Growth and reproduction of all living organisms depend on cell division. The process by which a parent cell divides into two daughter cells, passing on genetic information, is governed by the cell cycle.
The Cell Cycle
- 1.The cell cycle is the ordered sequence of events a cell goes through from one division to the next. It has two main phases:
- 2.Interphase — the preparatory phase (G1, S, G2)
- 3.M phase (Mitotic phase) — actual division (karyokinesis + cytokinesis)
- Interphase is NOT a resting phase; it is highly active:
- G1 phase (Gap 1): Cell grows; organelles increase; protein synthesis is active. Duration varies most between cells.
- S phase (Synthesis): DNA replication occurs. DNA content doubles (2C → 4C) but chromosome number stays the same.
- G2 phase (Gap 2): Cell continues to grow; proteins needed for division are synthesised; mitochondria replicate.
Some cells exit the cycle and enter G0 phase (quiescent state) — e.g., neurons and heart muscle cells in adults.
Mitosis
Mitosis produces two genetically identical daughter cells with the same chromosome number as the parent (diploid → diploid). It occurs in somatic (body) cells and is essential for growth and repair.
Stages of Mitosis (Karyokinesis):
Prophase: Chromatin condenses into visible chromosomes. Each chromosome consists of two chromatids joined at the centromere. The mitotic spindle begins to form; centrioles (in animal cells) move to poles; nuclear envelope starts to break down.
Metaphase: Chromosomes reach maximum condensation. They align at the metaphase plate (equatorial plate). Spindle fibres (kinetochore fibres) attach to the kinetochore of each chromatid.
Anaphase: Centromeres split. Sister chromatids separate and move to opposite poles due to shortening of spindle fibres. Each pole now receives one set of chromatids (now called chromosomes).
Telophase: Chromosomes reach poles and decondense. Nuclear envelope reforms around each set. Nucleolus reappears. Spindle fibres disappear.
- Cytokinesis follows karyokinesis:
- In animal cells: cleavage furrow forms by contraction of actin filaments, pinching the cell in two.
- In plant cells: a cell plate forms from vesicles derived from the Golgi apparatus, growing outward to form the new cell wall.
Significance of Mitosis
- Maintains genetic continuity and chromosome number
- Essential for growth and replacement of worn-out cells
- Produces genetically identical cells (clones)
- Enables asexual reproduction in many organisms
Meiosis
Meiosis occurs in reproductive cells (germ cells) to produce gametes. It reduces the chromosome number by half (diploid → haploid) and introduces genetic variation. It has two successive divisions: Meiosis I and Meiosis II.
- Meiosis I (Reductional division):
- Prophase I (longest and most complex stage) — divided into 5 sub-stages: Leptotene (chromosomes begin to condense), Zygotene (homologous chromosomes pair up — synapsis, forming bivalents), Pachytene (crossing over occurs between non-sister chromatids at chiasmata), Diplotene (chromosomes begin to separate but remain attached at chiasmata), Diakinesis (chromosomes fully condensed; chiasmata move towards ends — terminalisation; nuclear envelope breaks down).
- Metaphase I: Bivalents align at the equatorial plate; spindle fibres attach to kinetochores of homologues.
- Anaphase I: Homologous chromosomes (not chromatids) separate to opposite poles. Chromosome number is halved.
- Telophase I: Nuclear envelope may reform; cytokinesis may occur.
Meiosis II (Equational division):
Similar to mitosis — sister chromatids separate, producing 4 haploid daughter cells.
Significance of Meiosis
- Produces haploid gametes essential for sexual reproduction
- Crossing over in prophase I generates new combinations of alleles — major source of genetic variation
- Random segregation of homologues (independent assortment) also contributes to variation
Key Differences: Mitosis vs Meiosis
- Mitosis: 1 division → 2 diploid cells; no crossing over; somatic cells
- Meiosis: 2 divisions → 4 haploid cells; crossing over occurs; reproductive cells
Common mistakes
- Thinking interphase is a resting phase — it is metabolically very active.
- Confusing chromatid separation in mitosis (anaphase) with chromosome separation in meiosis I (anaphase I). In anaphase I, entire chromosomes (each with 2 chromatids) move apart.
- Saying DNA doubles in G2 — DNA doubles in S phase; G2 is growth after replication.
- Mixing up synapsis (pairing of homologues) with crossing over (actual exchange of segments) — synapsis happens in zygotene; crossing over in pachytene.
Summary
The cell cycle ensures faithful transmission of genetic material. Mitosis maintains the diploid number for growth and repair. Meiosis halves the chromosome number for sexual reproduction and generates diversity through crossing over and independent assortment. Both processes are tightly regulated to prevent errors.