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Q14:
Can there be mitosis without DNA replication in ‘S’ phase?
Solution :
Analysis of Mitotic Requirements
Mitosis is defined as an equational division, a process designed to distribute an identical set of chromosomes into two daughter nuclei. To determine if mitosis can occur without DNA replication in the $S$ phase, we must analyze the biochemical and mechanical prerequisites of the cell cycle.
Step 1: The Principle of Genome Equivalence
The fundamental goal of mitosis is the maintenance of genomic stability. A cell entering mitosis must possess a replicated genome ($4C$ DNA content, where $C$ is the haploid DNA content) to ensure that each daughter cell receives a diploid ($2C$) complement of genetic material. [Per the Cell Theory and the Principle of Genomic Continuity].
Step 2: Chromosomal Structure and Condensation
During the $S$ phase, DNA replication occurs such that each chromosome consists of two sister chromatids joined at the centromere.
- With replication: The cell enters prophase with $2n$ chromosomes, each consisting of two chromatids. Anaphase then separates these chromatids, resulting in two daughter cells with $2n$ chromosomes.
- Without replication: The cell would enter mitosis with chromosomes consisting of only a single chromatid.
Step 3: Mechanical Constraints of Anaphase
Mitotic spindle fibers (kinetochore microtubules) attach to the kinetochores located on the centromeres. The separation of chromatids is dependent on the cleavage of cohesin proteins that hold the replicated sister chromatids together. If DNA replication has not occurred, there are no "sister chromatids" to separate. Consequently, the segregation machinery would have no mechanism to generate two equivalent nuclei from one. [By the Mechanics of Spindle Assembly Checkpoint (SAC)].
Step 4: Theoretical Outcome of Replication-Free Mitosis
If a cell were to undergo mitosis without $S$ phase replication:
| Phase | Chromosomal State (Hypothetical) |
|---|---|
| Prophase | $2n$ chromosomes, each with one chromatid. |
| Anaphase | Failure to segregate; chromosome count in daughter cells would be effectively halved ($n$). |
| Outcome | Formation of aneuploid cells with only half the required genetic information (Haploidization). |
Step 5: Biological Justification
The cell cycle is strictly regulated by Cyclin-Dependent Kinases (CDKs). The transition from $G_1$ to $S$ is mandatory because the cell is genetically programmed to prevent mitosis unless the DNA is duplicated. A cell attempting mitosis without DNA replication would trigger the DNA Damage Checkpoint or fail the Spindle Assembly Checkpoint, leading to apoptosis rather than a successful division.
Final Solution: Mitosis cannot occur without DNA replication in the ‘S’ phase because the fundamental requirement of mitotic division is the separation of sister chromatids to ensure genomic integrity. Without replication, the cell lacks the requisite DNA quantity to produce two viable daughter cells, and the mechanical machinery of the spindle fibers would fail to achieve proper segregation, resulting in non-viable or aneuploid progeny.
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.
- 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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