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Q3:
Describe the events taking place during interphase.
Solution :
Biological Foundation: The Interphase Cycle
Interphase represents the preparatory phase of the cell cycle, occupying approximately 90–95% of the total cycle duration. It is categorized into three distinct sub-phases: $G_1$ phase, $S$ phase, and $G_2$ phase. [Per the cell cycle regulation theory].
Step 1: Gap 1 ($G_1$) Phase (Post-mitotic phase)
This phase occurs between the end of mitosis and the initiation of DNA replication. Key physiological events include:
- Metabolic Activity: The cell undergoes intense biochemical activity, including the synthesis of various enzymes, structural proteins, and RNA.
- Growth: There is a significant increase in cytoplasmic volume and organelle proliferation. [Justification: Preparation for subsequent DNA demand].
- Decision Point: The cell reaches the "Restriction Point" ($R$-point). If conditions are suboptimal, the cell may enter a quiescent stage known as the $G_0$ phase.
Step 2: Synthesis ($S$) Phase (DNA Replication)
This phase is dedicated to the semi-conservative replication of the cell's genetic material. The underlying mechanisms are:
- DNA Duplication: The amount of DNA per cell doubles ($2C \rightarrow 4C$), while the chromosome number remains constant ($2n$).
- Histone Synthesis: Concurrent with DNA replication, the cell synthesizes new histone proteins to facilitate the assembly of nucleosomes for the newly formed DNA strands.
- Centriole Duplication: In animal cells, the centriole pair within the centrosome duplicates to prepare for the formation of the mitotic spindle.
Step 3: Gap 2 ($G_2$) Phase (Pre-mitotic phase)
Following the $S$ phase, the cell undergoes a final preparatory check before mitosis begins. Key activities involve:
- Protein Synthesis: Continued synthesis of proteins, specifically tubulin, which is essential for the construction of the spindle apparatus. [Per the Microtubule Organizing Center theory].
- Energy Storage: High accumulation of ATP to fuel the energetically expensive processes of chromosome segregation.
- Checkpoint Control: The $G_2$ checkpoint monitors the integrity of the replicated DNA. If damage is detected, the cell cycle is arrested to allow for repair mechanisms.
Comparative Summary of Interphase Events
| Phase | Primary Characteristic | DNA Content |
|---|---|---|
| $G_1$ | Organelle growth and protein synthesis | $2C$ |
| $S$ | DNA replication and histone synthesis | $2C \rightarrow 4C$ |
| $G_2$ | Tubulin synthesis and error checking | $4C$ |
Final Solution: Interphase is the preparatory period of the cell cycle consisting of $G_1$ (growth), $S$ (DNA replication), and $G_2$ (protein synthesis and structural preparation), ensuring the cell is biologically and genetically equipped for successful nuclear and cytoplasmic division during the subsequent M-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.
- 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
- 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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