Cell Cycle and Cell Division — Class 11 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 11 Biology chapter "Cell Cycle and Cell Division" — 7 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 11 Biology chapter "Cell Cycle and Cell Division" — 7 important questions with detailed answers for CBSE b…
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NCERT Solutions for Class 11 Biology — Cell Cycle and Cell Division. Step-by-step answers to all 7 textbook questions from this chapter, written for CBSE board preparation and free to use.
Cell Cycle and Cell Division — All 7 Questions Solved
Q1. Define the cell cycle and name its two main phases.
The cell cycle is the series of events occurring in a cell between one division and the next. It includes the period of growth and DNA replication (interphase) and the period of cell division (M phase). The two main phases are:
1. Interphase:
- G1 phase (gap 1): Cell growth, normal metabolic activities, preparation for DNA synthesis. Duration ~20 hours in human somatic cells.
- S phase (synthesis): DNA replication; histone proteins synthesized. Chromosome number doubles. Duration ~7 hours.
- G2 phase (gap 2): Continued growth, protein synthesis (especially tubulin for spindle formation), preparation for mitosis. Duration ~4 hours.
- G0 phase: Quiescent state; some cells exit cell cycle (differentiated cells like neurons, muscle cells) and do not divide again.
2. M phase (mitotic phase):
- Mitosis: Nuclear division; segregation of replicated DNA to daughter cells.
- Cytokinesis: Cytoplasmic division; formation of two daughter cells.
Q2. What is mitosis? Describe the phases of mitosis with key events in each.
Mitosis is the process of nuclear division resulting in two daughter nuclei, each with identical chromosome number and genetic content as the parent cell. It maintains genetic continuity. Phases (in order):
1. Prophase:
- Chromosomes condense and become visible (each composed of two sister chromatids joined at centromere).
- Centrioles move to opposite poles of cell; aster (radiating fibers) forms around each centriole.
- Spindle fibers (microtubules) begin to form from centrosomes.
- Nuclear envelope breaks down; nucleolus disappears.
- Chromosomes become randomly scattered in cell.
2. Metaphase:
- Spindle apparatus complete; spindle fibers extend from poles to kinetochores (protein structures at centromeres).
- Chromosomes align at the cell's equatorial plate (metaphase plate) due to pulling forces from spindle fibers.
- Each chromosome attached to spindle fibers from both poles.
- Checkpoint: Cell ensures all chromosomes are properly attached before proceeding.
3. Anaphase:
- Sister chromatids separate at centromere; each chromatid now considered an individual chromosome.
- Separated chromosomes move toward opposite poles of cell (pulled by spindle fibers attached to kinetochores).
- Cell elongates; spindle fibers shorten, pulling chromosomes apart.
- Two identical sets of chromosomes move to opposite poles.
4. Telophase:
- Nuclear envelopes reform around separated chromosome sets at each pole.
- Chromosomes decondense; become less visible.
- Spindle apparatus disappears; asters dissolve.
- Nucleolus reappears in each forming nucleus.
- Cytokinesis (cytoplasmic division) begins.
Result: Two daughter cells, each genetically identical to parent cell; diploid cells produce diploid cells.
Q3. What is cytokinesis? Describe differences between plant and animal cytokinesis.
Cytokinesis is the division of cytoplasm, resulting in two daughter cells after mitosis or meiosis. It occurs during or after telophase. Cytokinesis is distinct from mitosis (nuclear division).
Animal cell cytokinesis (furrow cleavage):
1. Cleavage furrow forms at cell equator; deep groove pinches cell in two.
2. Contractile ring of actin and myosin filaments forms beneath cell membrane at equator.
3. Actin and myosin interact, contracting the ring; contracts like a drawstring, pulling cell membrane inward.
4. Furrow deepens until cell completely separates into two daughter cells.
5. Organelles and molecules distributed randomly between daughters.
6. Cell wall absent (not needed in animal cells).
7. Duration: 20-30 minutes in mammalian cells.
Plant cell cytokinesis (cell plate formation):
1. Cell plate (new cell wall and membrane) forms at equator.
2. Spindle fibers remain at cell equator, forming phragmoplast.
3. Golgi-derived vesicles accumulate at equator along spindle remnants.
4. Vesicles fuse, forming a membrane-bound compartment (cell plate).
5. Cell plate expands outward toward cell periphery; actin filaments and myosin may aid contraction.
6. Cellulose and other wall materials deposited; forms primary cell wall.
7. Middle lamella (pectin) forms between new cell walls, joining cells.
8. Two daughter cells separated by new cell wall; remain connected via plasmodesmata.
9. Cell wall provides rigidity; prevents random distribution of organelles.
10. Duration: 40-50 minutes in plant cells.
Key differences:
Feature | Animal | Plant
Mechanism | Cleavage furrow (contractile ring) | Cell plate formation
Involves | Actin-myosin ring; no wall formation | Golgi vesicles; cellulose deposition
Result | Two separate cells touching externally | Two cells joined by wall and plasmodesmata
Shape | Roughly spherical daughter cells | Polygonal cells due to wall pressure
Q4. What is meiosis? Explain its significance in sexual reproduction.
Meiosis is a type of cell division producing four genetically distinct haploid (n) cells from one diploid (2n) cell. Occurs in germ cells (reproductive organs) for gamete formation. Significance in sexual reproduction:
1. Reduction division:
- Reduces chromosome number from diploid (2n) to haploid (n).
- In humans: Diploid cell (46 chromosomes) → four haploid cells (23 chromosomes each).
- When gametes fuse during fertilization, diploid number restored in zygote.
2. Generates genetic variation:
- Crossing over (recombination): Exchange of genetic material between homologous chromosomes during prophase I; creates new combinations of alleles on same chromosome.
- Independent assortment: Random distribution of maternal and paternal chromosomes to gametes; each gamete has different combination.
- Result: Each gamete is genetically unique; offspring are different from parents and siblings (except identical twins).
3. Enables sexual reproduction:
- Two gametes (sperm and egg) fuse to form zygote.
- Zygote has genetic material from both parents; combines parental genes.
- Offspring receive genetic material from two parents, not just one (asexual reproduction).
4. Promotes evolution:
- Genetic variation in population provides raw material for natural selection.
- Beneficial traits favored; harmful traits eliminated.
- Enables species to adapt to changing environments.
5. Repairs DNA damage:
- Homologous chromosomes can repair double-strand breaks via recombination.
- Ensures genetic stability.
Compare to mitosis: Mitosis produces two identical diploid cells; maintains genetic continuity. Meiosis produces four different haploid cells; generates variation.
Q5. Describe the stages of meiosis I and meiosis II with key events (5-mark answer).
Meiosis consists of two divisions: meiosis I (reduction division) and meiosis II (similar to mitosis). Stages:
MEIOSIS I (Reduction division; separates homologous chromosomes):
1. Prophase I (longest and most complex phase):
- Homologous chromosomes pair up (synapsis); each pair called bivalent (or tetrad; four chromatids total).
- Crossing over (recombination): Homologous chromosomes exchange segments of DNA; creates new combinations of alleles.
- Recombination nodules mark sites of crossing over.
- Nuclear envelope breaks down; spindle forms.
- Chromosomes move toward metaphase plate.
2. Metaphase I:
- Bivalents (paired homologous chromosomes) align at metaphase plate.
- Orientation of each bivalent is random; maternal and paternal chromosomes randomly face each pole.
3. Anaphase I:
- Homologous chromosomes separate and move to opposite poles.
- Each pole receives one chromosome from each bivalent (one chromosome per homologous pair).
- Chromosome number halved; each chromosome still consists of two sister chromatids.
4. Telophase I:
- Nuclear envelopes form around separated chromosome sets.
- Cytokinesis occurs; two haploid cells formed.
- Brief interkinesis (resting stage) between meiosis I and meiosis II; no DNA replication.
MEIOSIS II (Similar to mitosis; separates sister chromatids):
1. Prophase II:
- Chromosomes (each with two sister chromatids) condense.
- Spindle forms; nuclear envelope breaks down.
- Similar to mitotic prophase but with half the chromosome number.
2. Metaphase II:
- Chromosomes (not bivalents) align at metaphase plate.
- Sister chromatids attached to spindle fibers from both poles.
3. Anaphase II:
- Sister chromatids separate at centromere; move to opposite poles.
- Now called individual chromosomes.
- Spindle fibers shorten; chromosomes pulled toward poles.
4. Telophase II:
- Nuclear envelopes form; chromosomes decondense.
- Cytokinesis occurs.
Result of meiosis: Four haploid cells (gametes in animals, spores in plants), each genetically unique due to crossing over and independent assortment.
Q6. Explain the significance of crossing over and independent assortment in generating genetic variation.
Genetic variation is the differences in alleles (gene versions) between organisms; essential for evolution and population diversity. Two mechanisms in meiosis generate variation:
1. Crossing over (recombination):
Mechanism:
- Occurs during prophase I of meiosis.
- Homologous chromosomes pair (synapsis) as bivalents.
- Non-sister chromatids exchange segments of DNA; physically break and rejoin.
- Genetic material from one chromatid swaps with corresponding region of homologous chromatid.
- Creates new combinations of alleles on same chromosome (linked genes).
Significance:
- Produces new allele combinations that did not exist in parents.
- Each crossover event is essentially random (can occur at different points on chromosome).
- Increases genetic diversity within gametes; siblings have different allele combinations.
- More crossovers on long chromosomes → more variation.
- Explains how traits can "recombine" in offspring different from either parent.
Example: Parent has alleles AB on one chromosome and ab on homologous chromosome. Crossing over can produce Ab and aB combinations in gametes—new combinations not in parent.
2. Independent assortment:
Mechanism:
- Occurs during metaphase I and anaphase I.
- Each homologous pair orients randomly at metaphase plate; maternal or paternal chromosome can face either pole.
- When separated in anaphase I, each gamete receives random mixture of maternal and paternal chromosomes.
- Not balanced; gametes do not necessarily get "half" from each parent.
Significance:
- Creates different combinations of chromosomes in each gamete.
- For each chromosome pair, gamete has 50% chance of receiving maternal or paternal chromosome.
- With n chromosome pairs, 2^n different combinations possible.
- Humans: 23 chromosome pairs → 2^23 = 8.4 million possible gamete combinations for each parent.
- Two gametes fusing: ~70 trillion different genetic combinations possible in human offspring.
- Explains why siblings from same parents are different (except identical twins from same zygote).
Combined significance:
Crossing over and independent assortment together ensure:
1. Genetic variation in offspring; no two individuals identical (except clones and identical twins).
2. Population has diverse allele frequencies; some alleles may be advantageous in certain environments.
3. Natural selection can act on variation; beneficial traits increase in population, harmful traits decrease.
4. Enables evolution; populations adapt to environmental changes.
5. Explains individual differences in traits (height, intelligence, disease resistance).
Without these mechanisms, asexual reproduction would produce identical offspring; no variation means no evolution.
Q7. Describe cell cycle checkpoints and their role in cancer prevention (5-mark answer).
Cell cycle checkpoints are control mechanisms ensuring cells progress through cell cycle only when conditions are favorable and DNA is intact. Checkpoints prevent damaged or abnormal cells from dividing. Major checkpoints and their role in cancer prevention:
1. G1/S checkpoint (Restriction point):
- Occurs between G1 and S phase.
- Checks if cell is ready for DNA synthesis; assesses cell size, nutrient availability, growth signals.
- Protein: Retinoblastoma protein (Rb) prevents S phase entry if conditions unfavorable.
- E2F transcription factor released only when Rb phosphorylated; allows S phase entry.
- Decision: If conditions poor, cell enters G0 (quiescence) and does not divide.
- Cancer relevance: Rb protein often mutated in cancers (e.g., retinoblastoma), removing checkpoint; allows uncontrolled entry into S phase.
2. G2/M checkpoint (G2 checkpoint):
- Occurs between G2 and M phase; just before mitosis.
- Checks if DNA is properly replicated and undamaged; verifies all centrosomes duplicated.
- Protein: p53 ("guardian of the genome") detects DNA damage; activates p21, preventing cyclin-CDK complex formation.
- CDKs (cyclin-dependent kinases) required for mitosis progression.
- If DNA damaged: p53 activates repair mechanisms or triggers apoptosis if damage irreparable.
- Decision: If DNA intact, cell proceeds to mitosis. If damaged, cell attempts repair or dies.
- Cancer relevance: p53 gene mutated in >50% of cancers; loss of p53 allows damaged DNA to be passed to daughter cells.
3. Spindle checkpoint (metaphase checkpoint):
- Occurs during metaphase; just before anaphase.
- Checks if all chromosomes properly attached to spindle fibers; monitors tension on kinetochores.
- Proteins: Mad1, Mad2, Bub3 (SAC proteins—spindle assembly checkpoint) prevent anaphase progression if chromosomes misaligned.
- Securin (holds sister chromatids together) degraded only when all kinetochores under tension.
- Decision: If chromosomes misaligned, checkpoint arrests cell; once aligned, securin degraded and sister chromatids separate.
- Cancer relevance: Checkpoint dysfunction allows aneuploidy (abnormal chromosome numbers); many cancers are aneuploid.
Regulatory mechanisms:
- Cyclin-CDK complexes drive cell cycle progression:
G1: Cyclin D-CDK4/6 → phosphorylates Rb → S phase entry.
S: Cyclin E-CDK2 → DNA replication.
G2: Cyclin A, B-CDK1 → mitotic preparations.
M: Cyclin B-CDK1 → mitosis.
- CDK inhibitors (p21, p27) halt cycle when activated by checkpoint sensors.
- Checkpoint proteins (p53, Rb, SAC proteins) act as "brakes."
Role in cancer prevention:
1. Prevents cells with DNA mutations from dividing; prevents mutation accumulation.
2. Forces damaged cells to repair DNA or undergo apoptosis; removes abnormal cells.
3. Ensures equal chromosome distribution; prevents aneuploid cells (which often are cancerous).
4. p53 activation by checkpoints triggers apoptosis if damage irreparable; prevents tumor formation.
5. Rb checkpoint prevents uncontrolled proliferation in response to growth signals.
Cancer development when checkpoints fail:
- Mutations in checkpoint genes (p53, Rb, CDK inhibitors) remove brakes on cell division.
- Cells with DNA damage continue dividing; mutations accumulate.
- Cells become immortal, accumulate oncogenes (promote growth) and lose tumor suppressors (suppress growth).
- Uncontrolled cell division leads to tumor formation and cancer.
- Approximately 90% of human cancers have p53 mutations, highlighting checkpoint importance.
Therapeutic implications: Cancer drugs target checkpoint regulators (e.g., CDK inhibitors) to halt cancer cell division.
Cell Cycle and Cell Division — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. What is the correct sequence of phases in the cell cycle?
- A. G1, S, G2, M
- B. M, G1, G2, S
- C. S, G1, G2, M
- D. G1, G2, S, M
Answer: A. G1, S, G2, M — The cell cycle consists of Interphase, which includes G1, S, and G2 phases, followed by the M-phase (mitosis or meiosis).
MCQ 2. During which phase of the cell cycle does DNA replication occur?
- A. G1 phase
- B. G2 phase
- C. S phase
- D. M phase
Answer: C. S phase — The S (synthesis) phase is characterized by the replication of DNA, where the amount of DNA per cell doubles.
MCQ 3. In which sub-stage of Prophase I of meiosis does crossing over between non-sister chromatids take place?
- A. Leptotene
- B. Zygotene
- C. Pachytene
- D. Diplotene
Answer: C. Pachytene — Crossing over, the exchange of genetic material between non-sister chromatids of homologous chromosomes, occurs during the pachytene stage of Prophase I.
MCQ 4. At what stage of mitosis do the chromosomes align themselves at the equatorial plate?
- A. Prophase
- B. Metaphase
- C. Anaphase
- D. Telophase
Answer: B. Metaphase — During metaphase, chromosomes become maximally condensed and align at the metaphase plate (equatorial plate) of the cell.
MCQ 5. Cells that do not divide further exit G1 phase and enter an inactive stage called:
- A. S phase
- B. G2 phase
- C. M phase
- D. G0 phase
Answer: D. G0 phase — Some cells, after exiting the G1 phase, enter a quiescent stage known as the G0 phase, where they remain metabolically active but no longer proliferate.
MCQ 6. Mitosis is essential for:
- A. Gamete formation
- B. Genetic variation
- C. Growth and repair
- D. Reduction of chromosome number
Answer: C. Growth and repair — Mitosis ensures the growth of multicellular organisms, replacement of dead cells, and repair of tissues by producing genetically identical daughter cells.
MCQ 7. Spindle fibers are primarily composed of which protein?
- A. Actin
- B. Myosin
- C. Tubulin
- D. Keratin
Answer: C. Tubulin — Spindle fibers are microtubules, which are polymers of alpha and beta tubulin proteins, responsible for chromosome movement during cell division.
MCQ 8. Meiosis I is characterized by the separation of:
- A. Sister chromatids
- B. Homologous chromosomes
- C. Non-homologous chromosomes
- D. Centromeres
Answer: B. Homologous chromosomes — During Anaphase I of meiosis, homologous chromosomes separate and move to opposite poles, while sister chromatids remain attached.
MCQ 9. How does cytokinesis differ in plant cells compared to animal cells?
- A. Plant cells form a cleavage furrow, animal cells form a cell plate
- B. Plant cells form a cell plate, animal cells form a cleavage furrow
- C. Both form a cleavage furrow
- D. Both form a cell plate
Answer: B. Plant cells form a cell plate, animal cells form a cleavage furrow — Plant cells form a cell plate in the center, which grows outwards to form a new cell wall, while animal cells divide by forming a cleavage furrow that deepens.
MCQ 10. If a diploid cell has 2n=16 chromosomes, how many chromosomes will each daughter cell have after mitosis?
- A. 8
- B. 16
- C. 32
- D. 4
Answer: B. 16 — Mitosis is an equational division, meaning the daughter cells receive the same number of chromosomes as the parent cell.
MCQ 11. A cell has 2n=4 chromosomes and 2C DNA content in G1 phase. What will be the chromosome number and DNA content after Meiosis I?
- A. n=2, 2C
- B. n=2, C
- C. 2n=4, 2C
- D. 2n=4, C
Answer: A. n=2, 2C — After Meiosis I, homologous chromosomes separate, resulting in cells that are haploid in terms of chromosome number (n=2), but each chromosome still consists of two sister chromatids, maintaining a DNA content of 2C.
MCQ 12. The division of the nucleus during cell division is known as:
- A. Cytokinesis
- B. Karyokinesis
- C. Plasmolysis
- D. Mitosis
Answer: B. Karyokinesis — Karyokinesis refers to the division of the nucleus, which precedes cytokinesis (division of the cytoplasm).
MCQ 13. Meiosis is crucial for:
- A. Maintaining diploidy in somatic cells
- B. Asexual reproduction
- C. Producing genetically identical cells
- D. Maintaining constant chromosome number across generations in sexually reproducing organisms
Answer: D. Maintaining constant chromosome number across generations in sexually reproducing organisms — Meiosis reduces the chromosome number by half, ensuring that when two gametes fuse during fertilization, the diploid state is restored, thus maintaining a constant chromosome number.
MCQ 14. Terminalization of chiasmata occurs during which stage of Prophase I?
- A. Zygotene
- B. Pachytene
- C. Diplotene
- D. Diakinesis
Answer: D. Diakinesis — Diakinesis is the final stage of meiotic prophase I, characterized by the complete terminalization of chiasmata, meaning the chiasmata move towards the ends of the homologous chromosomes.
MCQ 15. Which phase typically occupies more than 95% of the duration of the cell cycle?
- A. Prophase
- B. Metaphase
- C. Anaphase
- D. Interphase
Answer: D. Interphase — Interphase is the longest phase of the cell cycle, during which the cell grows, synthesizes DNA, and prepares for division.
MCQ 16. The splitting of the centromere and the separation of sister chromatids occur during:
- A. Metaphase
- B. Anaphase
- C. Telophase
- D. Prophase
Answer: B. Anaphase — During anaphase, the centromeres divide, and sister chromatids separate, moving to opposite poles of the cell as individual chromosomes.
MCQ 17. If the DNA content of a diploid cell in G1 phase is 'C', what will be the DNA content of each haploid cell after Meiosis II?
- A. C/2
- B. C
- C. 2C
- D. 4C
Answer: A. C/2 — If a diploid cell in G1 phase has a DNA content of 'C', then after the S phase, its DNA content doubles to '2C'. After Meiosis I, the DNA content in each daughter cell remains '2C' (though chromosome number is halved). After Meiosis II, sister chromatids separate, resulting in haploid cells with a DNA content of 'C/2'.
MCQ 18. Aster formation, characteristic of animal cell division, begins during:
- A. Prophase
- B. Metaphase
- C. Anaphase
- D. Telophase
Answer: A. Prophase — Aster formation, involving the radial microtubules, begins during prophase in animal cells as the centrioles move to opposite poles.
MCQ 19. How many cell divisions occur during meiosis?
- A. One
- B. Two
- C. Three
- D. Four
Answer: B. Two — Meiosis involves two sequential cycles of nuclear and cell division, Meiosis I and Meiosis II, but only a single cycle of DNA replication.
MCQ 20. Colchicine, an alkaloid, is a mitotic poison because it:
- A. Inhibits DNA replication
- B. Prevents spindle fiber formation
- C. Blocks cytokinesis
- D. Causes premature chromosome condensation
Answer: B. Prevents spindle fiber formation — Colchicine binds to tubulin and inhibits its polymerization into microtubules, thereby preventing the formation of the spindle apparatus and arresting cells in metaphase.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.