The continuity of life depends on the ability of cells to divide and produce new cells. This process ensures:
- Growth of organisms
- Repair and regeneration
- Reproduction
The sequence of events through which a cell passes to divide is called the cell cycle, while the process of division itself is known as cell division.
What is the Cell Cycle?
The cell cycle is an ordered sequence of events by which a cell grows, duplicates its DNA, and divides into two daughter cells.
Phases of Cell Cycle
The cell cycle is divided into two main phases:
- Interphase (Preparation phase)
- M Phase (Mitotic phase) (Division phase)
Interphase
Interphase is the longest phase (~95% of the cell cycle).
It is further divided into:
1. G₁ Phase (Gap 1)
- Cell grows in size
- Synthesis of proteins and RNA
- Cell prepares for DNA replication
2. S Phase (Synthesis)
- DNA replication occurs
- Chromosome number remains same, but DNA content doubles
- Histone proteins are synthesized
3. G₂ Phase (Gap 2)
- Further growth of cell
- Preparation for mitosis
- Synthesis of spindle proteins
Quiescent Stage (G₀ Phase)
- Cells exit the cell cycle
- Remain metabolically active but do not divide
- Example: Nerve cells
M Phase (Mitotic Phase)
This phase includes:
- Karyokinesis (division of nucleus)
- Cytokinesis (division of cytoplasm)
Mitosis (Equational Division)
Mitosis is a type of cell division where two identical daughter cells are formed, maintaining the same chromosome number as the parent cell.
Stages of Mitosis
1. Prophase
- Chromatin condenses into chromosomes
- Each chromosome has two sister chromatids
- Nuclear membrane starts disappearing
- Spindle fibers begin to form
2. Metaphase
- Chromosomes align at the equatorial plate
- Spindle fibers attach to centromeres
3. Anaphase
- Centromeres split
- Sister chromatids move to opposite poles
4. Telophase
- Chromosomes decondense
- Nuclear membrane reappears
- Two daughter nuclei formed
Cytokinesis
- Division of cytoplasm
- In plants: Cell plate formation
- In animals: Cleavage furrow
Significance of Mitosis
- Maintains chromosome number
- Growth and development
- Tissue repair
- Asexual reproduction
Meiosis (Reduction Division)
Meiosis is a type of cell division that produces four haploid cells from one diploid cell.
Occurs in reproductive cells.
Meiosis I (Reductional Division)
Prophase I (Longest phase)
Sub-stages:
- Leptotene – Chromosomes become visible
- Zygotene – Synapsis (pairing of homologous chromosomes)
- Pachytene – Crossing over occurs
- Diplotene – Chiasmata visible
- Diakinesis – Terminalization
Metaphase I
- Homologous chromosomes align at equator
Anaphase I
- Homologous chromosomes separate
- Chromosome number reduces
Telophase I
- Two haploid cells formed
Meiosis II (Mitotic-like Division)
Prophase II
- Chromosomes condense again
Metaphase II
- Chromosomes align at equator
Anaphase II
- Sister chromatids separate
Telophase II
- Four haploid daughter cells formed
Significance of Meiosis
- Maintains chromosome number across generations
- Introduces genetic variation
- Essential for sexual reproduction
Differences Between Mitosis and Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cells | 2 | 4 |
| Chromosome number | Same | Half |
| Genetic variation | No | Yes |
| Crossing over | Absent | Present |
Cell Cycle Checkpoints
Checkpoints ensure proper progression:
- G₁ checkpoint: Cell size and nutrients
- G₂ checkpoint: DNA replication accuracy
- M checkpoint: Chromosome alignment
Important Terms
- Chromosome: DNA + protein structure
- Chromatid: One half of chromosome
- Centromere: Joins sister chromatids
- Spindle fibers: Help in chromosome movement
Important NEET & CBSE Points
- Longest phase of cell cycle: Interphase
- Longest phase of mitosis: Prophase
- Crossing over occurs in: Pachytene
- Reduction in chromosome number: Meiosis I
- Equational division: Mitosis
Quick Revision Notes
- Cell cycle = Interphase + M phase
- Mitosis → identical cells
- Meiosis → variation
- Crossing over = genetic diversity
- Checkpoints regulate cycle
Frequently Asked Questions (FAQs)
Q1. What is the cell cycle?
Sequence of events leading to cell division.
Q2. Why is mitosis called equational division?
Because chromosome number remains the same.
Q3. What is crossing over?
Exchange of genetic material between homologous chromosomes.
Q4. Where does meiosis occur?
In reproductive cells.
Conclusion
Cell cycle and cell division are fundamental processes ensuring growth, repair, and reproduction. Understanding mitosis and meiosis is essential for mastering genetics and heredity topics in CBSE and NEET exams.