Cell Division (Mitosis & Meiosis)

Every living organism grows, repairs damaged tissues, and reproduces -- all through cell division. Your body produces millions of new cells every day. Understanding the two main types of cell division -- mitosis and meiosis -- is fundamental to biology.

The Cell Cycle

Before a cell divides, it goes through the cell cycle:

1. Interphase (longest phase): The cell grows, duplicates its DNA, and prepares for division
   • G1 phase: Cell growth
   • S phase: DNA replication
   • G2 phase: Preparation for mitosis
2. M phase (Mitotic phase): The cell actually divides

Mitosis

Mitosis produces two identical daughter cells with the same chromosome number as the parent cell (diploid, 2n). It is used for growth and repair.

Stages of Mitosis:

1. Prophase: Chromatin condenses into visible chromosomes. Nuclear membrane begins to break down. Spindle fibres form.

2. Metaphase: Chromosomes line up at the cell's equator (metaphase plate). Spindle fibres attach to centromeres.

3. Anaphase: Sister chromatids are pulled apart to opposite poles of the cell.

4. Telophase: Nuclear membranes reform around each set of chromosomes. Chromosomes decondense.

5. Cytokinesis: The cytoplasm divides, producing two daughter cells.

Result: 1 cell --> 2 identical diploid cells (2n)

Meiosis

Meiosis produces four non-identical daughter cells with half the chromosome number (haploid, n). It is used to form gametes (eggs and sperm).

Meiosis I (Reduction division):

• Homologous chromosomes pair up and exchange genetic material (crossing over)
• Homologous pairs separate
• Result: 2 haploid cells

Meiosis II (Similar to mitosis):

• Sister chromatids separate
• Result: 4 haploid cells

Result: 1 cell --> 4 non-identical haploid cells (n)

Key Differences

| Feature | Mitosis | Meiosis | |---------|---------|---------| | Daughter cells | 2 identical | 4 non-identical | | Chromosome number | Same (2n) | Half (n) | | Crossing over | No | Yes (Meiosis I) | | Purpose | Growth, repair | Gamete formation | | Where | Somatic cells | Reproductive cells |

Significance

• Mitosis: Growth from a single fertilized egg to a full organism, wound healing, replacement of worn-out cells
• Meiosis: Produces genetic variation through crossing over and independent assortment, essential for sexual reproduction and evolution

Nepal Connection

Nepal's rich biodiversity -- from tropical Terai to alpine Himalayas -- is a result of genetic variation produced through meiosis over millions of years. Understanding cell division also helps in agriculture (plant breeding) and medicine.

Key Takeaways

• Mitosis: 2n --> 2n (growth and repair), produces 2 identical cells
• Meiosis: 2n --> n (gamete formation), produces 4 non-identical cells
• Crossing over in meiosis creates genetic variation
• The cell cycle includes interphase (G1, S, G2) and M phase