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Q5:
Why is mitosis called equational division?
Solution :
Theoretical Basis of Mitosis
Mitosis, often termed somatic cell division, is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two separate nuclei. The nomenclature "equational division" is derived from the quantitative and qualitative preservation of genetic material across cell generations.
Step 1: Quantitative Analysis of DNA Content
During the Interphase ($S$-phase), the cell undergoes DNA replication. If the initial diploid quantity of DNA is denoted as $2C$, after $S$-phase, the quantity becomes $4C$. Mitosis ensures that through the process of chromosome segregation, these $4C$ units are distributed equally into two daughter cells, each receiving $2C$ of DNA [Per the Conservation of Genetic Information].
Step 2: Qualitative Analysis of Chromosomal Integrity
The ploidy level of a cell is maintained because mitosis involves the longitudinal splitting of sister chromatids at the centromere during Anaphase. Since each chromosome consists of two identical chromatids formed during replication, the separation ensures that each daughter cell receives an identical set of chromosomes to the parent cell.
| Stage | Chromosome Number | DNA Content |
|---|---|---|
| Parent Cell ($G_1$) | $2n$ | $2C$ |
| Post-Synthesis ($S$-Phase) | $2n$ | $4C$ |
| Daughter Cell (Post-Mitosis) | $2n$ | $2C$ |
Step 3: Visual Representation of Chromosome Segregation
Conclusion on Terminology
The term "equational" is justified by the following logical deductions:
- Identity: The genetic constitution of the two daughter nuclei is identical to that of the mother nucleus [Based on the high-fidelity mechanism of DNA polymerase].
- Ploidy Stability: The chromosomal number ($2n$) remains constant from parent to progeny.
- Balanced Partitioning: The mitotic apparatus (spindle fibers) ensures equal pulling forces, resulting in an exact numerical split of the chromatids.
Final Solution: Mitosis is called equational division because it preserves the original chromosome number ($2n$) and DNA content ($2C$) in both resulting daughter cells, ensuring the new cells are genetically identical to the parent cell.
More Questions from Class 11 Biology Cell Cycle and Division EXERCISES
- Q1: What is the average cell cycle span for a mammalian cell?
- Q2: Distinguish cytokinesis from karyokinesis.
- Q3: Describe the events taking place during interphase.
- Q4: What is $G_0$ (quiescent phase) of cell cycle?
- Q6(i): Name the stage of cell cycle at which one of the following events occur: (i) Chromosomes are moved to spindle equator.
- Q6(ii): Name the stage of cell cycle at which one of the following events occur: (ii) Centromere splits and chromatids separate.
- Q6(iii): Name the stage of cell cycle at which one of the following events occur: (iii) Pairing between homologous chromosomes takes place.
- Q6(iv): Name the stage of cell cycle at which one of the following events occur: (iv) Crossing over between homologous chromosomes takes place.
- Q7(a): Describe the following: (a) synapsis Draw a diagram to illustrate your answer.
- Q7(b): Describe the following: (b) bivalent Draw a diagram to illustrate your answer.
- Q7(c): Describe the following: (c) chiasmata Draw a diagram to illustrate your answer.
- Q8: How does cytokinesis in plant cells differ from that in animal cells?
- Q9: Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
- Q10: Distinguish anaphase of mitosis from anaphase I of meiosis.
- Q11: List the main differences between mitosis and meiosis.
- Q12: What is the significance of meiosis?
- Q13(i): Discuss with your teacher about (i) haploid insects and lower plants where cell-division occurs, and
- Q13(ii): Discuss with your teacher about (ii) some haploid cells in higher plants where cell-division does not occur.
- Q14: Can there be mitosis without DNA replication in ‘S’ phase?
- Q15: Can there be DNA replication without cell division?
- Q16(i): Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes ($N$) per cell
- Q16(ii): Analyse the events during every stage of cell cycle and notice how the following two parameters change (ii) amount of DNA content ($C$) per cell
CBSE Solutions for Class 11 Biology Cell Cycle and Division
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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