NCERT Solutions for Class 11 Biology Chapter 10 Cell Cycle and Cell Division

Short Note:

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

• Phases of Cell Cycle: Divided into two basic phases:

  1. Interphase (Resting Phase): Occupies over 95% of cell cycle duration. Cell prepares for division through cell growth and DNA replication.

    - G1 Phase (Gap 1): Interval between mitosis and initiation of DNA replication. Cell is metabolically active and continuously grows without DNA replication.

    - S Phase (Synthesis): DNA replication occurs (DNA amount doubles from 2C to 4C, but chromosome number remains 2n). Centriole duplicates in cytoplasm of animal cells.

    - G2 Phase (Gap 2): Proteins required for mitosis are synthesized while cell growth continues.

  2. G0 Phase (Quiescent Stage): Inactive metabolic stage where cells exit G1 phase and cease division unless called upon to divide (e.g., heart cells, neurons).

Short Note:

• Equational Division: Number of chromosomes in parent and daughter cells remains equal (2n → 2n).

• Karyokinesis (Nuclear Division Stages):

  - Prophase: Chromatin condenses into distinct chromosomes; nucleolus, nuclear envelope, and Golgi complex disappear; mitotic spindle begins forming.

  - Metaphase: Chromosomes align along the metaphase plate (equator); spindle fibers attach to kinetochores of sister chromatids.

  - Anaphase: Centromeres split, sister chromatids separate and move toward opposite poles driven by spindle fiber shortening.

  - Telophase: Chromosomes reach poles and decondense; nuclear membrane, nucleolus, and organelles reappear around each set of chromosomes.

• Cytokinesis (Cytoplasmic Division):

  - Animal Cells: Forms a cell furrow in the plasma membrane that deepens centripetally.

  - Plant Cells: Formation of a cell plate (precursor of middle lamella) growing centrifugally from the center outwards.

Short Note:

• Reductional Division: Special division in germ cells that reduces chromosome number by half (2n → n), producing four haploid daughter cells.

• Meiosis I:

  - Prophase I (Subdivided into 5 substages):

    1. Leptotene: Chromosomes become visible and compact.

    2. Zygotene: Homologous chromosomes pair up (synapsis) to form synaptonemal complexes and bivalents/tetrads.

    3. Pachytene: Crossing over occurs between non-sister chromatids of homologous chromosomes mediated by the enzyme recombinase.

    4. Diplotene: Dissolution of synaptonemal complex; homologous chromosomes separate except at crossover sites forming X-shaped structures called Chiasmata.

    5. Diakinesis: Terminalisation of chiasmata; nuclear membrane breaks down.

  - Metaphase I, Anaphase I (homologous chromosomes separate, sister chromatids remain associated), Telophase I.

• Interkinesis: Short-lived stage between Meiosis I and Meiosis II (no DNA replication occurs).

• Meiosis II: Similar to a normal mitosis, separating sister chromatids to produce 4 haploid cells.

Short Note:

• Significance of Mitosis:

  - Ensures genetic stability by producing identical daughter cells.

  - Essential for growth of multicellular organisms and continuous cell repair/replacement (e.g., blood cells, gut lining, epidermis).

• Significance of Meiosis:

  - Conserves specific chromosome number of sexually reproducing species across generations.

  - Introduces genetic variations through crossing over and independent assortment, driving evolution.