Cell Cycle & Cell Division
Basics of Cell Division
- Cell Division: The process by which new cells arise from pre-existing cells, essential for growth, reproduction, and repair.
- Kern Plasm Theory: Proposed by Hertwig; mitosis occurs due to disturbance in the Karyoplasmic Index (KI) or nucleocytoplasmic ratio.
- Karyoplasmic Index (KI): KI = Vn / (Vc - Vn) (where Vn = Volume of nucleus, Vc = Volume of cell, Vc - Vn = Volume of cytoplasm). A small cell has a high KI (efficient nuclear control). Cell growth decreases KI, triggering division to restore it NEET 2022.
Cell Cycle
Phases of Cell Cycle
- Cell Cycle: The sequence of events by which a cell duplicates its genome, synthesizes other constituents, and eventually divides.
- Correct sequence of stages: G1 → S → G2 → M NEET 2019, 2020.
- Average duration for a human cell is ~24 hours (Interphase > 95%, M-phase ~ 1 hour). Yeast completes it in 90 minutes.
Interphase (Resting Phase)
Period of preparation for division, involving continuous cell growth and DNA replication. It consists of three sub-phases:
- G1 Phase (Gap 1 / Post-mitotic phase):
- Interval between mitosis and initiation of DNA replication NEET 2020, 2021, 2024.
- Cell is metabolically active, continuously grows, synthesizes RNA and proteins, but does not replicate its DNA NEET 2020, 2024.
- Most organelle duplication and maximum cell growth occur here NEET 2020-Covid.
- S Phase (Synthesis Phase):
- Replication of nuclear DNA and synthesis of histone proteins take place AIPMT 1994, 1996, 2000, NEET 2016, 2024.
- Amount of DNA per cell doubles (e.g., from 2C to 4C) AIPMT 1999, NEET 2014, 2015.
- Crucial detail: Chromosome number remains the same (e.g., 2n remains 2n). If a fruit fly has 8 chromosomes in G1, it will still have 8 after S phase NEET 2023.
- In animal cells, the centriole/centrosome duplicates in the cytoplasm during S phase NEET 2021, 2023.
- If DNA replication is stalled, the cell predominantly activates the G1/S checkpoint NEET 2016.
- G2 Phase (Gap 2 / Pre-mitotic phase):
- Proteins (like tubulin for spindle fibres) are synthesized in preparation for mitosis NEET 2021, 2024.
- Cytoplasmic growth continues NEET 2020-Covid.
- G0 Phase (Quiescent Stage)
- Cells that do not divide further exit G1 phase and enter an inactive stage called G0 NEET 2019, 2020, 2021.
- Cells in G0 are metabolically active but do not proliferate unless called upon by the organism's requirements (e.g., heart cells) NEET 2024.
M-Phase (Mitotic Phase)
Characteristics of Mitosis
- Also known as Equational Division because chromosome number remains the same in parent and progeny cells.
- In animals, restricted to diploid somatic cells. In plants, seen in both haploid and diploid cells.
- Best material to study mitosis: Root tip (e.g., onion root tip) AIPMT 2002, 2004.
- Correct sequence of events: Condensation → Nuclear membrane disassembly → Arrangement at equator → Centromere division → Segregation → Telophase NEET 2017.
Stages of Karyokinesis (Nuclear Division)
| Phase | Key Events & Characteristics |
|---|---|
| Prophase | Chromosomal material condenses to form compact mitotic chromosomes. Centrosomes move to opposite poles and radiate asters. End of prophase: Golgi, ER, nucleolus, and nuclear envelope disappear AIPMT 2010. |
| Metaphase | Complete disintegration of nuclear envelope NEET 2022. Chromosomes are maximally condensed and align at the equator (Metaphase plate) NEET 2011, 2022. Best stage to study morphology/shape/size of chromosomes AIPMT 1994, NEET 2012. Spindle fibres attach to disc-shaped structures called kinetochores AIPMT 2000, NEET 2011, 2014, 2016, 2020, 2022. Chromosomes consist of two chromatids and one centromere AIPMT 1992. |
| Anaphase | Splitting of centromeres and separation of chromatids AIPMT 1991, NEET 2022, 2023. Chromatids (now daughter chromosomes) move to opposite poles NEET 2011. Note: Disruption here by colchicine (prevents spindle formation) doubles chromosome number/induces polyploidy AIPMT 1996, 2002, NEET 2023. Defective APC (Anaphase Promoting Complex) leads to failure of chromosome segregation NEET 2017. |
| Telophase | "Reverse Prophase". Chromosomes decondense and lose their identity NEET 2022. Nuclear envelope develops around chromosome clusters. Nucleolus, Golgi, and ER reform NEET 2013, 2023. |
Cytokinesis (Cytoplasmic Division)
- Animal Cells: Achieved by the appearance of a furrow in the plasma membrane (centripetal order - outside to inside) via actin/myosin contractile ring.
- Plant Cells: Achieved by cell-plate formation (centrifugal order - centre to periphery). The cell plate represents the middle lamella between adjacent cells.
- Exception: Failure of cytokinesis after karyokinesis results in a multinucleate condition called a syncytium (e.g., liquid endosperm in coconut).
Comparisons & Cytokinesis
Types of Cytokinesis: Centripetal vs Centrifugal
Cytokinesis marks the end of cell division by dividing the cytoplasm.
- Animal Cells (Centripetal): Achieved by the appearance of a furrow in the plasma membrane. A contractile ring of actin and myosin forms at the periphery. The furrow deepens from the outside to the inside (centripetal order) until the cytoplasm is divided in two.
- Plant Cells (Centrifugal): Plant cells have an inextensible cell wall, so they divide via cell plate formation. Golgi vesicles align at the equator and fuse from the centre to the periphery (centrifugal order).
- Key Detail: The cell plate acts as a precursor and represents the middle lamella (composed of calcium/magnesium pectate) between the walls of adjacent cells NEET 2023.
- Exception: If karyokinesis is not followed by cytokinesis, a multinucleate condition called a syncytium arises (e.g., liquid endosperm in coconut).
Significance of Cell Division
Significance of Mitosis
- Genetic Identicality: Produces diploid daughter cells with a genetic complement identical to the mother cell.
- Growth: Responsible for the growth and development of multicellular organisms (from a single-celled zygote).
- Nucleo-cytoplasmic Ratio: Cell growth disturbs the Karyoplasmic Index; mitosis restores the efficient nucleo-cytoplasmic ratio.
- Cell Repair: Vital for the continuous replacement of worn-out cells, specifically in the upper layer of the epidermis, lining of the gut, and blood cells.
- Continuous Plant Growth: Mitotic divisions in meristematic tissues (apical and lateral cambium) allow plants to grow continuously throughout their life.
Meiosis
Characteristics and Significance
- Reductional Division: Reduces chromosome number by half, producing four haploid daughter cells AIPMT 1988, NEET 2022.
- Involves two sequential cycles of nuclear/cell division (Meiosis I & II) but only a single cycle of DNA replication.
- Significance: Conserves specific chromosome number across generations in sexually reproducing organisms. Introduces genetic recombination/variations essential for evolution AIPMT 1994, 1998. Daughter cells differ from parent and each other due to crossing over, independent assortment, and segregation AIPMT 1991, 1993.
Meiosis I (Reductional Division)
Initiated after parental chromosomes replicate in S phase.
Prophase I: Highly complex, divided into 5 substages based on chromosomal behavior:
| Key Events & Distinguishing Features | |
|---|---|
| Leptotene | Chromosomes become gradually visible under the light microscope (compaction continues) NEET 2024. Bouquet stage. |
| Zygotene | Synapsis (pairing of homologous chromosomes) occurs AIPMT 2009, NEET 2015, 2020. Formation of the synaptonemal complex NEET 2024. Paired chromosomes form a bivalent or tetrad (clearly visible in next stage) NEET 2022. |
| Pachytene | Bivalent/tetrad clearly visible. Appearance of recombination nodules NEET 2022, 2023, 2024. Crossing over occurs between non-sister chromatids of homologous chromosomes AIPMT 1996, 2004, NEET 2016, 2024. Mediated by the enzyme recombinase NEET 2012, 2014, 2015, 2022. Recombination is completed by the end of this stage NEET 2024. |
| Diplotene | Dissolution of the synaptonemal complex NEET 2020, 2023, 2024. Separation of paired homologous chromosomes begins, forming X-shaped structures called chiasmata NEET 2024. Exception: Can last for months or years in oocytes of some vertebrates NEET 2022. |
| Diakinesis | Terminalisation of chiasmata NEET 2015, 2016, 2021, 2024. Chromosomes fully condensed; nucleolus and nuclear envelope disappear. |
- Metaphase I: Bivalents align at the equator. Microtubules attach to kinetochores of homologous chromosomes.
- Anaphase I: Homologous chromosomes separate (disjunction) and move to opposite poles AIPMT 1988, NEET 2012. Crucial detail: Sister chromatids remain associated at their centromeres (centromere does NOT split) NEET 2012, 2015.
- Telophase I: Nuclear membrane and nucleolus reappear. Followed by cytokinesis, forming a dyad of cells.
- Interkinesis: The gap/stage between Meiosis I and Meiosis II NEET 2024. Short-lived phase where NO DNA replication occurs.
Meiosis II (Equational Division)
Resembles normal mitosis.
- Prophase II: Initiated immediately after cytokinesis. Nuclear membrane disappears.
- Metaphase II: Chromosomes align at the equator.
- Anaphase II: Splitting of the centromere of each chromosome NEET 2021, 2023. Sister chromatids separate and move to opposite poles AIPMT 1993.
- Telophase II: Nuclear envelope encloses the four groups of chromosomes. Followed by cytokinesis, resulting in a tetrad of cells (four haploid daughter cells) NEET 2022.
Significance of Meiosis
- Conservation of Chromosome Number: Paradoxically, while meiosis is a reductional division, it is the exact mechanism that ensures the conservation of the specific chromosome number of a species across generations during sexual reproduction (n + n = 2n upon fertilization) AIPMT 1994.
- Genetic Variability: Crossing over and independent assortment increase genetic variability in the population from one generation to the next AIPMT 1991, 1993.
- Evolution: The variations produced are crucial for the adaptation and evolution of species AIPMT 1994.
Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Nature of Division | Equational (2n → 2n) | Reductional (2n → n) AIPMT 1988 |
| Cell Type | Somatic cells (diploid); rarely haploid (lower plants/social insects) | Germ cells (specialized diploid cells) |
| Number of Divisions | One single cycle of nuclear/cell division | Two sequential cycles (Meiosis I and Meiosis II) |
| Daughter Cells | Two genetically identical daughter cells | Four genetically distinct haploid cells AIPMT 1991 |
| Synapsis & Crossing Over | Absent | Present in Prophase I (Zygotene & Pachytene) AIPMT 1996, NEET 2016, 2024 |
| Centromere Splitting | Splits during Anaphase | Does NOT split in Anaphase I; splits in Anaphase II NEET 2021, 2023 |
Metaphase Vs Metaphase I Vs Metaphase II
| Feature | Metaphase (Mitosis) | Metaphase I (Meiosis I) | Metaphase II (Meiosis II) |
|---|---|---|---|
| Alignment at Equator | Individual chromosomes (dyads) align on a single metaphase plate NEET 2011, 2022. | Bivalents/Tetrads (homologous chromosome pairs) align on the double metaphase plate. | Individual chromosomes (dyads) align on a single metaphase plate (haploid number). |
| Spindle Fibre Attachment | Kinetochores of sister chromatids attach to spindle fibres from opposite poles AIPMT 2000, NEET 2016. | Kinetochores of homologous chromosomes attach to spindle fibres from opposite poles. | Kinetochores of sister chromatids attach to spindle fibres from opposite poles. |
| Chromatid Separation | Prepares for centromere splitting in Anaphase. | Prepares for homologous chromosome separation (centromere does NOT split) NEET 2012. | Prepares for centromere splitting in Anaphase II NEET 2023. |