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Q13(ii):
Discuss with your teacher about
(ii) some haploid cells in higher plants where cell-division does not occur.
Solution :
1. Biological Context: Haploidy in Higher Plants
In higher plants (Angiosperms and Gymnosperms), the life cycle is characterized by alternation of generations, where a diploid sporophyte ($2n$) undergoes meiosis to produce haploid ($n$) spores. While these haploid spores typically divide mitotically to form a multicellular gametophyte, specific cell types are programmed to terminate their cell cycle progression and differentiate into functional units without further division.
2. Key Examples of Non-Dividing Haploid Cells
The following table details the specific haploid cell types in higher plants that are terminal in their development:
| Cell Type | Location | Biological Justification |
|---|---|---|
| Mature Pollen Grains (Vegetative Cell) | Anther | The vegetative cell (tube cell) governs pollen tube growth. It undergoes no further division [Per the G₀ phase arrest mechanism]. |
| Synergids | Embryo Sac | Specialized for pollen tube guidance; these cells become metabolically inactive or degenerate post-fertilization [Per programmed cell senescence]. |
| Antipodal Cells | Chalazal end of Embryo Sac | These serve a transient nutritive role and eventually undergo apoptosis or cessation of function [Per developmental programming]. |
3. Physiological Mechanism of Division Arrest
The cessation of cell division in these haploid cells is primarily governed by the withdrawal from the cell cycle into the $G_0$ phase (quiescence) or via terminal differentiation. The transition is regulated by:
- Cyclin-Dependent Kinase (CDK) Inhibition: The absence of specific D-type cyclins prevents the phosphorylation of the Retinoblastoma ($Rb$) protein, thereby preventing the release of $E2F$ transcription factors required for the $S$-phase transition [By Regulation of Cell Cycle Control Points].
- Metabolic Specialization: Cells like the vegetative pollen cell prioritize protein synthesis and cytoskeletal transport over DNA replication [Per metabolic trade-off theory].
4. Visual Representation of Cell Cycle Arrest
5. Conclusion
In higher plants, haploid cells such as the vegetative pollen cell, synergids, and antipodal cells represent terminal differentiation. These cells exit the cell cycle to serve specific physiological or reproductive roles rather than contributing to proliferative growth. Their inability to divide is a genetically determined requirement to ensure the integrity and specific function of the gametophytic generation.
Final Solution: Examples of non-dividing haploid cells in higher plants include the vegetative cell of a mature pollen grain, synergid cells, and antipodal cells, all of which undergo terminal differentiation and exit the cell cycle into the $G_0$ phase.
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?
- Q5: Why is mitosis called equational division?
- 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
- 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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