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Cell Cycle and Cell Division

NCERT Class 11 · Biology Based on NCERT Class 11 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains how cells grow and divide to produce new cells, a fundamental process for growth, repair, and reproduction in all living organisms. It covers the sequential events of the cell cycle, including interphase and the M phase, and details the two main types of cell division: mitosis (equational division) and meiosis (reduction division). The one big idea a student must walk away with is that the cell cycle is a tightly regulated sequence of events that ensures accurate duplication and distribution of genetic material to daughter cells, with mitosis producing identical diploid cells for growth and repair, and meiosis producing genetically diverse haploid gametes for sexual reproduction.
Key Concepts (27)
Cell cycle
The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells.
e.g. A human cell in culture takes approximately 24 hours to complete one cell cycle, while yeast completes it in about 90 minutes.
Interphase
The phase between two successive M phases, representing more than 95% of the duration of the cell cycle, during which the cell prepares for division by undergoing cell growth and DNA replication in an orderly manner.
e.g. In a 24-hour human cell cycle, interphase lasts about 23 hours, during which the cell grows and replicates its DNA.
M Phase (Mitosis phase)
The phase of the cell cycle when actual cell division or mitosis occurs, including nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).
e.g. In a 24-hour human cell cycle, the M phase lasts for only about one hour.
G1 phase (Gap 1)
The interval between mitosis and initiation of DNA replication, during which the cell is metabolically active and continuously grows but does not replicate its DNA.
e.g. A cell in G1 phase has a diploid (2n) number of chromosomes and a 2C amount of DNA.
S phase (Synthesis)
The period during which DNA synthesis or replication takes place, doubling the amount of DNA per cell from 2C to 4C, while the chromosome number remains the same (2n).
e.g. In animal cells, during S phase, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm.
G2 phase (Gap 2)
The phase after S phase during which proteins are synthesised in preparation for mitosis while cell growth continues.
e.g. A cell in G2 phase has 4C DNA content and 2n chromosomes, ready to enter mitosis.
Quiescent stage (G0)
An inactive stage of the cell cycle entered by cells that do not divide further, exiting G1 phase; these cells remain metabolically active but no longer proliferate unless called on to do so.
e.g. Heart cells in adult animals are in G0 phase and do not exhibit division, while skin cells divide occasionally to replace lost cells.
Mitosis
A type of cell division, also called equational division, in which the number of chromosomes in the parent and progeny cells is the same, resulting in two daughter cells with identical genetic complement.
e.g. Mitosis in onion root tip cells (2n=16) produces daughter cells each with 16 chromosomes.
Karyokinesis
The division of the nucleus during cell division, involving the separation of daughter chromosomes; it includes prophase, metaphase, anaphase, and telophase.
e.g. During karyokinesis in mitosis, the replicated chromosomes are distributed to two daughter nuclei.
Cytokinesis
The division of the cytoplasm following karyokinesis, resulting in two daughter cells.
e.g. In animal cells, cytokinesis occurs by the formation of a furrow in the plasma membrane that deepens and joins in the centre.
Prophase
The first stage of karyokinesis, marked by the initiation of condensation of chromosomal material, movement of centrosomes to opposite poles, and disappearance of nucleolus and nuclear envelope.
e.g. At the end of prophase, chromosomes are seen as two chromatids attached at the centromere, and the mitotic apparatus (asters and spindle fibres) begins to form.
Metaphase
The second stage of mitosis, characterised by complete disintegration of the nuclear envelope, alignment of all chromosomes at the equator (metaphase plate), and attachment of spindle fibres to kinetochores.
e.g. At metaphase, each chromosome is made up of two sister chromatids held together by the centromere, with kinetochores serving as attachment sites for spindle fibres from opposite poles.
Anaphase
The stage of mitosis during which centromeres split, sister chromatids separate and move to opposite poles, with the centromere leading and arms trailing.
e.g. At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously, and daughter chromatids begin migration towards opposite poles.
Telophase
The final stage of karyokinesis, during which chromosomes that have reached their poles decondense, nuclear envelopes reform around each set, and nucleolus, Golgi complex, and ER reappear.
e.g. At telophase, individual chromosomes can no longer be seen, and two daughter nuclei are formed.
Centrosome
An organelle that undergoes duplication during S phase and, in prophase, begins to move towards opposite poles of the cell, radiating out microtubules called asters.
e.g. The two asters together with spindle fibres form the mitotic apparatus.
Kinetochore
Small disc-shaped structures at the surface of the centromeres that serve as the sites of attachment of spindle fibres to chromosomes.
e.g. During metaphase, one chromatid of each chromosome is connected by its kinetochore to spindle fibres from one pole, and its sister chromatid to fibres from the opposite pole.
Metaphase plate
The plane of alignment of chromosomes at metaphase, located at the equator of the cell.
e.g. All chromosomes come to lie at the metaphase plate during metaphase.
Meiosis
A specialised kind of cell division that reduces the chromosome number by half, resulting in the production of haploid daughter cells; it involves two sequential cycles of nuclear and cell division (meiosis I and meiosis II) but only a single cycle of DNA replication.
e.g. Meiosis occurs during gametogenesis in plants and animals, leading to the formation of haploid gametes.
Meiosis I
The first meiotic division, initiated after DNA replication, involving pairing of homologous chromosomes, recombination between non-sister chromatids, and separation of homologous chromosomes, resulting in two haploid cells.
e.g. Prophase I is longer and more complex than mitotic prophase, subdivided into leptotene, zygotene, pachytene, diplotene, and diakinesis.
Meiosis II
The second meiotic division, resembling mitosis, in which sister chromatids separate, resulting in four haploid daughter cells.
e.g. Meiosis II includes prophase II, metaphase II, anaphase II, and telophase II.
Synapsis
The process of pairing of homologous chromosomes during zygotene stage of prophase I.
e.g. Synapsis is accompanied by the formation of the synaptonemal complex.
Bivalent (or tetrad)
The complex formed by a pair of synapsed homologous chromosomes.
e.g. During pachytene, the four chromatids of each bivalent become distinct and appear as tetrads.
Crossing over
The exchange of genetic material between non-sister chromatids of homologous chromosomes, occurring during pachytene and mediated by the enzyme recombinase.
e.g. Crossing over leads to recombination of genetic material and is visible as chiasmata at diplotene.
Chiasmata
X-shaped structures formed at the sites of crossing over, visible during diplotene when homologous chromosomes begin to separate but remain linked at crossover points.
e.g. Chiasmata terminalise during diakinesis.
Interkinesis
The stage between meiosis I and meiosis II, generally short-lived, with no DNA replication occurring.
e.g. Interkinesis is followed by prophase II.
Syncytium
A multinucleate condition that arises when karyokinesis is not followed by cytokinesis.
e.g. Liquid endosperm in coconut is a syncytium.
Cell-plate
A simple precursor that forms in the centre of a plant cell during cytokinesis, representing the middle lamella between the walls of two adjacent cells.
e.g. In plant cells, wall formation starts in the centre and grows outward to meet the existing lateral walls.
Formulas & Equations (4)
DNA content: 2C → 4C during S phase
Reads as: DNA content changes from 2C to 4C during S phase
Variables: C = amount of DNA in a haploid cell (arbitrary unit)
Use when: When asked about changes in DNA content during the cell cycle, especially in G1, S, G2, and after M phase.
⚠ Students often confuse DNA content (C) with chromosome number (n). Remember: DNA content doubles (2C to 4C) in S phase, but chromosome number remains the same (2n).
Chromosome number: 2n (diploid) throughout interphase and mitosis
Reads as: Chromosome number remains 2n (diploid) throughout interphase and mitosis
Variables: n = haploid number of chromosomes
Use when: When asked about chromosome number in G1, S, G2, and after mitosis.
⚠ Students think chromosome number doubles after S phase. It does not — only DNA content doubles. Chromosome number remains 2n until anaphase when chromatids separate.
After meiosis: 2n → n (haploid)
Reads as: After meiosis, chromosome number is reduced from diploid (2n) to haploid (n)
Variables: n = haploid number of chromosomes
Use when: When explaining the reduction division aspect of meiosis or calculating gamete chromosome number.
⚠ Students forget that meiosis I reduces chromosome number (homologous separation), while meiosis II separates sister chromatids (like mitosis).
Cell cycle duration: Human cell ≈ 24 hours; Yeast ≈ 90 minutes
Reads as: A typical human cell cycle takes about 24 hours, while yeast takes about 90 minutes
Variables: Duration varies by organism and cell type
Use when: When comparing cell cycle duration across organisms or when asked for typical values.
⚠ Students think all cells divide every 24 hours. The text specifies 'human cells in culture' and notes variation.
How It Works
Cell Cycle (overall sequence)
  1. Cell begins in G1 phase: metabolically active, grows, does not replicate DNA.
  2. Cell enters S phase: DNA replication occurs, DNA content doubles from 2C to 4C, chromosome number remains 2n.
  3. Cell enters G2 phase: proteins synthesised, cell continues to grow, preparation for mitosis.
  4. Cell enters M phase: mitosis (karyokinesis and cytokinesis) occurs, producing two daughter cells.
  5. Daughter cells may re-enter G1 or enter G0 (quiescent stage) if they do not divide further.
💡 Interphase (G1, S, G2) accounts for >95% of the cell cycle duration; M phase is only about 1 hour in a 24-hour cycle.
Mitosis (Karyokinesis)
  1. Prophase: Chromosomal material condenses; centrosomes move to opposite poles; nucleolus and nuclear envelope disappear; mitotic apparatus (asters + spindle fibres) forms.
  2. Metaphase: Nuclear envelope completely disintegrates; chromosomes align at metaphase plate; spindle fibres attach to kinetochores of sister chromatids from opposite poles.
  3. Anaphase: Centromeres split simultaneously; sister chromatids (now daughter chromosomes) move to opposite poles with centromere leading and arms trailing.
  4. Telophase: Chromosomes decondense; nuclear envelope reforms around each set; nucleolus, Golgi complex, and ER reappear.
💡 Anaphase is the key stage where centromeres split and chromatids separate — this is the only time chromosome number temporarily doubles (from 2n to 4n) before cytokinesis.
Cytokinesis
  1. In animal cells: A furrow appears in the plasma membrane, gradually deepens, and ultimately joins in the centre, dividing the cell cytoplasm into two.
  2. In plant cells: Wall formation starts in the centre of the cell with the formation of a cell-plate (precursor of middle lamella), which grows outward to meet the existing lateral walls.
  3. Organelles like mitochondria and plastids get distributed between the two daughter cells.
💡 Animal cells use a cleavage furrow; plant cells use a cell-plate that grows outward. If karyokinesis is not followed by cytokinesis, a syncytium (multinucleate condition) forms, e.g., liquid endosperm in coconut.
Meiosis I (Reduction Division)
  1. Prophase I: Subdivided into five stages — Leptotene (chromosomes become visible), Zygotene (synapsis of homologous chromosomes, formation of synaptonemal complex, bivalents/tetrads), Pachytene (crossing over between non-sister chromatids via recombinase, recombination nodules visible), Diplotene (dissolution of synaptonemal complex, chiasmata visible), Diakinesis (terminalisation of chiasmata, chromosomes fully condensed, meiotic spindle assembled, nucleolus and nuclear envelope disappear).
  2. Metaphase I: Bivalent chromosomes align on the equatorial plate; microtubules from opposite poles attach to kinetochores of homologous chromosomes.
  3. Anaphase I: Homologous chromosomes separate (sister chromatids remain associated at centromeres) and move to opposite poles.
  4. Telophase I: Nuclear membrane and nucleolus reappear; cytokinesis follows, producing a dyad of cells (haploid, but each chromosome still has two sister chromatids).
💡 Crossing over occurs in pachytene between non-sister chromatids, not sister chromatids. Homologous chromosomes separate in anaphase I, not sister chromatids.
Meiosis II (Equational Division)
  1. Prophase II: Nuclear membrane disappears; chromosomes become compact again (simpler than prophase I).
  2. Metaphase II: Chromosomes align at the equator; microtubules from opposite poles attach to kinetochores of sister chromatids.
  3. Anaphase II: Centromeres split simultaneously; sister chromatids move to opposite poles.
  4. Telophase II: Chromosomes get enclosed by a nuclear envelope; cytokinesis follows, resulting in four haploid daughter cells (tetrad of cells).
💡 Meiosis II resembles mitosis — sister chromatids separate. No DNA replication occurs between meiosis I and II (interkinesis).
Exam Traps — Don't Make These Mistakes
⚠ Confusing DNA content (C) with chromosome number (n)
✗ After S phase, the chromosome number doubles from 2n to 4n.
→ DNA content doubles (2C to 4C), but chromosome number remains the same (2n) because sister chromatids are still attached at the centromere and count as one chromosome.
✓ After S phase, the DNA content doubles from 2C to 4C, but the chromosome number remains 2n (diploid) until anaphase when centromeres split.
⚠ Thinking interphase is a 'resting' phase
✗ Interphase is the resting phase of the cell cycle when the cell does nothing.
→ Interphase is metabolically very active — the cell grows, replicates DNA (S phase), synthesises proteins (G2), and prepares for division. It is called 'resting' only in the sense that no visible division occurs.
✓ Interphase is the phase between successive M phases during which the cell prepares for division by undergoing cell growth and DNA replication in an orderly manner; it is not a resting phase.
⚠ Mixing up anaphase of mitosis and anaphase I of meiosis
✗ In anaphase I of meiosis, sister chromatids separate and move to opposite poles.
→ In anaphase I, homologous chromosomes separate (each with two sister chromatids still attached at the centromere). Sister chromatids separate only in anaphase II of meiosis (and in anaphase of mitosis).
✓ In anaphase I of meiosis, homologous chromosomes separate while sister chromatids remain associated at their centromeres. In anaphase of mitosis and anaphase II of meiosis, centromeres split and sister chromatids separate.
⚠ Forgetting that crossing over occurs between non-sister chromatids
✗ Crossing over occurs between sister chromatids of homologous chromosomes.
→ Crossing over involves exchange of genetic material between non-sister chromatids of homologous chromosomes, not between sister chromatids (which are identical).
✓ Crossing over occurs between non-sister chromatids of homologous chromosomes during pachytene of prophase I, mediated by the enzyme recombinase.
⚠ Confusing cytokinesis in plant vs. animal cells
✗ In plant cells, cytokinesis occurs by the formation of a furrow in the plasma membrane.
→ Plant cells have a rigid cell wall, so they cannot form a cleavage furrow. Instead, they form a cell-plate that grows outward from the centre.
✓ In plant cells, cytokinesis occurs by the formation of a cell-plate in the centre of the cell that grows outward to meet the existing lateral walls. In animal cells, a furrow appears in the plasma membrane that deepens and joins in the centre.
⚠ Thinking all cells in the body divide constantly
✗ All cells in the body divide continuously throughout life.
→ Some cells (e.g., heart cells) enter G0 phase and do not divide. Others divide only occasionally as needed (e.g., skin cells to replace lost cells).
✓ Some cells in adult animals, like heart cells, do not exhibit division and enter the quiescent stage (G0). Many other cells divide only occasionally to replace cells lost due to injury or cell death.
Compare & Contrast
Type of division
Mitosis
Meiosis
Mitosis is equational division (chromosome number remains same, 2n → 2n), while meiosis is reduction division (chromosome number halved, 2n → n).
What separates
Anaphase of mitosis
Anaphase I of meiosis
In anaphase of mitosis, centromeres split and sister chromatids separate. In anaphase I of meiosis, homologous chromosomes separate while sister chromatids remain attached at centromeres.
Mechanism of cytoplasmic division
Cytokinesis in animal cells
Cytokinesis in plant cells
In animal cells, a furrow appears in the plasma membrane that deepens and joins in the centre. In plant cells, a cell-plate forms in the centre and grows outward to meet the existing lateral walls.
Complexity and events
Prophase of mitosis
Prophase I of meiosis
Prophase of mitosis is relatively simple with condensation of chromosomes and formation of mitotic apparatus. Prophase I of meiosis is longer and more complex, subdivided into five stages (leptotene, zygotene, pachytene, diplotene, diakinesis) involving synapsis, crossing over, and chiasmata formation.
Exam Focus
Marks pattern: Typically 2-3 MCQs (1 mark each), 1-2 short answer questions (2-3 marks), and 1 long answer question (5 marks) on mitosis or meiosis stages. Total approximately 8-12 marks.
Most asked: Phases of cell cycle (G1, S, G2, M) and their key events · Stages of mitosis (prophase, metaphase, anaphase, telophase) with characteristic events · Meiosis I vs. Meiosis II — key differences and events · Crossing over and its significance · Cytokinesis in plant vs. animal cells · Significance of mitosis and meiosis · Changes in chromosome number (n) and DNA content (C) during cell cycle
Don't skip: Draw and label stages of mitosis or meiosis (5 marks diagram question); Compare and contrast mitosis and meiosis (3-5 marks); Describe the events of prophase I of meiosis with sub-stages (3-5 marks); Explain changes in chromosome number and DNA content during cell cycle (3 marks)
One-Line Revision (read aloud before the exam)
  1. The cell cycle consists of interphase (G1, S, G2) and M phase (mitosis and cytokinesis).
  2. Interphase accounts for more than 95% of the cell cycle duration and is a period of preparation, not rest.
  3. DNA replication occurs only during S phase, doubling DNA content from 2C to 4C but keeping chromosome number at 2n.
  4. Cells that do not divide enter the quiescent stage (G0) and remain metabolically active.
  5. Mitosis is equational division producing two genetically identical diploid daughter cells.
  6. The four stages of karyokinesis in mitosis are prophase, metaphase, anaphase, and telophase.
  7. In anaphase of mitosis, centromeres split and sister chromatids move to opposite poles.
  8. Cytokinesis in animal cells occurs by cleavage furrow; in plant cells, by cell-plate formation.
  9. Meiosis involves two divisions (meiosis I and II) but only one round of DNA replication.
  10. Meiosis I is reduction division where homologous chromosomes separate; meiosis II is equational where sister chromatids separate.
  11. Prophase I of meiosis is subdivided into leptotene, zygotene, pachytene, diplotene, and diakinesis.
  12. Crossing over occurs between non-sister chromatids during pachytene, mediated by recombinase.
  13. Synapsis is the pairing of homologous chromosomes during zygotene, forming bivalents or tetrads.
  14. Chiasmata are X-shaped structures visible at diplotene, representing sites of crossing over.
  15. Meiosis produces four haploid daughter cells, increasing genetic variability through recombination.
  16. Mitosis is significant for growth, repair, and maintaining nucleo-cytoplasmic ratio.
  17. Meiosis ensures conservation of chromosome number across generations in sexually reproducing organisms.
  18. If karyokinesis is not followed by cytokinesis, a multinucleate syncytium forms, e.g., coconut liquid endosperm.

MCQs — 10 Questions with Answers

Q1. During which phase of the cell cycle does DNA replication occur in animal cells?

  • A. G1 phase
  • B. S phase ✓
  • C. G2 phase
  • D. M phase

Answer: B — The S or Synthesis phase is the period during which DNA replication takes place, causing DNA content to double from 2C to 4C.

Q2. If a cell in G1 phase has 2n = 16 chromosomes and 2C DNA content, what will be its DNA content after S phase?

  • A. 2C with 8 chromosomes
  • B. 2C with 16 chromosomes
  • C. 4C with 16 chromosomes ✓
  • D. 4C with 32 chromosomes

Answer: C — During S phase, DNA content doubles (2C → 4C) but chromosome number remains unchanged at 2n = 16, as sister chromatids are still joined at the centromere.

Q3. Which of the following structures radiates out microtubules called asters during prophase?

  • A. Kinetochore
  • B. Centromere
  • C. Centrosome ✓
  • D. Nucleolus

Answer: C — The centrosome, which duplicates during S phase, moves towards opposite poles and radiates microtubules called asters to form the mitotic apparatus.

Q4. What is the main characteristic of cells in the G0 phase?

  • A. They are actively synthesizing DNA
  • B. They are undergoing mitosis
  • C. They remain metabolically active but do not divide ✓
  • D. They are preparing proteins for cell division

Answer: C — The G0 or quiescent phase is an inactive stage where cells remain metabolically active but no longer proliferate unless called upon by organismal requirements.

Q5. In a human cell culture, if the cell cycle duration is 24 hours and M phase lasts 1 hour, what percentage of the cell cycle is spent in interphase?

  • A. 4.17%
  • B. 50%
  • C. 95.8% ✓
  • D. 99%

Answer: C — Interphase duration = 24 - 1 = 23 hours; percentage = (23/24) × 100 = 95.8%, confirming interphase occupies more than 95% of the cell cycle.

Q6. Which statement is INCORRECT regarding metaphase of mitosis?

  • A. Nuclear envelope is completely disintegrated
  • B. Chromosomes are maximally condensed and clearly visible
  • C. Chromosomes are aligned at the metaphase plate at the cell equator
  • D. Sister chromatids have already separated and moved to opposite poles ✓

Answer: D — Sister chromatids separate during Anaphase, not Metaphase; in Metaphase they are still attached at the centromere and aligned at the equator.

Q7. During which stage of mitosis do the centrosomes move towards opposite poles of the cell?

  • A. Metaphase
  • B. Anaphase
  • C. Telophase
  • D. Prophase ✓

Answer: D — Prophase is marked by the movement of duplicated centrosomes towards opposite poles of the cell, where they radiate asters and spindle fibres.

Q8. Both statements: (A) Interphase is called the resting phase because no cell activity occurs, and (B) Mitosis is called equational division because daughter cells have the same number of chromosomes as the parent cell.

  • A. Both A and B are correct
  • B. A is correct, B is incorrect
  • C. A is incorrect, B is correct ✓
  • D. Both A and B are incorrect

Answer: C — Statement A is incorrect because interphase is metabolically very active with continuous cell growth and DNA replication; Statement B is correct as mitosis maintains chromosome number (2n → 2n).

Q9. In plant cells, where does DNA replication begin during S phase?

  • A. In the cytoplasm
  • B. In the nucleus ✓
  • C. In the nucleolus
  • D. In the centrosome

Answer: B — In animal cells (and by extension plant cells), DNA replication begins in the nucleus during S phase, while centriole duplication occurs in the cytoplasm.

Q10. HOTS: A biologist observes that in a particular organism, cells can divide by mitosis even when they are haploid. This organism is most likely a:

  • A. Mammal, as mammals allow haploid mitotic divisions
  • B. Plant, as plants show mitotic divisions in both haploid and diploid cells ✓
  • C. Fungus, as fungi always produce haploid cells
  • D. Male insect like a honeybee, as male insects are naturally haploid

Answer: B — The passage states plants can show mitotic divisions in both haploid and diploid cells (like during alternation of generations), whereas in most animals mitosis is restricted to diploid somatic cells with rare exceptions like male honeybees.

Flashcards

What is the cell cycle?

The sequence of events by which a cell duplicates its genome, synthesizes other cell constituents, and divides into two daughter cells.

What is the M phase?

The phase when actual cell division (mitosis) occurs, lasting about 1 hour and including karyokinesis followed by cytokinesis.

How long is interphase compared to M phase in human cells?

Interphase lasts more than 95% of the 24-hour cell cycle duration, while M phase lasts only about 1 hour.

What happens during S phase?

DNA replication occurs, causing DNA content to double from 2C to 4C, though chromosome number remains unchanged.

What is the G0 phase?

A quiescent or inactive stage where cells exit the cell cycle, remain metabolically active, but no longer divide.

What are sister chromatids held together by?

The centromere, which also contains disc-shaped structures called kinetochores.

What is the mitotic apparatus?

The structure formed by centrosomes radiating microtubules called asters plus spindle fibres together.

Why is metaphase the best stage to study chromosome morphology?

Because chromosomes are maximally condensed and spread through the cytoplasm, making them clearly visible under the microscope.

What happens to organelles like Golgi and ER during prophase?

They completely disintegrate, along with the nuclear envelope and nucleolus.

Why is mitosis called equational division?

Because the number of chromosomes in parent and progeny cells remains the same (2n → 2n).

Important Board Questions

Define the S phase of the cell cycle and state what happens to DNA content and chromosome number during this phase. [2 marks]

S phase is Synthesis phase when DNA replication occurs. State that DNA content doubles from 2C to 4C, but chromosome number remains unchanged (2n stays 2n) because sister chromatids remain joined at centromere.

Explain the structure of a metaphase chromosome and describe the arrangement of chromosomes during metaphase of mitosis. Why is metaphase considered the ideal stage for studying chromosome morphology? [5 marks]

Describe that metaphase chromosome consists of two sister chromatids joined at centromere with kinetochores at the surface. Explain that chromosomes are aligned at the metaphase plate (cell equator) and are maximally condensed. State that all chromosomes are spread through cytoplasm, making their individual morphology clearly visible under microscope, unlike other stages.

With reference to the cell cycle phases, explain how a single cell is able to grow into a large multicellular organism consisting of millions of cells. Include in your answer the duration and significance of interphase in relation to M phase. [6 marks]

Explain that a single cell undergoes cycles of growth (interphase: G1+S+G2) and division (M phase), with each parental cell dividing to form two daughter cells. Each daughter cell can itself grow and divide repeatedly. State that interphase (lasting >95% of 24-hour cycle) involves cell growth and DNA replication, preparing the cell for accurate division. M phase (~1 hour) ensures equational division maintaining chromosome number. Emphasize that this coordinated, controlled process of repeated cycles allows exponential increase in cell number and thus organism growth. Include that DNA replication during S phase and protein synthesis during G2 are critical for producing daughter cells with intact genomes.

Assertion-Reason Questions for Cell Cycle and Cell Division

Q1. Assertion (A):

The S phase of interphase is the period during which DNA synthesis and replication occur, causing the DNA content to double from 2C to 4C.

Reason (R):

During S phase, the chromosome number increases from 2n to 4n to accommodate the replicated DNA.

Answer: C — A is true but R is false — The assertion is correct—DNA content doubles during S phase while chromosome number remains constant. However, the reason is false because chromosome number does not increase during S phase; it remains at 2n. Sister chromatids are held at the centromere, so the chromosome count stays the same.

Q2. Assertion (A):

Prophase is marked by the condensation of chromosomal material and the movement of centrosomes to opposite poles of the cell.

Reason (R):

The centrosome duplication that began during S phase results in two centrosomes that move apart and form the spindle apparatus.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why centrosomes move to opposite poles during prophase. The duplication in S phase allows two centrosomes to exist and separate during M phase, establishing the spindle apparatus.

Q3. Assertion (A):

Metaphase is the stage of mitosis when chromosomes are maximally condensed and most easily observed under a microscope.

Reason (R):

During metaphase, the nuclear envelope disintegrates and chromosomes align at the metaphase plate, which is the optimal stage for studying chromosome morphology.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both the assertion and reason are true. Metaphase is indeed when chromosomes are most visible and condensed. However, the reason is not the correct explanation—the maximum condensation itself (not the alignment) is why they are most easily observed. The alignment is a consequence of metaphase, not the cause of visibility.

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Case-Based Questions (CBQ) for Cell Cycle and Cell Division

Case Study:

A medical research team studied a tumor in a 45-year-old patient. They analyzed cell samples from the tumor and found that many cells had a DNA content of 8C instead of the normal 4C found after S phase. Additionally, these cells were entering mitosis prematurely without completing proper DNA damage checkpoints. The researchers hypothesized that a mutation in cell cycle checkpoint genes was allowing uncontrolled cell division. When they examined the dividing cells, they counted 92 sister chromatids in one cell preparing for division, which is double the normal 46 chromosomes seen in healthy G1 phase cells.

Q1. What does a DNA content of 8C indicate about these tumor cells compared to normal cells at G1 phase?

  • A. The tumor cells have undergone two complete rounds of DNA replication without cell division ✓
  • B. The tumor cells are in G2 phase with doubled DNA content
  • C. The tumor cells have four times the normal DNA content suggesting genomic instability
  • D. The tumor cells have completed mitosis but failed to divide cytoplasm

Answer: A

Q2. If the 92 sister chromatids counted were from a single tumor cell preparing for mitosis, how many chromosomes would this cell have?

  • A. 46 chromosomes, each consisting of 2 sister chromatids ✓
  • B. 92 chromosomes, each consisting of 1 chromatid
  • C. 92 chromosomes, each consisting of 2 sister chromatids
  • D. 46 sister chromatids representing half the normal chromosome complement

Answer: A

Q3. Why is the premature entry into mitosis without completing DNA damage checkpoints particularly dangerous for the organism?

  • A. It ensures rapid cell division needed for tissue repair
  • B. It allows cells with damaged or replicated DNA to divide and pass mutations to daughter cells ✓
  • C. It shortens the cell cycle time which is beneficial for the patient
  • D. It prevents the accumulation of mutations in tumor cells

Answer: B

Q4. If normal human cells have 46 chromosomes and divide with an equational division producing 46 chromosomes in each daughter cell, what would be expected from these 8C tumor cells if they completed mitosis?

  • A. Four daughter cells each with 46 chromosomes and 4C DNA content
  • B. Two daughter cells each with 92 chromosomes and 4C DNA content ✓
  • C. Two daughter cells each with 46 chromosomes and 4C DNA content
  • D. One daughter cell with 92 chromosomes and 8C DNA content

Answer: B

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