Class 11 Biology Notes Chapter 10 (Cell cycle and cell division) – Biology Book

Alright class, let's dive into one of the most fundamental processes of life: how cells grow and divide. Chapter 10, 'Cell Cycle and Cell Division,' is crucial not just for understanding biology but also frequently appears in various government exams. Pay close attention to the phases, the key events in each, and the significance of Mitosis and Meiosis.

Chapter 10: Cell Cycle and Cell Division - Detailed Notes

1. The Cell Cycle:

• Definition: The sequence of events by which a cell duplicates its genome, synthesizes other constituents of the cell, and eventually divides into two daughter cells.
• Coordination: Cell growth (cytoplasmic increase) is a continuous process, but DNA synthesis occurs only during one specific stage. Replicated chromosomes are then distributed to daughter nuclei in a complex, ordered series of events.
• Duration: Varies between organisms and cell types (e.g., human cell divides approx. every 24 hours, yeast takes about 90 minutes).

2. Phases of the Cell Cycle:
The cell cycle is broadly divided into two main phases:

• A. Interphase:

  • The 'resting phase' is a misnomer; it's actually a period of intense growth and preparation for division.
  • Occupies >95% of the total cell cycle duration.
  • Sub-divided into three further phases:
    • G1 Phase (Gap 1):
      • Interval between mitosis (M phase) and initiation of DNA replication (S phase).
      • Cell is metabolically active and grows continuously.
      • Synthesizes RNA, proteins, and organelles.
      • Decides whether to divide or enter the quiescent stage (G0).
    • S Phase (Synthesis):
      • DNA replication occurs. The amount of DNA per cell doubles (from 2C to 4C if diploid), but the chromosome number remains the same (e.g., 2n).
      • In animal cells, centriole duplication also occurs in the cytoplasm.
      • Histone protein synthesis occurs.
    • G2 Phase (Gap 2):
      • Cell growth continues.
      • Proteins (like tubulin for spindle fibres) are synthesized in preparation for mitosis.
      • RNA synthesis continues.
      • Cell prepares to enter the M phase.

• Quiescent Stage (G0):

  • Cells that do not divide further (e.g., heart cells, neurons) exit the G1 phase and enter an inactive stage called G0.
  • Cells in G0 remain metabolically active but no longer proliferate unless called upon to do so depending on the requirement of the organism.

• B. M Phase (Mitosis Phase):

  • Represents the actual cell division phase.
  • Starts with nuclear division (Karyokinesis) and usually ends with the division of cytoplasm (Cytokinesis).
  • Most dramatic period of the cell cycle involving major reorganization of virtually all components of the cell.

3. Mitosis (Equational Division):

• Occurs in somatic (body) cells.

• Ensures that daughter cells receive the same number of chromosomes as the parent cell (hence, equational division).

• Important for growth, repair, and asexual reproduction in some organisms.

• Divided into four stages (PMAT):

  • i. Prophase:

    • Initiation of condensation of chromosomal material. Chromosomes become visible as distinct structures.
    • Each chromosome consists of two sister chromatids attached at the centromere.
    • Centrioles (in animal cells) move towards opposite poles.
    • Initiation of the assembly of the mitotic spindle (microtubules).
    • Nuclear envelope, nucleolus, Golgi complex, and ER start to disappear.

  • ii. Metaphase:

    • Nuclear envelope completely disintegrates.
    • Chromosomes are maximally condensed and most clearly visible.
    • Chromosomes align at the equator of the cell, forming the metaphase plate.
    • Spindle fibres from opposite poles attach to the kinetochores (disc-shaped structures at the surface of centromeres) of each chromosome.

  • iii. Anaphase:

    • Centromeres split, and sister chromatids separate.
    • Separated sister chromatids (now considered individual chromosomes) move towards opposite poles.
    • Spindle fibres shorten, pulling the chromosomes apart.
    • The cell elongates.

  • iv. Telophase:

    • Chromosomes reach their respective poles and start decondensing.
    • They lose their individuality and collect as a mass (chromatin).
    • Nuclear envelope reforms around the chromosome clusters at each pole.
    • Nucleolus, Golgi complex, and ER reappear.

• Cytokinesis:

  • Division of the cytoplasm, usually begins during late anaphase or telophase.
  • In Animal Cells: Occurs by the formation of a cleavage furrow in the plasma membrane, which deepens and eventually joins in the centre, dividing the cell cytoplasm into two.
  • In Plant Cells: Occurs by the formation of a cell plate. Vesicles from the Golgi apparatus accumulate at the equator, fuse, and form the cell plate, which grows outwards to meet the existing lateral walls, dividing the cell into two. The cell plate represents the middle lamella.

• Significance of Mitosis:

  • Growth of multicellular organisms.
  • Cell repair and replacement (e.g., skin cells, gut lining).
  • Maintains genetic stability (same chromosome number and type).
  • Asexual reproduction in some organisms (e.g., budding in Hydra, vegetative propagation in plants).

4. Meiosis (Reductional Division):

• Occurs in diploid germ cells (reproductive cells) during gamete formation (gametogenesis) in sexually reproducing organisms and spore formation in plants.

• Involves two sequential cycles of nuclear and cell division (Meiosis I and Meiosis II) but only a single cycle of DNA replication.

• Reduces the chromosome number by half (from diploid 2n to haploid n).

• Introduces genetic variation.

• Meiosis I (Reductional Division): Separates homologous chromosomes.

  • Prophase I: Longest and most complex phase. Subdivided into 5 stages:
    • Leptotene: Chromosomes start condensing and become visible.
    • Zygotene: Pairing of homologous chromosomes (synapsis) begins. Paired chromosomes are called bivalents or tetrads. A protein structure called the synaptonemal complex forms between homologous chromosomes.
    • Pachytene: Synapsis is complete. Crossing over (exchange of genetic material between non-sister chromatids of homologous chromosomes) occurs at recombination nodules. This leads to genetic recombination. Chromosomes remain linked at sites of crossing over (chiasmata).
    • Diplotene: Synaptonemal complex dissolves. Homologous chromosomes start separating but remain attached at the chiasmata (X-shaped structures). In oocytes of some vertebrates, diplotene can last for months or years.
    • Diakinesis: Chiasmata terminalisation (move towards the ends). Chromosomes fully condensed. Meiotic spindle assembles. Nucleolus disappears, and the nuclear envelope breaks down.
  • Metaphase I: Bivalents (pairs of homologous chromosomes) align on the equatorial plate (double metaphase plate). Spindle fibres from opposite poles attach to the kinetochores of homologous chromosomes (one pole attaches to one chromosome of the pair, the opposite pole to the other).
  • Anaphase I: Homologous chromosomes separate and move to opposite poles. Sister chromatids remain attached at their centromeres. This is where the chromosome number is halved.
  • Telophase I: Nuclear membrane and nucleolus may reappear. Chromosomes may undergo some decondensation. Cytokinesis follows, resulting in two haploid cells (dyad). The stage between Meiosis I and Meiosis II is called interkinesis (generally short-lived, no S phase).

• Meiosis II (Equational Division): Separates sister chromatids. Resembles mitosis.

  • Prophase II: Nuclear envelope disappears (if formed in Telophase I). Chromosomes condense again.
  • Metaphase II: Individual chromosomes (each with two sister chromatids) align at the equatorial plate. Spindle fibres from opposite poles attach to the kinetochores of sister chromatids.
  • Anaphase II: Centromeres split. Sister chromatids separate and move to opposite poles as individual chromosomes.
  • Telophase II: Chromosomes arrive at poles, decondense. Nuclear envelope reforms. Cytokinesis follows, resulting in four haploid daughter cells (tetrad).

• Significance of Meiosis:

  • Formation of haploid gametes essential for sexual reproduction.
  • Maintains a constant chromosome number across generations in sexually reproducing organisms.
  • Introduces genetic variation through:
    • Crossing over (recombination) during Pachytene of Prophase I.
    • Independent assortment of homologous chromosomes during Anaphase I.

Key Differences Summarized:

Multiple Choice Questions (MCQs):

1. During which phase of the cell cycle does DNA synthesis or replication take place?
   (a) G1 phase
   (b) S phase
   (c) G2 phase
   (d) M phase

2. The complex formed by a pair of synapsed homologous chromosomes is called a:
   (a) Kinetochore
   (b) Bivalent
   (c) Centromere
   (d) Chromatid

3. Crossing over, the exchange of genetic material between non-sister chromatids, occurs during which stage of Prophase I?
   (a) Leptotene
   (b) Zygotene
   (c) Pachytene
   (d) Diplotene

4. In which stage of mitosis do chromosomes align at the equator of the cell, forming the metaphase plate?
   (a) Prophase
   (b) Metaphase
   (c) Anaphase
   (d) Telophase

5. Separation of homologous chromosomes occurs during:
   (a) Anaphase of Mitosis
   (b) Anaphase I of Meiosis
   (c) Anaphase II of Meiosis
   (d) Metaphase I of Meiosis

6. Cytokinesis in plant cells occurs primarily through the formation of a:
   (a) Cleavage furrow
   (b) Cell plate
   (c) Synaptonemal complex
   (d) Kinetochore

7. Cells that have exited the cell cycle and entered an inactive state are said to be in the:
   (a) S phase
   (b) G2 phase
   (c) M phase
   (d) G0 phase

8. Meiosis results in the formation of:
   (a) Two diploid cells
   (b) Two haploid cells
   (c) Four diploid cells
   (d) Four haploid cells

9. Which of the following is NOT a significance of mitosis?
   (a) Growth of multicellular organisms
   (b) Repair of tissues
   (c) Production of gametes in animals
   (d) Asexual reproduction

10. The stage where sister chromatids separate during meiosis is:
    (a) Anaphase I
    (b) Metaphase I
    (c) Anaphase II
    (d) Telophase I

Answer Key:

1. (b)
2. (b)
3. (c)
4. (b)
5. (b)
6. (b)
7. (d)
8. (d)
9. (c)
10. (c)

Study these notes thoroughly. Remember the sequence of events, the specific occurrences in each phase (especially Prophase I), and the fundamental differences between mitosis and meiosis. Good luck with your preparation!