Class 11 Biology Notes Chapter 10 (Chapter 10) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 10: Cell Cycle and Cell Division. This is a fundamental chapter, crucial not just for your Class 11 understanding but also for various government exams where Biology is a component. Pay close attention to the details of each phase and the differences between mitosis and meiosis.

Chapter 10: Cell Cycle and Cell Division - Detailed Notes

1. Introduction

• Cell Division: The process by which a parent cell divides into two or more daughter cells. It's essential for growth, repair, and reproduction.
• Cell Cycle: The sequence of events by which a cell duplicates its genome, synthesizes other constituents, and eventually divides into two daughter cells. It's a genetically controlled process.
• Duration: Varies between organisms and cell types (e.g., human cell divides approx. every 24 hours, yeast in about 90 minutes).

2. Phases of the Cell Cycle

The cell cycle is divided into two main phases:

• A. Interphase: The 'resting' or preparatory phase between two successive M phases. The cell grows and prepares for division. It constitutes >95% of the cell cycle duration.

  • G1 Phase (Gap 1):
    • Interval between mitosis and initiation of DNA replication.
    • Cell is metabolically active and grows continuously.
    • Synthesizes RNA, proteins, and organelles.
    • Decides whether to continue to S phase or enter G0.
    • Checkpoint: G1 checkpoint ensures conditions are favourable for DNA synthesis.
  • S Phase (Synthesis):
    • DNA replication (synthesis) occurs.
    • Amount of DNA per cell doubles (from 2C to 4C), but the chromosome number remains the same (2n).
    • In animal cells, centriole duplication occurs in the cytoplasm.
    • Histone protein synthesis also occurs.
  • G2 Phase (Gap 2):
    • Cell growth continues.
    • Proteins (like tubulin for spindle fibres) are synthesized in preparation for mitosis.
    • Organelles like mitochondria and chloroplasts may divide.
    • Checkpoint: G2 checkpoint ensures DNA replication is complete and checks for DNA damage before entering M phase.

• B. M Phase (Mitosis Phase): The actual cell division phase. Most dramatic period involving major reorganization. It includes:

  • Karyokinesis: Nuclear division.
  • Cytokinesis: Cytoplasmic division.

• G0 Phase (Quiescent Stage):

  • 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.

3. M Phase - Mitosis (Equational Division)

• Occurs in somatic cells (and germline stem cells).

• Ensures daughter cells have the same chromosome number as the parent cell (2n -> 2n).

• Divided into four stages:

  • a) Prophase:
    • Initiation of condensation of chromosomal material, becoming compact mitotic chromosomes (visible under light microscope).
    • Chromosomes seen to be composed of two sister chromatids attached at the centromere.
    • Centrosomes (which duplicated during S phase) move towards opposite poles.
    • Microtubules radiate out (asters). Spindle fibres start forming.
    • Nuclear envelope, Golgi complex, ER, and nucleolus start disappearing.
  • b) Metaphase:
    • Nuclear envelope completely disintegrates.
    • Chromosomes spread throughout the cytoplasm.
    • Condensation of chromosomes is completed; they are thickest and shortest, clearly visible. Morphology best studied here.
    • Chromosomes align at the equator (metaphase plate).
    • Spindle fibres from opposite poles attach to the kinetochores (disc-shaped structures at the centromere) of each chromosome. One kinetochore for each sister chromatid.
  • c) Anaphase:
    • Centromeres split simultaneously.
    • Sister chromatids separate and are now called daughter chromosomes.
    • Daughter chromosomes move towards opposite poles, pulled by the shortening spindle fibres. Centromeres lead the way.
    • Chromosome number temporarily doubles within the cell.
  • d) Telophase:
    • Chromosomes reach their respective poles.
    • Chromosomes decondense and lose their individuality, becoming chromatin material again.
    • Nuclear envelope reforms around the chromosome clusters at each pole.
    • Nucleolus, Golgi complex, and ER reappear.
    • Essentially the reverse of prophase.

• Cytokinesis: Division of cytoplasm.

  • Animal Cells: Appearance of a furrow in the plasma membrane, which gradually deepens and joins in the centre, dividing the cytoplasm into two (Cleavage furrow method).
  • Plant Cells: Formation of a cell plate starting in the centre and growing outwards towards the lateral walls. The cell plate represents the middle lamella between the walls of the two adjacent cells. (Cell plate method).
  • Note: In some organisms (like liquid endosperm in coconut), karyokinesis is not immediately followed by cytokinesis, leading to a multinucleate condition (syncytium).

4. Significance of Mitosis

• Growth: Increases the number of cells in multicellular organisms.
• Repair: Replaces old, worn-out, or damaged cells (e.g., skin cells, gut lining cells, blood cells).
• Asexual Reproduction: In some lower organisms and vegetative propagation in plants.
• Maintains genetic stability and chromosome number in somatic cells.
• Restores the nucleo-cytoplasmic ratio.

5. Meiosis (Reductional Division)

• Occurs in diploid germ cells (meiocytes) during gamete formation (gametogenesis) in sexually reproducing organisms. Also for 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 (2n -> n), producing four haploid daughter cells.

• A. Meiosis I (Reductional Division): Homologous chromosomes separate.

  • Prophase I: Longest and most complex phase. Subdivided into 5 stages:
    • Leptotene: Chromosomes start condensing, become visible. Compaction continues.
    • Zygotene: Pairing of homologous chromosomes (synapsis) begins. Paired chromosomes are called bivalents or tetrads. Synaptonemal complex forms between homologous chromosomes.
    • Pachytene: Synapsis is complete. Bivalents are clearly visible as tetrads (4 chromatids). Crossing over occurs between non-sister chromatids of homologous chromosomes at recombination nodules. This leads to genetic recombination. Enzyme involved: Recombinase.
    • Diplotene: Dissolution of the synaptonemal complex. Homologous chromosomes start separating but remain attached at sites of crossing over, forming X-shaped structures called chiasmata. Can last for months or years in oocytes of some vertebrates.
    • Diakinesis: Terminalisation of chiasmata (chiasmata shift towards the ends). Chromosomes fully condensed. Meiotic spindle assembles. Nucleolus disappears, nuclear envelope breaks down.
  • Metaphase I:
    • Bivalents align on the equatorial plate (double metaphase plate).
    • Microtubules 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 stage reduces the chromosome number from diploid to haploid (disjunction).
  • Telophase I:
    • Nuclear membrane and nucleolus reappear (may be short-lived or absent in some cases).
    • Cytokinesis follows, resulting in two haploid cells (dyad of cells).
    • Chromosomes may undergo some decondensation.

• Interkinesis: A short stage between Meiosis I and Meiosis II. No DNA replication occurs here.

• B. Meiosis II (Equational Division): Sister chromatids separate. Resembles mitosis.

  • Prophase II:
    • Initiated immediately after cytokinesis, often before chromosomes fully elongate.
    • Nuclear membrane disappears (if formed in Telophase I).
    • Chromosomes become compact again.
  • Metaphase II:
    • Chromosomes align at the equator (single metaphase plate).
    • Microtubules from opposite poles attach to the kinetochores of sister chromatids.
  • Anaphase II:
    • Simultaneous splitting of the centromere of each chromosome (which were holding sister chromatids together).
    • Sister chromatids separate and move towards opposite poles as individual chromosomes.
  • Telophase II:
    • Chromosomes arrive at opposite poles.
    • Decondensation occurs.
    • Nuclear envelope reforms around each group of chromosomes.
    • Cytokinesis follows, resulting in four haploid daughter cells (tetrad of cells).

6. Significance of Meiosis

• Formation of Gametes: Produces haploid gametes essential for sexual reproduction.
• Maintenance of Chromosome Number: Halving the chromosome number in gametes ensures the diploid number is restored upon fertilization.
• Genetic Variation: Crossing over during Pachytene and independent assortment of homologous chromosomes during Anaphase I lead to new combinations of genes, which is crucial for evolution.

7. Comparison: Mitosis vs Meiosis

8. Regulation of Cell Cycle

• The cell cycle is tightly regulated by checkpoints (G1, G2, M) that monitor the order, integrity, and fidelity of events.
• Progression through the cycle is driven by specific proteins called Cyclins and enzymes called Cyclin-dependent Kinases (Cdks).
• Cyclins bind to Cdks, activating them to phosphorylate target proteins that initiate or regulate key events (like DNA replication, mitosis). Cyclin levels fluctuate cyclically.

9. Amitosis

• Direct cell division where the nucleus constricts and divides, followed by cytoplasmic division.
• No spindle formation or distinct chromosome changes.
• Occurs in some lower organisms, specialized cells, or old/abnormal cells. Results in unequal distribution of chromatin.

Multiple Choice Questions (MCQs)

1. During which phase of the cell cycle does DNA replication occur?
   a) G1 phase
   b) S phase
   c) G2 phase
   d) M phase

2. Synapsis, the pairing of homologous chromosomes, occurs during which stage of meiosis?
   a) Leptotene
   b) Zygotene
   c) Pachytene
   d) Diplotene

3. If a diploid cell (2n) with 10 chromosomes undergoes mitosis, how many chromosomes will each daughter cell have?
   a) 5
   b) 10
   c) 20
   d) 40

4. Crossing over, which leads to genetic recombination, takes place between:
   a) Sister chromatids of the same chromosome
   b) Non-sister chromatids of homologous chromosomes
   c) Sister chromatids of non-homologous chromosomes
   d) Non-sister chromatids of non-homologous chromosomes

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. In plant cells, cytokinesis occurs by the formation of a:
   a) Cleavage furrow
   b) Cell plate
   c) Synaptonemal complex
   d) Kinetochore

7. Cells that are metabolically active but do not proliferate are said to be in which phase?
   a) G1 phase
   b) S phase
   c) G0 phase
   d) G2 phase

8. Which stage of mitosis is best for studying the morphology (shape and size) of chromosomes?
   a) Prophase
   b) Metaphase
   c) Anaphase
   d) Telophase

9. Meiosis II is often referred to as equational division because:
   a) Homologous chromosomes separate
   b) The chromosome number is halved
   c) Sister chromatids separate, maintaining the ploidy level established in Meiosis I
   d) DNA replication occurs just before it

10. The key regulators of the cell cycle progression are:
    a) Hormones and enzymes
    b) Cyclins and Cyclin-dependent kinases (Cdks)
    c) Actin and Myosin
    d) Histones and DNA polymerase

Answer Key:

1. b) S phase
2. b) Zygotene
3. b) 10
4. b) Non-sister chromatids of homologous chromosomes
5. b) Anaphase I of Meiosis
6. b) Cell plate
7. c) G0 phase
8. b) Metaphase
9. c) Sister chromatids separate, maintaining the ploidy level established in Meiosis I
10. b) Cyclins and Cyclin-dependent kinases (Cdks)

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