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Short Notes Cell Cycle and Cell Division - Short Notes for NEET
Cell Cycle
- Definition: Sequence of events between one cell division and the next
- Duration: Approximately 24 hours in human cells
Phases of Cell Cycle
| Phase | Sub-phase | Duration | Events |
|---|---|---|---|
| Interphase (95% of cycle) "Non-dividing phase" | G1 (Gap 1) | 9-11 hours | Cell growth; organelle duplication; RNA and protein synthesis; metabolically active |
| S (Synthesis) | 8-10 hours | DNA replication; chromosome duplication; histone protein synthesis; centrosome duplication | |
| G2 (Gap 2) | 4-5 hours | Continued cell growth; protein synthesis for mitosis; energy storage | |
| M Phase (5% of cycle) | Karyokinesis + Cytokinesis | 1 hour | Nuclear division + Cytoplasmic division |
| G0 Phase | Quiescent stage | Variable | Cells exit cycle; metabolically active but non-dividing (neurons, heart cells) |
Cell Cycle Regulation
- Checkpoints: Control mechanisms to ensure proper cell division
- G1 checkpoint: Checks cell size, nutrients, growth signals, DNA damage
- G2 checkpoint: Checks DNA replication completion and damage
- M checkpoint (Spindle checkpoint): Checks chromosome attachment to spindle
- Cyclins and CDKs: Proteins that regulate cell cycle progression
MITOSIS
Definition and Significance
- Mitosis: Equational division - produces two identical daughter cells with same chromosome number as parent
- Occurs in: Somatic (body) cells
- Chromosome number: Remains same (2n → 2n + 2n)
Significance of Mitosis
- Growth of multicellular organisms
- Repair and regeneration of tissues
- Asexual reproduction
- Maintains genetic stability
- Cell replacement (skin, blood cells)
Stages of Mitosis (Karyokinesis)
| Stage | Key Events |
|---|---|
| PROPHASE (Longest phase) | • Chromatin condenses into visible chromosomes (2 sister chromatids joined at centromere) • Centrioles move to opposite poles • Spindle fibres begin to form • Nuclear envelope starts to break down • Nucleolus disappears |
| METAPHASE (Shortest phase) | • Chromosomes align at metaphase plate/equatorial plane • Centromeres lie on equator • Spindle fibres attach to kinetochores (at centromeres) • Nuclear envelope completely disappears • Maximum condensation of chromosomes - best stage to study chromosome morphology |
| ANAPHASE | • Centromeres split • Sister chromatids separate and move to opposite poles • Chromosomes move by shortening of spindle fibres • V, L, J, or I shaped chromosomes depending on centromere position • Each pole receives equal number of chromosomes |
| TELOPHASE | • Chromosomes reach poles and begin to decondense • Nuclear envelope reforms around each set of chromosomes • Nucleolus reappears • Spindle fibres disappear • Two daughter nuclei formed |
Cytokinesis
| Type | Mechanism | Occurs in |
|---|---|---|
| Animal cells | Cleavage/Furrowing - centripetal (periphery to center) Contractile ring of actin and myosin forms cleavage furrow | Animal cells |
| Plant cells | Cell plate formation - centrifugal (center to periphery) Vesicles from Golgi fuse to form cell plate in middle; grows outward | Plant cells (due to rigid cell wall) |
MEIOSIS
Definition and Significance
- Meiosis: Reductional division - produces four haploid daughter cells from one diploid cell
- Occurs in: Germ cells (reproductive cells) to form gametes
- Chromosome number: Halved (2n → n + n + n + n)
- Divisions: Two consecutive divisions - Meiosis I and Meiosis II
Significance of Meiosis
- Maintains constant chromosome number across generations
- Formation of gametes for sexual reproduction
- Introduces genetic variation through:
- Crossing over (recombination)
- Independent assortment of chromosomes
- Basis of heredity and evolution
Meiosis I (Reductional Division)
| Stage | Key Events |
|---|---|
| PROPHASE I (Longest and most complex) | Divided into 5 sub-stages: 1. Leptotene: Chromosome condensation begins; appear as thin threads 2. Zygotene:Synapsis - pairing of homologous chromosomes; forms bivalent/tetrad; synaptonemal complex forms 3. Pachytene:Crossing over occurs - exchange of genetic material between non-sister chromatids; recombination nodules visible 4. Diplotene: Synaptonemal complex dissolves; homologues begin to separate but remain attached at chiasmata (sites of crossing over); chromosomes fully condensed 5. Diakinesis: Terminalisation of chiasmata; chromosomes maximally condensed; nuclear envelope breaks; nucleolus disappears; spindle forms |
| METAPHASE I | • Bivalents align at metaphase plate • Homologous pairs (not individual chromosomes) at equator • Spindle fibres attach to centromeres • Independent assortment occurs |
| ANAPHASE I | • Homologous chromosomes separate and move to opposite poles • Centromeres do NOT split • Each chromosome still has 2 sister chromatids • Reduction in chromosome number occurs (2n → n) |
| TELOPHASE I | • Two haploid nuclei formed • Nuclear envelope may reform • Chromosomes may partially decondense • Cytokinesis occurs - two haploid cells formed |
Interkinesis
- Short phase between Meiosis I and II
- NO DNA replication (unlike interphase)
- May or may not be present
Meiosis II (Equational Division)
| Stage | Key Events |
|---|---|
| PROPHASE II | • Chromosomes condense again • Nuclear envelope breaks down • Spindle apparatus forms • Similar to mitotic prophase but cells are haploid |
| METAPHASE II | • Individual chromosomes align at metaphase plate • Spindle fibres attach to kinetochores • Similar to mitotic metaphase |
| ANAPHASE II | • Centromeres split • Sister chromatids separate and move to opposite poles • Similar to mitotic anaphase |
| TELOPHASE II | • Four haploid nuclei formed • Nuclear envelope reforms • Chromosomes decondense • Cytokinesis produces four haploid cells (gametes) |
Comparison: Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Type of division | Equational | Reductional |
| Number of divisions | One | Two (Meiosis I & II) |
| Daughter cells produced | 2 | 4 |
| Chromosome number | Same as parent (2n → 2n) | Half of parent (2n → n) |
| Genetic composition | Identical to parent | Different from parent (variation) |
| Occurs in | Somatic cells | Germ cells |
| Crossing over | Absent | Present (Prophase I) |
| Synapsis | Absent | Present (Zygotene) |
| Function | Growth, repair, asexual reproduction | Gamete formation, sexual reproduction |
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FAQs on Short Notes Cell Cycle and Cell Division - Short Notes for NEET
| 1. What are the main phases of the cell cycle? |  |
Ans. The cell cycle consists of four main phases: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis). During G1, the cell grows and prepares for DNA replication. The S phase involves the synthesis of DNA, where each chromosome is duplicated. In G2, the cell continues to grow and prepares for mitosis. Finally, during the M phase, the cell divides its copied DNA and cytoplasm to form two daughter cells.
| 2. What is mitosis and how does it differ from meiosis? | |
Ans. Mitosis is the process of cell division that results in two genetically identical daughter cells, each with the same number of chromosomes as the parent cell. It consists of several stages: prophase, metaphase, anaphase, and telophase. Meiosis, on the other hand, is a specialised form of cell division that occurs in gametes, resulting in four genetically diverse daughter cells, each with half the number of chromosomes of the parent cell. This process includes two rounds of division, meiosis I and meiosis II.
| 3. What role do checkpoints play in the cell cycle? | |
Ans. Checkpoints are regulatory mechanisms in the cell cycle that ensure proper progression through the cycle. They monitor the integrity of DNA, the completion of DNA replication, and the attachment of chromosomes to the spindle apparatus. Key checkpoints occur at the G1, G2, and M phases. If errors or damage are detected, checkpoints can halt the cycle to allow for repair or trigger apoptosis if the damage is irreparable.
| 4. What is the significance of the S phase in the cell cycle? | |
Ans. The S phase is crucial because it is the period during which DNA replication occurs. Each chromosome is duplicated, resulting in two sister chromatids per chromosome, which are essential for accurate segregation during mitosis. This phase ensures that when a cell divides, each daughter cell receives an exact copy of the genetic material, maintaining genetic continuity.
| 5. How do external factors influence the cell cycle? | |
Ans. External factors such as nutrients, growth factors, and environmental conditions can significantly influence the cell cycle. For example, the presence of growth factors can stimulate cells to progress from the G1 phase to the S phase, promoting cell division. Conversely, insufficient nutrients or unfavourable conditions can lead to cell cycle arrest, preventing cell division until conditions improve. This regulation is vital for maintaining tissue homeostasis and preventing uncontrolled cell proliferation.
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