arrow_backBack to Study Resources

Chapter 6: Cell Division

Form 4 Biology Bab 6: Cell Division

6.1 Cell Cycle and Mitosis

The cell cycle is an ordered sequence of events that involves cell growth and nuclear division to produce two new daughter cells.

Phases of the Cell Cycle

The cell cycle consists of two major phases: Interphase and M Phase (Mitotic Phase).

  • Interphase (Longest Phase - non-dividing stage):
    • $G_1$ Phase (First Gap): Cell grows in size, synthesizes proteins and cytoplasmic organelles (e.g., mitochondria, ribosomes). Chromosomes are uncoiled as chromatin.
    • S Phase (Synthesis): DNA replication occurs. Each chromosome duplicates to form two identical sister chromatids joined at the centromere.
    • $G_2$ Phase (Second Gap): Cell continues to grow and accumulates energy (ATP) required for cell division; spindle fibers begin to organize.
  • M Phase (Division Phase):
    • Mitosis (Nuclear Division): Divided into four continuous stages: Prophase, Metaphase, Anaphase, and Telophase (PMAT).
    • Cytokinesis: Division of the cytoplasm to form two distinct daughter cells.

Stages of Mitosis

  • Prophase: Chromatin condenses and thickens into visible chromosomes. Centrioles move to opposite poles and form spindle fibers. The nuclear membrane and nucleolus disintegrate.
  • Metaphase: Chromosomes align at the equator plane (metaphase plate). Spindle fibers attach to the centromeres of each chromosome.
  • Anaphase: Centromeres divide. Spindle fibers shorten, pulling sister chromatids apart toward opposite poles. Once separated, chromatids are called daughter chromosomes.
  • Telophase: Daughter chromosomes reach opposite poles. Chromosomes uncoil back into thin chromatin threads. Nuclear envelope and nucleoli reform. Spindle fibers disappear.

Cytokinesis

  • Animal Cells: Microfilaments contract around the equator to form a cleavage furrow, which deepens until the cell pinches into two daughter cells.
  • Plant Cells: Membrane-bound vesicles line up at the center to form a cell plate, which grows outward to fuse with the cell wall, dividing the cell into two.

Significance of Mitosis

  • Produces genetically identical daughter cells with the same diploid chromosome number ($2n$) as the parent cell.
  • Essential for growth, replacement of dead/damaged tissues (repair and regeneration), and asexual reproduction in unicellular organisms (e.g., Amoeba, yeast).

6.2 Meiosis

Meiosis is a specialized form of nuclear division that reduces the chromosome number by half, producing four non-identical haploid ($n$) gametes (sex cells) from a single diploid ($2n$) germ cell.

Key Concepts in Meiosis

  • Homologous Chromosomes: Pairs of matching chromosomes (one paternal, one maternal) with identical structural features and gene loci.
  • Diploid ($2n$): Cells containing two complete sets of chromosomes (e.g., human somatic cells = 46 chromosomes).
  • Haploid ($n$): Cells containing a single set of chromosomes (e.g., human gametes = 23 chromosomes).

Stages of Meiosis

Meiosis consists of two sequential nuclear divisions: Meiosis I and Meiosis II.

Meiosis I (Reduction Division)

  • Prophase I: Homologous chromosomes pair up (synapsis) to form a bivalent / tetrad. Non-sister chromatids cross over at points called chiasmata, exchanging genetic material (crossing over). Nuclear membrane disappears.
  • Metaphase I: Homologous chromosome pairs align randomly along the metaphase plate (independent assortment). Spindle fibers attach to centromeres.
  • Anaphase I: Spindle fibers contract, separating homologous chromosome pairs and pulling whole chromosomes to opposite poles (sister chromatids remain attached).
  • Telophase I & Cytokinesis: Chromosomes reach poles; cytoplasm divides to yield two haploid ($n$) daughter cells.

Meiosis II (Similar to Mitosis)

  • Prophase II: Spindle apparatus forms in each of the two haploid cells; nuclear envelope breaks down.
  • Metaphase II: Individual chromosomes line up randomly at the metaphase plate.
  • Anaphase II: Centromeres divide. Sister chromatids are pulled apart toward opposite poles as daughter chromosomes.
  • Telophase II & Cytokinesis: Nuclear membranes reform; cytoplasm divides, producing a total of four genetically non-identical haploid ($n$) gametes.

Significance of Meiosis

  • Maintains constant diploid chromosome numbers across generations after fertilization.
  • Generates genetic variation through:
    • Crossing over during Prophase I.
    • Independent assortment of homologous chromosomes during Metaphase I.
    • Random fertilization of male and female gametes.

6.3 Issues Related to Cell Division on Human Health

Uncontrolled Mitosis (Cancer)

  • Mutations in genes regulating the cell cycle lead to rapid, uncontrolled mitotic cell division without stopping at checkpoints.
  • This results in the formation of an abnormal mass of cells called a tumor:
    • Benign Tumor: Non-cancerous mass that stays localized and does not spread.
    • Malignant Tumor (Cancer): Cancerous mass that invades neighboring tissues and can spread to distant organs via blood/lymph (metastasis).
  • Causes (Carcinogens): Radiation (UV, X-rays), chemicals (tar in tobacco smoke), viruses (HPV), genetic factors.

Abnormalities in Meiosis (Nondisjunction)

  • Nondisjunction: Failure of homologous chromosomes (in Anaphase I) or sister chromatids (in Anaphase II) to separate properly.
  • Leads to gametes with abnormal chromosome numbers ($n+1$ or $n-1$).
  • Down Syndrome (Trisomy 21): Caused by nondisjunction of chromosome 21 during gametogenesis. Fertilization with a normal gamete results in an individual with 47 chromosomes ($46 + 1$ extra chromosome 21).
    • Symptoms: Slanted eyes, flat nasal bridge, broad hands, short stature, and varying degrees of intellectual disability.
Sponsored