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ICSE Class 10 · Biology · Chapter 3

Structure of Chromosomes, Cell Cycle and Cell Division

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Chromosomes - Structure and Types

  • Chromosome: Thread-like structure made of DNA and proteins (histones) carrying genetic information. Visible during cell division.
  • Discovery: Discovered by Waldeyer (1888). Name derived from 'chroma' (color) and 'soma' (body) as they take up stains.
  • Structure: Each chromosome has: (1) Chromatids - two identical halves joined at centromere, (2) Centromere - constriction point holding chromatids, (3) Telomeres - protective caps at ends.
  • Chromatin vs Chromosome: Chromatin is loosely coiled DNA-protein complex in non-dividing cells. During division, chromatin condenses to form chromosomes.
  • Types by Centromere Position: (1) Metacentric - centromere in middle (V-shaped), (2) Sub-metacentric - centromere slightly off-center (L-shaped), (3) Acrocentric - centromere near one end (J-shaped), (4) Telocentric - centromere at end (I-shaped).
  • Chromosome Number: Species-specific. Humans: 46 (23 pairs), Fruit fly: 8, Rice: 24, Dog: 78.
  • Autosomes vs Sex Chromosomes: Autosomes control body characteristics (44 in humans). Sex chromosomes determine sex (XX in females, XY in males).
  • Diagram Importance: Structure of chromosome showing chromatids, centromere, and arms is crucial for ICSE.

DNA - The Genetic Material

  • DNA (Deoxyribonucleic Acid): Hereditary material carrying genetic information. Located in nucleus (also in mitochondria and chloroplasts).
  • Structure: Double helix discovered by Watson and Crick (1953). Two polynucleotide chains twisted around each other.
  • Components: (1) Sugar - deoxyribose, (2) Phosphate group, (3) Nitrogenous bases - Purines (Adenine, Guanine) and Pyrimidines (Thymine, Cytosine).
  • Base Pairing: Chargaff's Rule - A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds). Complementary base pairing.
  • Functions: (1) Storage of genetic information, (2) Replication - making copies for cell division, (3) Transmission of traits from parents to offspring, (4) Protein synthesis through transcription and translation.
  • Gene: Segment of DNA coding for a specific protein or trait. Humans have ~20,000-25,000 genes.

Cell Cycle - Phases and Regulation

  • Cell Cycle: Series of events from one cell division to the next. Divided into Interphase and M phase (Mitotic phase).
  • Interphase (90% of cycle): Cell grows and prepares for division. Subdivided into: (1) G₁ phase (Gap 1) - cell growth, protein synthesis, (2) S phase (Synthesis) - DNA replication, chromosome duplication, (3) G₂ phase (Gap 2) - further growth, organelle duplication, preparation for mitosis.
  • M Phase (10% of cycle): Mitosis (nuclear division) + Cytokinesis (cytoplasmic division).
  • G₀ Phase: Quiescent phase. Cells exit cycle and stop dividing. Example: Nerve cells, muscle cells.
  • Checkpoints: Control points ensuring proper cell division: (1) G₁ checkpoint - checks cell size and nutrients, (2) G₂ checkpoint - checks DNA replication, (3) M checkpoint - checks chromosome attachment to spindle.
  • Duration: Varies by cell type. Human cells: 24 hours (G₁: 11h, S: 8h, G₂: 4h, M: 1h).
  • Importance: Growth, repair, reproduction, maintaining chromosome number.

Mitosis - Equational Division

  • Mitosis: Type of cell division producing two identical daughter cells with same chromosome number as parent. Occurs in somatic (body) cells.
  • Significance: (1) Growth of organism, (2) Repair and regeneration of tissues, (3) Asexual reproduction, (4) Maintains chromosome number.
  • Phases of Mitosis: Prophase → Metaphase → Anaphase → Telophase (PMAT).
  • Prophase: (1) Chromatin condenses into chromosomes, (2) Each chromosome has 2 sister chromatids, (3) Centrioles move to opposite poles, (4) Nuclear membrane disintegrates, (5) Spindle fibers form.
  • Metaphase: (1) Chromosomes align at cell equator (metaphase plate), (2) Spindle fibers attach to centromeres, (3) Chromosomes most visible and condensed.
  • Anaphase: (1) Sister chromatids separate and move to opposite poles, (2) Centromeres divide, (3) Spindle fibers shorten, (4) Cell elongates.
  • Telophase: (1) Chromosomes reach poles and decondense, (2) Nuclear membrane reforms around each set, (3) Spindle disappears, (4) Two nuclei form.
  • Cytokinesis: Division of cytoplasm. In animal cells: cleavage furrow forms. In plant cells: cell plate forms.
  • Diagram Importance: Stages of mitosis with labeled diagrams are frequently asked in ICSE exams.

Meiosis - Reductional Division

  • Meiosis: Type of cell division producing four daughter cells with half the chromosome number. Occurs in reproductive cells to form gametes.
  • Significance: (1) Maintains chromosome number across generations, (2) Produces genetic variation through crossing over and random assortment, (3) Essential for sexual reproduction.
  • Two Divisions: Meiosis I (reductional) and Meiosis II (equational).
  • Meiosis I: (1) Prophase I - longest phase, homologous chromosomes pair (synapsis), crossing over occurs, (2) Metaphase I - bivalents align at equator, (3) Anaphase I - homologous chromosomes separate, (4) Telophase I - two haploid cells form.
  • Crossing Over: Exchange of genetic material between non-sister chromatids of homologous chromosomes during Prophase I. Increases genetic variation.
  • Meiosis II: Similar to mitosis. Sister chromatids separate. Results in four haploid cells.
  • Comparison: Meiosis I reduces chromosome number (diploid to haploid). Meiosis II is like mitosis (sister chromatids separate).
  • Diagram Importance: Stages of meiosis showing chromosome behavior and crossing over are crucial.

Mitosis vs Meiosis - Key Differences

  • Location: Mitosis in somatic cells; Meiosis in germ cells (reproductive organs).
  • Number of Divisions: Mitosis - one division; Meiosis - two divisions.
  • Daughter Cells: Mitosis - 2 identical diploid cells; Meiosis - 4 non-identical haploid cells.
  • Chromosome Number: Mitosis maintains (2n → 2n); Meiosis reduces (2n → n).
  • Genetic Variation: Mitosis - no variation (clones); Meiosis - variation through crossing over and random assortment.
  • Function: Mitosis for growth and repair; Meiosis for gamete formation.
  • Synapsis: Absent in mitosis; Present in meiosis (pairing of homologous chromosomes).
  • Crossing Over: Does not occur in mitosis; Occurs in Prophase I of meiosis.
  • Duration: Mitosis is shorter; Meiosis is longer (especially Prophase I).

Significance of Cell Division

  • Growth: Multicellular organisms grow by increasing cell number through mitosis. Single cell (zygote) → trillions of cells.
  • Repair and Regeneration: Damaged tissues repaired by mitosis. Example: Wound healing, bone fracture repair.
  • Reproduction: (1) Asexual reproduction through mitosis (binary fission, budding, fragmentation), (2) Sexual reproduction through meiosis (gamete formation).
  • Maintaining Chromosome Number: Mitosis maintains constant chromosome number. Meiosis reduces it in gametes; fertilization restores it.
  • Genetic Variation: Meiosis creates variation essential for evolution and adaptation through crossing over and independent assortment.
  • Cell Replacement: Old and dead cells replaced. Example: RBCs replaced every 120 days, skin cells every 2-4 weeks.

Abnormalities in Cell Division

  • Cancer: Uncontrolled cell division due to loss of cell cycle regulation. Cells divide continuously forming tumors.
  • Tumor: Mass of cells. Benign (non-cancerous, localized) or Malignant (cancerous, spreads).
  • Causes: Mutations in genes controlling cell cycle (oncogenes, tumor suppressor genes), carcinogens (tobacco, UV radiation, chemicals).
  • Chromosomal Abnormalities: Errors during meiosis causing: (1) Aneuploidy - abnormal chromosome number, (2) Down syndrome (Trisomy 21) - extra chromosome 21, (3) Turner syndrome (45,X) - missing X chromosome in females, (4) Klinefelter syndrome (47,XXY) - extra X chromosome in males.
  • Non-disjunction: Failure of chromosomes to separate during meiosis, causing aneuploidy.
  • Polyploidy: More than two sets of chromosomes. Common in plants, rare in animals.

Quiz

Test your knowledge

Multiple choice

  1. Question 1 of 20Knowledge
    The number of chromosomes in human somatic cells is:
  2. Question 2 of 20Knowledge
    The phase of cell cycle where DNA replication occurs is:
  3. Question 3 of 20Knowledge
    Mitosis produces:
  4. Question 4 of 20Knowledge
    Meiosis produces:
  5. Question 5 of 20Knowledge
    Crossing over occurs during:
  6. Question 6 of 20Knowledge
    The constriction point on a chromosome where sister chromatids are joined is called:
  7. Question 7 of 20Knowledge
    The double helix structure of DNA was discovered by:
  8. Question 8 of 20Knowledge
    In DNA, adenine pairs with:
  9. Question 9 of 20Knowledge
    The longest phase of mitosis is:
  10. Question 10 of 20Comprehension
    Chromosomes are most visible and condensed during:
  11. Question 11 of 20Knowledge
    Cytokinesis in plant cells occurs by:
  12. Question 12 of 20Assertion-Reasoning
    Assertion (A): Meiosis is called reductional division. Reason (R): Meiosis reduces chromosome number from diploid to haploid.
  13. Question 13 of 20Knowledge
    The pairing of homologous chromosomes during meiosis is called:
  14. Question 14 of 20Knowledge
    Down syndrome is caused by:
  15. Question 15 of 20Application
    Which cells remain in G₀ phase and do not divide?
  16. Question 16 of 20Knowledge
    The sex chromosomes in human males are:
  17. Question 17 of 20Comprehension
    Mitosis is important for:
  18. Question 18 of 20Knowledge
    The number of chromosomes in human gametes is:
  19. Question 19 of 20Comprehension
    Uncontrolled cell division leads to:
  20. Question 20 of 20Knowledge
    The phase where chromosomes align at the equator is:

0 of 20 answered