CELL CYCLE AND CELL DIVISION
NCERT Masterclass & NEET Biology Study Module
1. The Cell Cycle & Interphase
The sequence of events by which a cell duplicates its genome, synthesizes other cell constituents, and eventually divides into two daughter cells is termed the cell cycle.
- Interphase: The resting phase. It lasts for more than 95% of the duration of the cell cycle. Divided into $G_1$, $S$, and $G_2$ phases.
- $G_1$ Phase (Gap 1): Cell is metabolically active and continuously grows but does not replicate its DNA.
- S Phase (Synthesis): DNA replication occurs. The amount of DNA doubles (from 2C to 4C), but the chromosome number remains the same (2n). In animal cells, the centriole duplicates in the cytoplasm.
- $G_2$ Phase (Gap 2): Proteins (like tubulin) are synthesized in preparation for mitosis while cell growth continues.
- $G_0$ Phase (Quiescent Stage): Cells that do not divide further exit $G_1$ phase to enter an inactive stage. They remain metabolically active but no longer proliferate (e.g., heart cells).
2. M Phase (Mitosis)
Mitosis is called equational division because the number of chromosomes in the parent and progeny cells remains the same.
- Prophase: Chromatin condenses to form discrete chromosomes. Centrosomes move to opposite poles. Spindle fibers begin to form. Nucleolus, Golgi, ER, and nuclear envelope disappear.
- Metaphase: Chromosomes align at the equator (metaphase plate). Spindle fibers attach to the kinetochores (disc-shaped structures at the surface of centromeres). This is the best stage to study chromosome morphology.
- Anaphase: Centromeres split, chromatids separate and move to opposite poles. Chromatids represent chromosomes of the future daughter nuclei.
- Telophase: Chromosomes decondense at poles. Nuclear envelope reforms around chromosome clusters. Nucleolus, Golgi, and ER reappear.
3. Cytokinesis & Significance of Mitosis
- Animal Cells: Achieved by the appearance of a furrow in the plasma membrane that deepens and joins in the center.
- Plant Cells: Starts with the formation of a cell plate at the center, which grows outward to meet the lateral walls. The cell plate represents the middle lamella.
Significance: Growth of multicellular organisms, cell repair (gut lining, blood cells), and maintenance of nucleo-cytoplasmic ratio.
4. Meiosis I (Reductional Division)
Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms. It involves two sequential cycles (Meiosis I and II) but only a single cycle of DNA replication.
Stages of Prophase I:
Prophase I is typically longer and more complex, subdivided into 5 phases:
- Leptotene: Compaction of chromosomes.
- Zygotene: Homologous chromosomes pair up (Synapsis). Accompanied by the formation of the synaptonemal complex. The paired chromosomes form a bivalent or tetrad.
- Pachytene: Crossing over occurs between non-sister chromatids of homologous chromosomes. It is an enzyme-mediated process (enzyme: Recombinase).
- Diplotene: Dissolution of the synaptonemal complex. The X-shaped structures formed by crossing over are called Chiasmata. (In oocytes of some vertebrates, diplotene can last for months or years).
- Diakinesis: Terminalisation of chiasmata. Nuclear envelope breaks down.
5. Meiosis II & Significance
The stage between the two meiotic divisions is called interkinesis (no DNA replication occurs here). Meiosis II is strictly an equational division, just like mitosis.
Significance of Meiosis: Conserves the specific chromosome number of each species across generations. Increases genetic variability in the population, which is crucial for evolution.
← Back to NEET Resource Hub🚀 NEET CELL CYCLE MEGA QUIZ (100 MCQ)
Solve the 5 parts below to master Mitosis, Meiosis, DNA Content, and Prophase I.
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