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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
Solution :
Initial Parameters & Definitions
The cell cycle is a regulated sequence of events leading to cellular replication. The parameter $C$ represents the amount of DNA content in a haploid genome. In a diploid somatic cell ($2n$), the initial DNA content is defined as $2C$. We track the variations of $C$ across the distinct phases: $G_1$, $S$, $G_2$, and $M$ phases.
Step 1: The Gap 1 ($G_1$) Phase
During the $G_1$ phase, the cell undergoes metabolic growth and prepares for DNA replication. As the cell has not yet initiated the synthesis of new genetic material:
- DNA content: Remains $2C$.
- [Per the definition of the post-mitotic phase in cell cycle regulation].
Step 2: The Synthesis ($S$) Phase
During the $S$ phase, the process of DNA replication occurs. The cell synthesizes an exact copy of each chromosome.
- Process: The DNA content doubles as each chromatid is replicated.
- DNA content transition: Increases linearly from $2C$ to $4C$.
- [Per the semi-conservative replication mechanism of DNA].
Step 3: The Gap 2 ($G_2$) Phase
Following replication, the cell enters $G_2$ where it prepares for mitosis by synthesizing proteins and organelles.
- DNA content: Remains stable at $4C$.
- [Per the state of post-replicative stability].
Step 4: The Mitotic (M) Phase
The M-phase involves the separation of replicated chromosomes (sister chromatids) into two daughter nuclei.
- Prophase to Anaphase: The DNA content remains at $4C$.
- Anaphase: Sister chromatids separate, and the cell is considered to possess two sets of chromosomes.
- Telophase/Cytokinesis: The cell physically divides, splitting the $4C$ content equally into two daughter cells.
- Final DNA content per daughter cell: $2C$.
- [Per the Law of Segregation and the mechanics of the mitotic spindle].
Graphical Representation of DNA Content ($C$) Over Time
Summary Table of DNA Content
| Phase | DNA Content |
|---|---|
| $G_1$ | $2C$ |
| $S$ | $2C \rightarrow 4C$ |
| $G_2$ | $4C$ |
| Mitosis (until Anaphase) | $4C$ |
| Post-Cytokinesis | $2C$ (per daughter cell) |
Final Solution: The DNA content per cell remains $2C$ during $G_1$, increases to $4C$ during the $S$ phase, remains $4C$ through $G_2$ and the majority of mitosis, and returns to $2C$ in each daughter cell upon the completion of cytokinesis.
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
- 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
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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