Introduction: The Continuity of Life
The ability of organisms to reproduce their own kind is a defining characteristic of life. This continuity is based on cellular reproduction, or cell division. In this lesson, we will deeply explore the two main types of cell division in eukaryotes: Mitosis, used for growth and asexual reproduction, and Meiosis, essential for sexual reproduction and the creation of genetic variation.
Part 1: The Cell Cycle and Mitosis
1.1 The Cell Cycle: A Cell's Lifetime
The Cell Cycle tracks a cell's life from its formation until its division into two daughter cells. It is divided into two main phases: Interphase (the period of growth and DNA replication) and the M Phase (the division period). The cycle is strictly governed by a molecular control system involving Checkpoints.
Diagram: Components of the Eukaryotic Cell Cycle
- Interphase (Non-dividing period): A period of vigorous growth and metabolic activity.
- G₁ Phase: Cell growth, protein synthesis, and metabolic function. The critical G₁ Checkpoint determines if the cell proceeds to division.
- S Phase (Synthesis): DNA Replication occurs. Each chromosome is duplicated to form two identical Sister Chromatids, joined at the Centromere.
- G₂ Phase: Cell prepares for M phase, ensures DNA is replicated correctly (G₂ Checkpoint).
- M Phase (Mitotic Phase): Includes Mitosis (nuclear division) and Cytokinesis (cytoplasmic division).
1.2 Mitosis: Division for Growth and Repair
Mitosis is the process of nuclear division, resulting in two Diploid (2n) daughter cells that are genetically identical to the parent cell. It is a continuous process conventionally broken down into five stages.
Purpose of Mitosis: Growth of multicellular organisms, tissue repair, and asexual reproduction.
Mitosis Stages (Detailed SVG Diagram, 2n=4)
Key Events in Mitosis (Two pairs of chromosomes: one large, one small)
- Prophase / Prometaphase:
- Chromatin fibers condense into visible chromosomes (each with two sister chromatids).
- The nuclear envelope fragments. Spindle microtubules attach to the Kinetochores (protein structures at the centromeres).
- Metaphase:
- All chromosomes align at the Metaphase Plate (an imaginary plane equidistant from the two spindle poles).
- This is the M Checkpoint, ensuring all kinetochores are properly attached before proceeding to separation.
- Anaphase:
- The shortest stage. Cohesin proteins are cleaved, allowing Sister Chromatids to separate and move toward opposite poles. Each separated chromatid is now considered a full-fledged chromosome.
- Telophase & Cytokinesis:
- Genetically identical daughter nuclei form at opposite ends as nuclear envelopes re-form.
- Cytokinesis (cytoplasm division) occurs, forming two separate daughter cells. (e.g., via Cleavage Furrow in animal cells).
Part 2: Meiosis and Genetic Variation
2.1 Meiosis: Division for Sexual Reproduction
Meiosis is a specialized cell division that reduces the chromosome number by half, creating four Haploid (n) cells (gametes) from one Diploid (2n) parent cell. It is crucial for sexual reproduction.
Meiosis involves two consecutive divisions: Meiosis I and Meiosis II, with no DNA replication occurring between them.
2.2 Meiosis I: Separation of Homologous Chromosomes (Reductional Division)
This is the reductional division where the chromosome number is halved. The key events that generate genetic diversity occur here.
Meiosis I: Key Events and Sources of Variation (2n=4)
Mechanisms Generating Genetic Variation (Homologous Chromosomes shown in Red and Blue)
- Prophase I: Homologous chromosomes pair up (Synapsis) to form Bivalents (or Tetrads). Crossing Over occurs, where non-sister chromatids exchange DNA segments at points called Chiasmata. This creates recombinant chromosomes.
- Metaphase I: Homologous pairs (bivalents) align along the Metaphase Plate. The orientation of each pair is random, which is the physical basis for Independent Assortment.
- Anaphase I: Homologous chromosomes separate and move toward opposite poles. Crucially, Sister Chromatids remain attached at the centromere.
- Telophase I & Cytokinesis: Two Haploid (n) cells form, but each chromosome still consists of two sister chromatids.
2.3 Meiosis II: Separation of Sister Chromatids (Equational Division)
Meiosis II is functionally similar to Mitosis, but it starts with haploid cells and no further DNA replication occurs (no interkinesis/Interphase between Meiosis I and II). This is the Equational Division that separates the sister chromatids.
- Metaphase II: The chromosomes (each still composed of two sister chromatids) align at the metaphase plate.
- Anaphase II: Sister Chromatids separate and move toward opposite poles (identical movement to mitotic anaphase).
- Result: The final outcome is four Haploid (n) daughter cells, each genetically unique due to crossing over and independent assortment.
Part 3: Comparing Mitosis and Meiosis
Despite being both forms of cell division, their purposes and outcomes are fundamentally different.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Production of Gametes for sexual reproduction |
| Number of Divisions | One | Two (Meiosis I and Meiosis II) |
| Chromosome Number | Daughter cells are Diploid (2n) and genetically identical to the parent cell. | Daughter cells are Haploid (n) and genetically unique from the parent cell and each other. |
| Genetic Variation | Does not produce genetic variation. | Produces genetic variation through Crossing Over and Independent Assortment. |
| Synapsis/Crossing Over | Does not occur. | Occurs during Prophase I between homologous chromosomes. |
| Structure Separated in Anaphase | Sister Chromatids | Anaphase I: Homologous Chromosomes / Anaphase II: Sister Chromatids |
Part 4: Practice Quiz
Test your knowledge of Mitosis and Meiosis.