Cell Cycle and Cell Division ISC Class 11: Mitosis and Meiosis

What Cell Cycle and Cell Division is about

Weightage: Cell Cycle and Cell Division belongs to the unit Cell: Structure and Function, which CISCE weights at 15 of the 70 marks in the ISC Class XI Biology Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Biology syllabus, examination year 2028, Class XI unit weightage

Key concepts

C-value and DNA content through the cycle

The C-value is the amount of DNA contained in an unreplicated haploid chromosome set, i.e. in the nucleus of a gamete. It is a fixed amount for a species and is often given in picograms. Using it, we can track DNA through the cycle without confusing DNA amount with chromosome number. For a diploid organism (2n): in G1 the nucleus has 2n chromosomes and 2C DNA; in S phase DNA replicates, so at the end of S and through G2 it has 2n chromosomes and 4C DNA (each chromosome now has two sister chromatids joined at the centromere). After mitosis each daughter returns to 2n, 2C. In meiosis, after the S phase (4C), meiosis I gives two cells each with n chromosomes and 2C (each chromosome still has two chromatids), and meiosis II gives four cells each with n and 1C.

Worked example

A G1 nucleus of a plant contains 12 pg of DNA. How much DNA is present in (i) G2 (ii) a cell just after meiosis I (iii) a gamete?

Try it first, then show the solutionSolution

1. G1 = 2C = 12 pg, so C = 6 pg
2. (i) G2 = 4C = 24 pg; (ii) after meiosis I = 2C = 12 pg; (iii) gamete = 1C = 6 pg

Answer: 24 pg; 12 pg; 6 pg

Phases of the cell cycle: G1, S, G2, M and G0

A typical human cell in culture divides about once in 24 hours, of which mitosis takes only about an hour; yeast can complete a cycle in about 90 minutes. Interphase is the preparation phase, not a resting phase: (1) G1 (gap 1): between the end of mitosis and the start of DNA replication; the cell is metabolically active, grows and makes RNA and proteins, but does not replicate DNA. (2) S (synthesis): DNA replication; DNA doubles from 2C to 4C but the chromosome number stays 2n. In animal cells the centriole also duplicates in the cytoplasm. (3) G2 (gap 2): proteins needed for mitosis (such as tubulin) are made and growth continues. M phase: karyokinesis (division of the nucleus) followed by cytokinesis (division of the cytoplasm). G0 (quiescent stage): some cells exit the cycle from G1 and stop dividing, though they stay metabolically active; e.g. many adult heart muscle cells and nerve cells. Some G0 cells, such as liver cells, can be called back into the cycle when needed (after injury). In adult animals mitosis is usually seen only in diploid somatic cells, but plants can show mitosis in both haploid and diploid cells.

Worked example

In a root tip, 1000 cells are counted and 50 are in M phase. If the cell cycle lasts 24 hours, estimate the duration of M phase.

Try it first, then show the solutionSolution

1. The fraction of cells in a phase is roughly the fraction of time spent in it: 50/1000 = 0.05
2. Duration = 0.05 × 24 h = 1.2 h

Answer: About 1.2 hours (72 minutes)

Mitosis: prophase and metaphase

Mitosis follows S and G2. Prophase: (1) the chromatin condenses so that chromosomes become visible; each has two sister chromatids attached at the centromere; (2) in animal cells the centrosome (already duplicated) separates and the two move towards opposite poles, each radiating asters; (3) the mitotic spindle of microtubules begins to form; (4) by the end, the nucleolus, nuclear envelope, Golgi complex and ER are no longer seen. Metaphase: (1) the nuclear envelope has fully disappeared, so chromosomes lie free in the cytoplasm; (2) chromosomes are at their most condensed, which makes metaphase the best stage to study chromosome shape and number (karyotype); (3) spindle fibres attach to the kinetochores, the small disc-shaped structures at each centromere; (4) chromosomes line up on the equator, forming the metaphase plate, with one chromatid of each chromosome facing each pole.

Worked example

Why is colchicine, which blocks spindle formation, used before counting chromosomes in root-tip squashes?

Try it first, then show the solutionSolution

1. Without a spindle, chromosomes cannot move to the poles, so cells stop at metaphase
2. Metaphase chromosomes are maximally condensed and spread, making them easiest to count and study

Answer: It arrests cells at metaphase, the best stage for counting chromosomes

Mitosis: anaphase, telophase and cytokinesis

Anaphase: (1) the centromere of each chromosome splits, and the sister chromatids separate; each chromatid is now a daughter chromosome; (2) the daughter chromosomes are pulled to opposite poles by shortening spindle fibres, centromere first with the arms trailing (so they appear V-, J- or rod-shaped depending on centromere position). For a 2n = 16 cell, there are briefly 32 chromosomes in the cell during anaphase. Telophase: (1) chromosomes reach the poles and decondense into chromatin; (2) the nuclear envelope reforms around each group; (3) the nucleolus, Golgi and ER reappear. Cytokinesis: in animal cells, a furrow appears in the plasma membrane and deepens until it pinches the cell into two. In plant cells, which have a rigid wall, a cell plate forms in the centre from Golgi vesicles and grows outwards to meet the side walls; it becomes the middle lamella, and cell walls are laid on either side. Organelles such as mitochondria and plastids are shared between daughters. If karyokinesis is not followed by cytokinesis, a multinucleate cell (syncytium) results, e.g. the liquid endosperm of a tender coconut.

Worked example

Why does a plant cell divide by a cell plate rather than a furrow?

Try it first, then show the solutionSolution

1. Plant cells are surrounded by a rigid cellulose wall that cannot be pinched in
2. So a new wall (cell plate) is built from the centre outwards, starting with vesicles from the Golgi

Answer: The rigid cell wall prevents furrowing

Significance of mitosis

(1) Produces genetically identical diploid daughter cells, keeping the chromosome number constant in somatic cells. (2) Growth of multicellular organisms by increasing cell number. (3) Restores the nucleus-to-cytoplasm ratio: as a cell grows, a single nucleus cannot control a large mass of cytoplasm, so it divides. (4) Repair and replacement: cells of the upper layer of skin, the gut lining and blood are continually replaced. (5) Asexual and vegetative reproduction in many organisms (e.g. budding in yeast, vegetative propagation). (6) Mitosis in meristems (apical and lateral cambium) gives continuous growth of plants throughout life.

Worked example

A single zygote divides by mitosis five times, every cell dividing each time. How many cells result, and how do their chromosomes compare with the zygote's?

Try it first, then show the solutionSolution

1. Cell number doubles each round: 2⁵ = 32
2. Mitosis is equational, so each cell has the same chromosome number and genetic information as the zygote

Answer: 32 genetically identical diploid cells

Meiosis I: prophase I and its five substages

Meiosis occurs in diploid meiocytes (in animals during gamete formation; in plants during spore formation). Prophase I is long and complex. (1) Leptotene: chromosomes condense and become visible as long, thin threads; compaction continues through the stage. (2) Zygotene: homologous chromosomes begin to pair, a process called synapsis, aided by a protein framework, the synaptonemal complex; each pair is a bivalent (also called a tetrad, as it has four chromatids). (3) Pachytene: the four chromatids of each bivalent are clearly visible; recombination nodules appear where crossing over occurs, the exchange of segments between non-sister chromatids of homologous chromosomes, catalysed by the enzyme recombinase; it is the longest phase of prophase I. (4) Diplotene: the synaptonemal complex dissolves and homologues begin to separate, except at the points of crossing over, which appear as X-shaped chiasmata. In the oocytes of some vertebrates, diplotene can last for months or years. (5) Diakinesis: chiasmata move towards the ends (terminalisation); chromosomes are fully condensed; the spindle assembles; the nucleolus disappears and the nuclear envelope breaks down, marking the change to metaphase I.

Worked example

At which substage of prophase I does crossing over occur, and when do its results first become visible?

Try it first, then show the solutionSolution

1. Crossing over (exchange between non-sister chromatids) takes place in pachytene at recombination nodules
2. The results are seen in diplotene as chiasmata when the homologues start to separate

Answer: Occurs in pachytene; chiasmata become visible in diplotene

Meiosis I (later stages) and meiosis II

Metaphase I: bivalents line up on the equatorial plate; spindle fibres from opposite poles attach to the kinetochores of the homologous chromosomes. The orientation of each bivalent is random, which leads to independent assortment. Anaphase I: homologous chromosomes separate and move to opposite poles; sister chromatids stay together at their centromeres. This halves the chromosome number: it is the reductional step. Telophase I: nuclear membrane and nucleolus reappear; cytokinesis gives a dyad of cells. Interkinesis: a short gap between the two divisions; no DNA replication occurs. Meiosis II is like mitosis (equational): prophase II (chromosomes condense again, nuclear membrane disappears), metaphase II (chromosomes on the equator; spindle fibres attach to kinetochores of sister chromatids), anaphase II (centromeres split; sister chromatids move to opposite poles), telophase II (four nuclei form, followed by cytokinesis into a tetrad of four haploid cells).

Worked example

For a cell with 2n = 16, give the number of bivalents at metaphase I and the number of chromosomes and chromatids in each cell after telophase I.

Try it first, then show the solutionSolution

1. 16 chromosomes form 16 ÷ 2 = 8 bivalents
2. After meiosis I each cell has n = 8 chromosomes, each still with two chromatids, so 16 chromatids

Answer: 8 bivalents; 8 chromosomes and 16 chromatids per cell

Significance of meiosis and differences from mitosis

Significance: (1) Halves the chromosome number in gametes so that fertilisation restores the diploid number: the chromosome number stays constant from generation to generation in sexually reproducing organisms. (2) Produces genetic variation through crossing over (new combinations on one chromosome) and random (independent) assortment of homologues at metaphase I; with n = 23 in humans there are $2^{23}$ (over 8 million) possible combinations from assortment alone. (3) Variation is the raw material for evolution and adaptation. Differences: mitosis occurs in somatic cells, meiosis in meiocytes (germ cells); mitosis has one division giving two cells, meiosis has two divisions after one replication giving four; mitosis is equational (2n → 2n), meiosis reductional (2n → n); mitosis has a short, simple prophase with no pairing, meiosis has a long prophase I with synapsis, crossing over and chiasmata; in mitotic anaphase chromatids separate, in anaphase I homologues separate; daughter cells are identical after mitosis but genetically different after meiosis.

Worked example

Suppose meiosis were replaced by mitosis in gamete formation in humans. What would happen to the chromosome number over two generations?

Try it first, then show the solutionSolution

1. Gametes would be diploid (46); fertilisation would give 92 in the next generation
2. In the following generation the number would double again to 184

Answer: It would double every generation (46 → 92 → 184), which is why meiosis is essential

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

• Cell Cycle Phases and Mitosis
• Meiosis: Stages and Significance

Cell Cycle and Cell Division: key facts

DNA content through mitosis (diploid cell)

$G_1:2n,2C\to S\to G_2:2n,4C\to \text{mitosis}\to 2\times (2n,2C)$

DNA doubles in S; chromosome number does not.

DNA content through meiosis

$2n,4C\stackrel{\text{meiosis I}}{\to }2\times (n,2C)\stackrel{\text{meiosis II}}{\to }4\times (n,1C)$

No replication during interkinesis.

Time spent in a phase

$t_{\text{phase}}=\frac{\text{cells in phase}}{\text{total cells}}\times T_{\text{cycle}}$

Valid for an unsynchronised, steadily dividing population.

Cell number after mitotic generations

$N=2^k\text{cells after }k\text{ rounds};\text{division events}=N-1$

From one cell, every cell dividing each round.

Prophase I order

$\text{leptotene}\to \text{zygotene}\to \text{pachytene}\to \text{diplotene}\to \text{diakinesis}$

Condense, pair, cross, separate (chiasmata), terminalise.

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 school exam 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 A Tea'.

  Use it when: Order of mitosis.

• Lazy Zebras Prefer Delicious Dinners

  Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.

  Use it when: Prophase I order.

• Zip, Pass, Detach

  Zygotene zips homologues (synapsis); Pachytene passes segments (crossing over); Diplotene detaches except at chiasmata.

  Use it when: What happens in each substage.

• C counts DNA, n counts chromosomes

  S doubles C but not n; meiosis I halves n; meiosis II halves C.

  Use it when: DNA calculations.

• Anaphase I splits pairs, Anaphase II splits sisters

  Homologues part in I; chromatids part in II (as in mitosis).

  Use it when: Meiosis vs mitosis.

Cell Cycle and Cell Division: exam-level solved questions

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

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 1 + 1 + 2 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: Saying chromosome number doubles in S phase

  DNA amount and chromosome number are confused.

  Wrong working

  After S phase a 2n = 16 cell has 32 chromosomes.

  Right working

  It still has 16 chromosomes, each with two chromatids; DNA goes from 2C to 4C.

  ✓Do this: Count centromeres to count chromosomes.

  Usually costs: 1-2 marks.

• ✗Mistake: Calling interphase a resting phase

  Nothing visible happens to chromosomes.

  Wrong working

  Interphase is the resting phase of the cell.

  Right working

  Interphase is the most active preparation phase: growth, DNA replication and protein synthesis.

  ✓Do this: Only G0 is a non-dividing state, and even it is metabolically active.

  Usually costs: 1 mark.

• ✗Mistake: Putting crossing over in zygotene or diplotene

  Neighbouring substages are mixed up.

  Wrong working

  Crossing over occurs in diplotene.

  Right working

  Crossing over occurs in pachytene; chiasmata become visible in diplotene.

  ✓Do this: Pachytene = Pass segments.

  Usually costs: 1 mark.

• ✗Mistake: Saying meiosis II is reductional

  The halving is attributed to the final step.

  Wrong working

  Chromosome number halves in meiosis II.

  Right working

  Meiosis I is reductional; meiosis II is equational, like mitosis.

  ✓Do this: Homologues part in I.

  Usually costs: 1 mark.

• ✗Mistake: Diagrams with changing chromosome numbers

  Careless drawing across stages.

  Wrong working

  Metaphase with 4 chromosomes, anaphase with 3 at one pole.

  Right working

  Keep the count consistent: 2n at metaphase, 2n at each pole in anaphase.

  ✓Do this: Decide 2n before drawing and count every time.

  Usually costs: 1-2 marks per diagram.

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 · MCQExam level

   Synapsis of homologous chromosomes occurs in:
   (a) leptotene
   (b) zygotene
   (c) pachytene
   (d) diakinesis

   Hint

   Pairing.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   The enzyme involved in crossing over is:
   (a) ligase
   (b) recombinase
   (c) helicase
   (d) polymerase

   Hint

   Recombination.

   Answer

   (b)

3. Q3 · 1 mark · Very short answerExam level

   Name the structures to which spindle fibres attach.

   Hint

   On the centromere.

   Answer

   Kinetochores

4. Q4 · 1 mark · Very short answerExam level

   What is a syncytium? Give an example.

   Hint

   No cytokinesis.

   Answer

   A multinucleate cell formed when karyokinesis is not followed by cytokinesis, e.g. liquid endosperm of coconut.

5. Q5 · 2 marks · Short answerExam level

   State two events of G1 and two of G2.

   Hint

   Growth and preparation.

   Answer

   G1: cell growth; RNA and protein synthesis (no DNA replication). G2: synthesis of proteins for mitosis such as tubulin; continued growth.

6. Q6 · 2 marks · Short answerHarder exam level

   Distinguish between anaphase of mitosis and anaphase I of meiosis.

   Hint

   What separates?

   Answer

   Mitotic anaphase: centromeres split and sister chromatids move to opposite poles. Anaphase I: homologous chromosomes, each still with two chromatids, move to opposite poles; centromeres do not split.

7. Q7 · 2 marks · Short answerHarder exam level

   What are chiasmata? In which stage are they first seen and what is terminalisation?

   Hint

   X-shapes.

   Answer

   X-shaped points where homologous chromosomes remain joined after crossing over; first seen in diplotene. Terminalisation is the movement of chiasmata towards the ends of the chromosomes during diakinesis.

8. Q8 · 3 marks · Short answerHarder exam level

   State four points of significance of mitosis.

   Hint

   Growth, repair...

   Answer

   Growth of multicellular organisms; repair and replacement of cells (skin, gut lining, blood); maintains chromosome number and genetic identity; restores nucleo-cytoplasmic ratio; asexual and vegetative reproduction; growth from meristems in plants.

9. Q9 · 3 marks · Short answerHarder exam level

   State three points of significance of meiosis.

   Hint

   Constancy and variation.

   Answer

   Keeps chromosome number constant across generations by halving it in gametes; creates genetic variation by crossing over and independent assortment; provides variation for evolution and adaptation.

10. Q10 · 3 marks · Short answerHarder exam level

    A cell has 2n = 24. Give the number of (i) bivalents at pachytene (ii) chromatids at metaphase I (iii) chromosomes in each cell after meiosis II.

    Hint

    Pairs and halves.

    Answer

    (i) 12 bivalents (ii) 48 chromatids (iii) 12 chromosomes.

11. Q11 · 5 marks · Long answerHarder exam level

    (a) Describe the four phases of the cell cycle and G0. [3]
    (b) Define C-value and show how the DNA content changes in each phase. [2]

    Hint

    G1, S, G2, M.

    Answer

    (a) G1: growth, RNA and protein synthesis; S: DNA replication and centriole duplication; G2: proteins for mitosis made; M: karyokinesis and cytokinesis; G0: cells exit from G1 and stay non-dividing but active. (b) C-value is the DNA content of an unreplicated haploid set. G1 2C, S 2C → 4C, G2 4C, after M each daughter 2C.

12. Q12 · 5 marks · Long answerHarder exam level

    (a) Draw labelled diagrams of metaphase and anaphase of mitosis in an animal cell with 2n = 4. [3]
    (b) Why is mitosis called equational division? [2]

    Hint

    Keep 4 chromosomes.

    Answer

    (a) Metaphase: four chromosomes on the equator, each with two chromatids, spindle fibres from centrioles at both poles to kinetochores. Anaphase: four daughter chromosomes moving to each pole, V/J shaped. (b) Each daughter cell receives the same number and kind of chromosomes as the parent, so chromosome number is unchanged (2n → 2n).

13. Q13 · 2 marks · Short answerHarder exam level

    In a tissue with a 20-hour cell cycle, 10% of cells are in S phase at any moment. How long does S phase last?

    Hint

    Proportion × cycle.

    Answer

    0.10 × 20 h = 2 hours.

14. Q14 · 3 marks · Short answerHOTS / competency

    A drug blocks DNA replication. In which phase will treated cells accumulate, and why does this make it useful against fast-growing tumours but harmful to hair follicles and the gut lining?

    Hint

    Dividing cells.

    Answer

    Cells cannot complete S phase, so they accumulate at the G1/S boundary or in early S. Tumour cells divide rapidly, so they are hit hard. Hair follicle and gut lining cells also divide rapidly for normal replacement, so they are harmed too, while non-dividing cells in G0 are largely spared.

15. Q15 · 3 marks · Short answerHOTS / competency

    A mutant plant forms gametes in which homologues fail to pair in zygotene. Predict two consequences.

    Hint

    No bivalents.

    Answer

    Without synapsis there is no crossing over, so less variation. Without bivalents, homologues are not properly held together for orderly separation in anaphase I, so they move to the poles randomly; gametes get unbalanced chromosome numbers and many are non-viable, reducing fertility.

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.Longest phase of the cell cycle?Show answer

   Interphase (G1 usually the longest part).

2. 2.Phase where centriole duplicates?Show answer

   S phase.

3. 3.Best stage to count chromosomes?Show answer

   Metaphase.

4. 4.Plant cytokinesis?Show answer

   Cell plate formation.

5. 5.Protein framework for synapsis?Show answer

   Synaptonemal complex.

6. 6.Stage of crossing over?Show answer

   Pachytene.

7. 7.Gap between meiosis I and II?Show answer

   Interkinesis (no DNA replication).

8. 8.Reductional division?Show answer

   Meiosis I.

Quick revision checklist

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

• ☐Can you define C-value?
• ☐Can you give n and C for a cell at G1, G2 and after each meiotic division?
• ☐Can you describe G0 with an example?
• ☐Can you list the events of each mitotic stage?
• ☐Can you contrast plant and animal cytokinesis?
• ☐Can you describe all five substages of prophase I?
• ☐Can you explain how meiosis creates variation?
• ☐Can you give five differences between mitosis and meiosis?

Cell Cycle and Cell Division: frequently asked questions