splits, so sister chromatids separate; each is now a daughter chromosome.
• Spindle fibres shorten and pull the daughter chromosomes to opposite poles (they look V- or J-shaped as they move, centromere first).

Telophase:

• Daughter chromosomes reach the poles and uncoil into chromatin.
• The nuclear membrane and nucleolus reappear around each set.
• The spindle fibres disappear.

Cytokinesis:

• Animal cell: a furrow (constriction) forms in the cell membrane at the equator and deepens inwards until the cell divides.
• Plant cell: a cell plate forms in the middle (from vesicles) and grows outwards to the walls, because the rigid cell wall cannot constrict.

Worked example

Name the stage in which (a) sister chromatids move to opposite poles, (b) the spindle fibres disappear.

Try it first, then show the solutionSolution

1. (a) The centromere splits and chromatids are pulled apart in anaphase.
2. (b) The spindle breaks down when new nuclei form in telophase.

Answer: (a) Anaphase (b) Telophase

Meiosis: a reduction division

Meiosis occurs in the reproductive cells (in the testes and ovaries of animals; in the anthers and ovules of flowering plants) to form gametes (or spores).

• One diploid (2n) cell divides twice to form four haploid (n) cells.
• The chromosome number is halved: this is why meiosis is called reduction division.
• When two gametes fuse at fertilisation, the diploid number is restored in the zygote, so the chromosome number stays constant from generation to generation.

The detailed stages of meiosis are not in the 2027 syllabus; know the outcome and its significance.

Worked example

A dog has 78 chromosomes in each body cell. How many will be in (a) a skin cell formed by mitosis, (b) a sperm, (c) the zygote?

Try it first, then show the solutionSolution

1. Mitosis keeps the number: 78.
2. Meiosis halves it for gametes: 39.
3. Fertilisation joins 39 + 39 = 78.

Answer: (a) 78 (b) 39 (c) 78

Homologous chromosomes and crossing over

Homologous chromosomes are a pair of chromosomes, one from the father and one from the mother, that are alike in size, shape and position of the centromere, and carry genes for the same characters at the same positions (loci).

During meiosis, homologous chromosomes pair up. Crossing over is the exchange of segments between non-sister chromatids of a homologous pair. The points where they cross are called chiasmata (singular: chiasma).

Crossing over produces new combinations of genes on the chromosomes, so the gametes, and hence the offspring, show variation. This is one reason why siblings (other than identical twins) differ.

Worked example

Why do children of the same parents look different from one another?

Try it first, then show the solutionSolution

1. Gametes are formed by meiosis.
2. Crossing over and the random sorting of homologous chromosomes give each gamete a new combination of genes.
3. Each child develops from a different pair of gametes.

Answer: Meiosis (with crossing over) produces genetically different gametes, giving variation.

Significance and differences: mitosis versus meiosis

Significance of mitosis: growth of the body; repair of injured tissues and replacement of worn-out cells (skin, blood); asexual (vegetative) reproduction; keeps the chromosome number and genetic make-up the same in all body cells.

Significance of meiosis: forms gametes; keeps the chromosome number constant across generations; produces variation through crossing over, which helps evolution.

Feature	Mitosis	Meiosis
Where	somatic (body) cells	reproductive cells (gamete mother cells)
Divisions	one	two
Daughter cells	two	four
Chromosome number	same as parent (2n → 2n)	halved (2n → n)
Pairing of homologues	absent	present
Crossing over	absent	present
Daughter cells	genetically identical	genetically different
Role	growth, repair	gamete formation, variation

Worked example

Give two differences between mitosis and meiosis based on the number of daughter cells and their genetic make-up.

Try it first, then show the solutionSolution

1. Mitosis: two daughter cells; meiosis: four.
2. Mitosis: daughter cells identical to the parent; meiosis: daughter cells genetically different (crossing over).

Answer: As above.

Cell Cycle and Cell Division in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Stages of Mitosis
• Mitosis and Meiosis: Differences and Significance
• The Cell Cycle: Interphase and M Phase

Cell Cycle and Cell Division: key facts

Chromosome number in mitosis

$2n\to 2n+2n$

Two identical diploid cells.

Chromosome number in meiosis

$2n\to n+n+n+n$

Four haploid cells.

Fertilisation restores diploidy

$n(\text{sperm})+n(\text{egg})=2n(\text{zygote})$

Humans: 23 + 23 = 46.

Order of the cell cycle

$G_1\to S\to G_2\to M$

Interphase = G1 + S + G2.

Cell Cycle and Cell Division: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Cell Cycle and Cell Division

The question patterns to expect, and the method that works for each.

Tricks to remember Cell Cycle and Cell Division

Short memory aids. Say each one aloud once, then use it on a question the same day.

• PMAT

  Prophase, Metaphase, Anaphase, Telophase: 'Please Make Another Twin'.

  Use it when: Order of mitosis stages.

• M for Middle, A for Apart, T for Two nuclei

  Metaphase: middle. Anaphase: apart. Telophase: two nuclei form.

  Use it when: Identifying stages.

• Meiosis Makes Me (gametes)

  MEiosis makes gametes, which make ME; MItosis makes MIrror copies.

  Use it when: Which division is which.

• Furrow in, plate out

  Animal cells pinch IN with a furrow; plant cells build a plate OUT to the walls.

  Use it when: Cytokinesis differences.

Cell Cycle and Cell Division: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 6-mark question split into 1 + 2 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Cell Cycle and Cell Division

The slips examiners see most often, and what to write instead.

• ✗Mistake: Wrong chromosome number in gametes

  CISCE examiners' comments, 2024: candidates confused the chromosome numbers in body cells and gametes when asked about sperms of an animal.

  Wrong working

  Body cell 48, so each sperm has 48.

  Right working

  Sperms are formed by meiosis: 48 ÷ 2 = 24.

  ✓Do this: Gametes are always half the body number.

  Usually costs: 1 mark.

• ✗Mistake: Misnaming anaphase

  CISCE examiners' comments, 2024: for a diagram of anaphase, candidates wrote metaphase or prophase, did not say chromatids are pulled to the poles, and gave wrong chromosome counts.

  Wrong working

  Stage: metaphase.

  Right working

  Anaphase: sister chromatids are separated and pulled to opposite poles.

  ✓Do this: Chromosomes moving apart = anaphase; give the reason in those words.

  Usually costs: 1-2 marks.

• ✗Mistake: Telophase versus anaphase

  CISCE examiners' comments, 2025: for the phase in which spindle fibres disappear, candidates wrote anaphase instead of telophase.

  Wrong working

  Spindle disappears in anaphase.

  Right working

  Spindle disappears in telophase.

  ✓Do this: Anaphase uses the spindle; telophase removes it.

  Usually costs: 1 mark.

• ✗Mistake: Poor metaphase diagram

  CISCE examiners' comments, 2025: candidates drew a plant cell instead of an animal cell, did not show duplicated chromosomes at the equator, omitted centrioles and labels, or drew anaphase.

  Wrong working

  Rectangular cell with single-stranded chromosomes scattered.

  Right working

  Round animal cell, centrioles at poles, spindle, four double-chromatid chromosomes on the equator.

  ✓Do this: Follow the draw-and-label checklist; label at least four parts.

  Usually costs: Up to 3 marks.

• ✗Mistake: Centrosome for centromere

  CISCE examiners' comments, 2024: candidates wrote centrosome for the point where duplicated chromatids are joined.

  Wrong working

  Chromatids are joined at the centrosome.

  Right working

  Chromatids are joined at the centromere.

  ✓Do this: Centro-MERE joins chromatids; centro-SOME holds centrioles.

  Usually costs: 1 mark.

Practice questions on Cell Cycle and Cell Division

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   The number of chromosomes in a human egg is:
   (a) 46
   (b) 23
   (c) 44
   (d) 92

   Hint

   Haploid.

   Answer

   (b)

2. Q2 · 1 mark · MCQBoard level

   In which phase does DNA replication take place?
   (a) G1
   (b) S
   (c) G2
   (d) M

   Hint

   Synthesis.

   Answer

   (b)

3. Q3 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): Mitosis produces genetically identical daughter cells.
   Reason (R): Crossing over occurs during mitosis.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Crossing over is a meiosis event.

   Answer

   (c)

4. Q4 · 3 marks · Very short answerBoard level

   Name: (a) the point where two chromatids are joined; (b) the structure formed in the middle of a dividing plant cell; (c) the division that forms gametes.

   Hint

   One word each.

   Answer

   (a) Centromere (b) Cell plate (c) Meiosis

5. Q5 · 2 marks · Short answerHarder board level

   Describe the events of prophase in an animal cell.

   Hint

   Four events.

   Answer

   Chromatin condenses into chromosomes (two chromatids each); nuclear membrane and nucleolus disappear; centrioles move to opposite poles; spindle fibres begin to form.

6. Q6 · 2 marks · Short answerHarder board level

   A cell with 10 chromosomes undergoes meiosis. How many cells are formed, and how many chromosomes will each have? How many chromatids does each chromosome have at the start of the division, and why?

   Hint

   Remember the S phase.

   Answer

   Four cells, 5 chromosomes each. At the start each chromosome has two chromatids because DNA was replicated in the S phase.

7. Q7 · 3 marks · Short answerBoard level

   Give the significance of mitosis (any three points).

   Hint

   Growth, repair...

   Answer

   Growth of the organism; repair and replacement of cells; asexual reproduction; maintains the same chromosome number in all body cells.

8. Q8 · 2 marks · Short answerHarder board level

   Explain how meiosis and fertilisation together keep the chromosome number constant in a species.

   Hint

   Halve then join.

   Answer

   Meiosis halves the number in gametes (2n → n); fertilisation fuses two gametes (n + n = 2n), restoring the diploid number in the zygote, generation after generation.

9. Q9 · 3 marks · Case-basedHOTS / competency

   A diagram shows a cell with no nuclear membrane and chromosomes lined up in the middle; the cell has a thick outer wall and no centrioles. (a) Name the stage. (b) Is it a plant or an animal cell? Give two reasons. (c) How will its cytoplasm divide?

   Hint

   Wall, no centrioles.

   Answer

   (a) Metaphase (b) Plant: cell wall present; no centrioles (c) By a cell plate forming in the middle and growing outwards.

10. Q10 · 2 marks · Short answerHOTS / competency

    Suggest why cancer cells, which divide by mitosis without control, produce a lump (tumour) but no new type of tissue.

    Hint

    Identical copies.

    Answer

    Uncontrolled mitosis produces many cells genetically identical to the parent cell, so they pile up as a mass of similar cells rather than forming a new type of tissue.

11. Q11 · 2 marks · Short answerHarder board level

    Differentiate between a chromatid and a chromosome at metaphase.

    Hint

    One of two halves.

    Answer

    At metaphase, a chromosome consists of two identical sister chromatids joined at a centromere; a chromatid is one of these two halves, which becomes a daughter chromosome after the centromere splits in anaphase.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Three stages of interphase?Show answer

   G1, S and G2.

2. 2.When is DNA replicated?Show answer

   S phase.

3. 3.Stage where chromosomes lie at the equator?Show answer

   Metaphase.

4. 4.Stage where centromeres split?Show answer

   Anaphase.

5. 5.Why is meiosis called reduction division?Show answer

   It halves the chromosome number (2n → n).

6. 6.Define homologous chromosomes.Show answer

   A pair of similar chromosomes, one from each parent, carrying genes for the same characters at the same loci.

7. 7.Define crossing over.Show answer

   Exchange of segments between non-sister chromatids of homologous chromosomes during meiosis.

8. 8.Cytokinesis in plant cells?Show answer

   By cell plate formation.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you state one event in G1, S and G2?
• ☐Can you identify each stage of mitosis from a description or diagram?
• ☐Can you draw metaphase in an animal cell with four chromosomes?
• ☐Can you give chromosome numbers for gametes, body cells and zygotes?
• ☐Can you give four differences between mitosis and meiosis?

Cell Cycle and Cell Division: frequently asked questions