Cell Division in Biological Systems - Lecture 2.3

Learning Objectives

• Cell Division in Prokaryotes
• Cell Division in Eukaryotes
• Compare Mitosis and Meiosis
• Explain Meiosis' Role in Sexual Reproduction
• Impact of Meiosis on Genetic Variability

Chromosomal Structure

• Chromatin: DNA and proteins forming chromosomes.
• Chromosomes: Tightly coiled DNA-protein complex; single DNA molecule.
• Chromatids: Identical copies of a chromosome (sister chromatids).
• Centromere: Area of attachment for sister chromatids.

Cell Division Overview

• Definition: Process where a parent cell divides into two daughter cells.
  • Unicellular organisms (like prokaryotes): Cell division = reproduction.
  • Multicellular organisms: Cell division facilitates growth and tissue repair.
• Daughter Cells: Typically genetically identical to each other and the parent cell.

Prokaryotic Cell Division

• Binary Fission Process:
  1. DNA Replication: Chromosomes replicate from a single origin of replication.
  2. Cell Growth: Increase in cell size before division.
  3. Cell Splitting: Cell membrane pinches inwards to form two daughter cells.

Eukaryotic Cell Division

• Types:
  • Mitosis: Typical cellular growth and repair.
  • Meiosis: Essential for sexual reproduction, includes additional steps.

The Cell Cycle

• Cycle Overview: Includes interphase and mitotic phase (M)
  • Interphase: ~23 hours in the human cell cycle.
  • G1 Phase: Cell growth, normal function.
  • S Phase: DNA replication.
  • G2 Phase: Prepare for mitosis (growth, organelle duplication).
  • Mitosis Duration: Less than 1 hour.

Mitosis Process (PPMAT)

• Phases:
  • Prophase: Chromosomes condense, spindle formation.
  • Prometaphase: Nuclear envelope breakdown, chromosomal alignment.
  • Metaphase: Chromosomes align at the metaphase plate.
  • Anaphase: Sister chromatids separate and move to opposite poles.
  • Telophase: Nuclear envelope reforms around separated chromatids.
• Cytokinesis: Division of the cytoplasm, forming two identical daughter cells.
  • Animal Cells: Use actin microfilaments forming a cleavage furrow.
  • Plant Cells: Form a cell plate to separate daughter cells.

Meiosis Overview

• Purpose: Produces gametes for sexual reproduction.
• Gamete Formation: Cells with half the chromosome number (haploid - 1n).
• Meiosis Process:
  • Meiosis I: Homologous chromosomes separate, reducing chromosome number.
  • Meiosis II: Similar to mitosis; sister chromatids separate.
• Outcome: 4 genetically diverse haploid cells.

Genetic Variability in Meiosis

• Advantages:
  • Crossing Over: Exchange of genetic material between homologous chromosomes.
  • Independent Assortment: Random distribution of maternal and paternal chromosomes.

Comparison: Mitosis vs. Meiosis

• Mitosis:
  • Produces 2 identical diploid cells.
• Meiosis:
  • Produces 4 non-identical haploid gametes.

Cancer and Cell Division

• Errors in the cell cycle can result in uncontrolled cell growth leading to cancer, a disease of mitosis.

Types of Sexual Life Cycles

• Animals: Diploid multicellular adults; haploid gametes.
• Plants/Algae: Alternation of generations between diploid and haploid stages.
• Fungi/Protists: Dominant haploid life cycle.

Conclusion Notes

• Prokaryotic cell division = binary fission.
• Eukaryotic cell division = mitosis (growth) and meiosis (sex).
• Cell cycle includes interphase (23 hours) and mitosis (<1 hour).
• Mitosis leads to identical cells; meiosis results in genetic diversity, crucial for evolution.

Key Readings

• Campbell’s Biology, 12th Edition: Chapters on cell division and meiosis.