Overview of Cell Division

• Importance of Cell Division
  • Crucial for all living organisms.
  • Involves processes like DNA replication and cell growth.

Coordinated Processes

• Cell Division, DNA Replication, and Cell Growth
  • Must occur in a coordinated manner.
  • Ensures correct division and formation of progeny cells.

Cell Cycle Definition

• Definition of Cell Cycle
  • Sequence of events:
    • Duplicates the cell’s genome.
    • Synthesizes other cell constituents.
    • Divides into two daughter cells.

Continuous Cell Growth

• Cell Growth in the Cell Cycle
  • Continuous process, especially in terms of cytoplasmic increase.

DNA Synthesis Timing

• Specific Stage for DNA Synthesis
  • DNA synthesis occurs during a specific stage in the cell cycle.
  • Coordination with other processes is essential.

Chromosome Distribution

• Distribution of Replicated Chromosomes
  • After DNA replication, chromosomes are replicated.
  • Complex series of events during cell division.
  • Chromosomes are distributed to daughter nuclei.

Genetic Control

• Genetic Control of Cell Cycle Events
  • Events in the cell cycle are under genetic control.
  • Ensures precise regulation and coordination.

PHASES OF CELL CYCLE

Typical Eukaryotic Cell Cycle

• Cell Cycle Duration
  • Varies among organisms and cell types.
  • Human cells in culture: approximately 24 hours.
  • Yeast: about 90 minutes for cell cycle progression.

Two Basic Phases

• Division of Cell Cycle
  • Interphase
    • Significant phase between two successive M phases.
  • M Phase (Mitosis)
    • Represents actual cell division.
    • M Phase duration: about one hour in a 24-hour cell cycle.

Interphase Details

• Interphase Duration
  • Lasts more than 95% of the cell cycle.
  • Prepares for division through cell growth and DNA replication.

Three Sub-Phases of Interphase

1. G1 Phase (Gap 1)
   • Interval between mitosis and DNA replication initiation.
   • Metabolically active, continuous growth, no DNA replication.
2. S Phase (Synthesis)
   • DNA synthesis or replication occurs.
   • DNA amount per cell doubles (e.g., 2C to 4C).
   • Chromosome number remains the same (2n).
3. G2 Phase (Gap 2)
   • Proteins synthesized for mitosis preparation.
   • Cell growth continues.

Mitotic Cell Division

• M Phase Details
  • Starts with Karyokinesis
    • Nuclear division, separation of daughter chromosomes.
  • Ends with Cytokinesis
    • Division of cytoplasm.

Special Considerations

• Quiescent Stage (G0)
  • Some cells exit G1 phase into G0, an inactive stage.
  • Metabolically active but no proliferation unless needed.
• Mitotic Cell Division in Animals
  • Mainly in diploid somatic cells.
  • Exceptions: Haploid cells (e.g., male honey bees).
• Plants and Mitotic Divisions
  • Occur in both haploid and diploid cells.

M PHASE

Overview

• Dramatic Period
  • Major reorganization of cell components.
  • Equational division: Chromosome number in parent and progeny cells is the same.

Karyokinesis (Nuclear Division)

• Four Stages of Karyokinesis
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase

Prophase

• Initiation of Chromosomal Condensation
  • Follows S and G2 phases of interphase.
  • Chromosomal material condenses.
  • Centrosome moves towards opposite poles.
  • Characteristics:
    • Formation of compact mitotic chromosomes.
    • Chromosomes composed of two chromatids attached at the centromere.

Metaphase

• Disintegration of Nuclear Envelope
  • Chromosomes spread through the cytoplasm.
  • Condensation completed.
  • Morphology of chromosomes easily studied.
  • Characteristics:
    • Chromosomes at equator with sister chromatids held by the centromere.
    • Kinetochores on centromeres attach to spindle fibers.
    • Chromosomes align at the metaphase plate.

Anaphase

• Simultaneous Chromatid Separation
  • Chromosomes split, chromatids move to opposite poles.
  • Centromeres split.
  • Characteristics:
    • Centromeres split, chromatids separate.
    • Chromatids move to opposite poles.

Telophase

• Final Stage of Karyokinesis
  • Chromosomes decondense, lose individuality.
  • Chromosomes cluster at opposite spindle poles.
  • Characteristics:
    • Chromosomes cluster at spindle poles.
    • Nuclear envelope forms around chromosome clusters.
    • Nucleolus, golgi complex, and ER reform.

Cytokinesis

• Cell Division Completion
  • Separation of cytoplasm into two daughter cells.
  • Animal Cells:
    • Furrow formation in the plasma membrane.
  • Plant Cells:
    • Wall formation from the center outwards.
  • Organelles distribution between daughter cells.
  • Some organisms exhibit multinucleate conditions without cytokinesis (e.g., liquid endosperm in coconut).

SIGNIFICANCE OF MITOSIS

Restriction to Diploid Cells

• Mitosis Type: Equational Division
  • Primarily in diploid cells.
  • Exceptions in lower plants and some social insects where haploid cells also divide.

Importance in Organism’s Life

• Understanding the Significance
  • Crucial for the life of an organism.
  • Examples of haploid and diploid insects studied.

Production of Diploid Daughter Cells

• Genetic Complement in Mitosis
  • Results in diploid daughter cells.
  • Genetic identity is maintained.

Contribution to Multicellular Organisms

• Growth of Multicellular Organisms
  • Mitosis is essential for growth.
  • Cell growth disturbs the nucleo-cytoplasmic ratio.
  • Cell division restores the ratio.

Cell Repair

• Mitosis for Cell Repair
  • The upper epidermis, gut lining, and blood cells are constantly replaced.
  • Mitotic divisions contribute to cell repair.

Continuous Growth in Plants

• Meristematic Tissues and Plant Growth
  • Meristematic tissues (apical and lateral cambium) undergo mitotic divisions.
  • Result: Continuous growth of plants throughout their life.

duction and Gamete Formation

• Fusion of Gametes
  • Involves two gametes, each with a complete haploid set of chromosomes.
  • Gametes formed from specialized diploid cells.
  • Meiosis reduces chromosome number by half.
  • Ensures production of haploid daughter cells.

Phases of Meiosis

• Two Sequential Cycles
  • Meiosis I and Meiosis II
  • Single cycle of DNA replication.

Meiosis I

Prophase I

• Complex Stages
  • Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.
  • Chromosomes become visible, and pairing (synapsis) occurs.
  • Formation of synaptonemal complex, bivalent or tetrad formation.
  • Recombination nodules, crossing over, and recombinase involvement.
  • Chiasmata formation at diplotene.
  • Terminalisation of chiasmata at diakinesis, spindle assembly.

Metaphase I

• Alignment of Bivalent Chromosomes
  • Align on the equatorial plate.
  • Microtubules attach to kinetochore of homologous chromosomes.

Anaphase I

• Separation of Homologous Chromosomes
  • Homologous chromosomes separate.
  • Sister chromatids remain associated at centromeres.

Telophase I

• Nuclear Reappearance and Cytokinesis
  • Nuclear membrane and nucleolus reappear.
  • Cytokinesis results in dyad of cells.
  • Interkinesis follows, no DNA replication.

Meiosis II

Prophase II

• Initiated after Cytokinesis
  • Resembles a normal mitosis.
  • The nuclear membrane disappears, and chromosomes compact.

Metaphase II

• Alignment of Chromosomes
  • Chromosomes align at the equator.
  • Microtubules attach to kinetochores of sister chromatids.

Anaphase II

• Separation of Sister Chromatids
  • Centromere splits, and sister chromatids move to opposite poles.

Telophase II

• Nuclear Reformation and Cytokinesis
  • A nuclear envelope forms around groups of chromosomes.
  • Cytokinesis results in the tetrad of haploid daughter cells.

SIGNIFICANCE OF MEIOSIS

Conservation of Chromosome Number

• Across Generations
  • Ensures specific chromosome number in each species.
  • Vital for maintaining genetic stability.
  • Achieved through the reduction of chromosome number by half during meiosis.

Paradoxical Reduction for Stability

• Meiotic Reduction
  • Process paradoxically reduces chromosome number.
  • Essential for preserving species-specific characteristics.
  • Counterintuitively supports the stability of the genetic makeup.

Genetic Variability

• Increased Genetic Diversity
  • Meiosis enhances genetic variability.
  • Results from recombination and independent assortment of chromosomes.
  • Crucial for the process of evolution.

Evolutionary Significance

• Importance for Evolution
  • Genetic variations are fundamental for evolution.
  • Meiosis introduces variations through the shuffling of genetic material.
  • Supports adaptability and survival in changing environments.