Chapter 10: Cell Cycle and Cell Division Application Questions | biology Class 11 CBSE

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Chapter 10: Cell Cycle and Cell Division Application Questions | biology Class 11 CBSE | ByteNotes.in

Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 1

Applied Concept

A cell has 2n = 16 chromosomes and 2C DNA content at the end of M phase. What will be the chromosome number and DNA content at G1, after S phase, and at G2? What change occurs in DNA content but NOT in chromosome number during S phase?

At G1: chromosome number = 16 (2n), DNA content = 2C. No DNA replication has occurred; the cell only grows metabolically.
After S phase: chromosome number remains 16 (2n), DNA content doubles to 4C because each chromosome is replicated to form two sister chromatids still joined at the centromere.
At G2: chromosome number = 16 (2n), DNA content = 4C. G2 involves protein synthesis for mitosis but no further replication.
The change during S phase is that DNA content doubles (2C to 4C) while chromosome number remains unchanged — this is because replication produces sister chromatids that are joined and counted as one chromosome.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 2

Applied Concept

With reference to cytokinesis, explain why a plant cell cannot use the same mechanism as an animal cell to divide its cytoplasm. What alternative mechanism does it use?

Animal cells use cleavage by a plasma membrane furrow, but plant cells are enclosed by a rigid, relatively inextensible cell wall that prevents inward furrowing of the membrane.
Instead, plant cells form a cell-plate in the centre of the cell; this starts with the fusion of Golgi-derived vesicles at the equatorial plane during telophase.
The cell-plate grows centrifugally outward until it meets the existing lateral walls, becoming the future middle lamella between the two daughter cell walls.
This mechanism ensures that division occurs without mechanically distorting the existing cell wall, maintaining cell integrity while successfully partitioning the cytoplasm.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 3

Applied Concept

A scientist observes cells under a microscope and notices some cells with chromosomes maximally condensed and all aligned at the equatorial plane. Some cells show chromosomes moving apart to two poles. Identify the stages and state one key event that distinguishes each stage.

Cells with maximally condensed chromosomes aligned at the equatorial plane are in Metaphase — the nuclear envelope has already disintegrated and chromosomes are attached via kinetochores to spindle fibres from both poles.
Cells with chromosomes moving to opposite poles are in Anaphase — centromeres have split, sister chromatids have separated, and daughter chromosomes migrate with centromere at the leading edge.
Metaphase is the best stage for studying chromosome morphology because chromosomes are maximally condensed and individually distinct at this stage.
Anaphase is the stage that ensures equal genetic distribution: simultaneous centromere splitting guarantees each daughter nucleus receives exactly one copy of each chromosome.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 4

Applied Concept

A student cuts the root tip of an onion (which has 2n = 16 chromosomes) and prepares a slide. She expects to see all four stages of mitosis. In which region of the root tip will she find dividing cells, and why? Predict the chromosome number she will observe in a cell in metaphase.

Dividing cells will be found in the apical meristem of the root tip — this is the region of the root that contains meristematic cells capable of continuous mitotic division throughout the plant's life.
Meristematic cells are actively proliferating and cycling through all stages of the cell cycle; cells further up the root have differentiated and exited the cycle.
In a cell at metaphase, the student will observe 16 chromosomes (2n = 16) aligned at the metaphase plate, each composed of two sister chromatids joined at the centromere.
The chromosome number does not change during mitosis; the same 2n = 16 count is maintained in daughter cells, confirming that mitosis is equational division.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 5

Applied Concept

With reference to the cell cycle, explain why skin cells and gut lining cells need to divide frequently, while heart cells in adults do not. What stage does each cell type enter in the cell cycle?

Skin cells (upper epidermis) and gut lining cells are constantly lost due to mechanical wear and digestive activity respectively; they must be replaced continuously to maintain tissue function.
These cells undergo frequent mitosis — a key role of mitosis described in the chapter as cell repair and replacement; they remain in an active cell cycle.
Heart cells in adult animals, once formed, are long-lived and are not subject to the same rate of mechanical replacement; they exit G1 and enter the G0 (quiescent) stage.
G0 cells remain metabolically active and functional but no longer proliferate unless specifically stimulated by the organism's needs; this is why heart regeneration after injury is very limited.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 6

Applied Concept

During prophase I of meiosis, a student observes X-shaped structures connecting paired chromosomes under an electron microscope. Name these structures, explain how they formed, and state their biological significance.

The X-shaped structures are chiasmata (singular: chiasma), visible during the diplotene sub-stage of Prophase I when the synaptonemal complex dissolves.
Chiasmata form at the sites of crossing over that occurred during pachytene — they are the physical manifestation of genetic material exchange (recombination) between non-sister chromatids of homologous chromosomes.
Biologically, chiasmata represent points of genetic recombination where new allele combinations are created on chromosomes — this is the molecular basis of increased genetic variability.
Chiasmata also physically hold homologous chromosomes together after dissolution of the synaptonemal complex, maintaining bivalent integrity until chromosomes are properly oriented at Metaphase I for reductional separation.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 7

Applied Concept

If a cell undergoes meiosis starting with 2n = 8 chromosomes and 2C DNA, what will be the chromosome number and DNA content in each of the four daughter cells at the end of Meiosis II?

After Meiosis I (reductional division), each of the two cells (dyad) has n = 4 chromosomes; DNA content is 2C per cell since each chromosome still has two chromatids but chromosome number is halved.
Interkinesis follows — no DNA replication occurs, so each cell still has n = 4 chromosomes and 2C DNA content entering Meiosis II.
After Meiosis II (equational division), centromeres split and sister chromatids separate; each of the four haploid daughter cells has n = 4 chromosomes and 1C DNA content.
Thus, starting from 2n = 8 and 2C, each of the four final daughter cells has 4 chromosomes and 1C DNA — demonstrating both the reductional and the equational aspects of meiosis.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 8

Applied Concept

A biology student claims: 'Since meiosis reduces chromosome number, it would eventually lead to extinction of a species over generations.' Evaluate this claim using your knowledge of the life cycle of sexually reproducing organisms.

The claim is incorrect. Meiosis does reduce chromosome number by half (producing haploid gametes), but this reduction is balanced by fertilisation, which fuses two haploid gametes to restore the diploid number.
The chapter explicitly states that meiosis ensures the haploid phase in the life cycle, while fertilisation restores the diploid phase — these two processes work in tandem to maintain species-specific chromosome numbers across generations.
If only meiosis occurred without fertilisation, the claim would have merit; but the complementary relationship between meiosis and fertilisation is what conserves chromosome numbers generation after generation.
Far from causing extinction, meiosis actually promotes species survival by generating genetic diversity (through crossing over and independent assortment), providing variation that is the raw material for evolution and adaptation.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 9

Applied Concept

With reference to the chapter, explain why the stage between the two meiotic divisions is called 'interkinesis' rather than 'interphase', and what crucial difference exists between the two.

Interkinesis is the stage between Meiosis I and Meiosis II; it is short-lived and the chromosomes do not return to the fully extended state typical of interphase nuclei.
The critical distinction is that NO DNA replication occurs during interkinesis, whereas interphase (specifically the S phase) is characterised by DNA replication that doubles DNA content.
Because there is no S phase during interkinesis, cells entering Meiosis II have the same DNA content as they had at the end of Meiosis I — they do not re-synthesise DNA.
This absence of replication during interkinesis is what enables meiosis to produce haploid cells: only one round of replication occurs for two rounds of division, ensuring chromosome number is halved overall.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 10

Applied Concept

A cell biologist treats dividing animal cells with a drug that prevents spindle fibre formation. Predict what will happen during mitosis and explain the consequence for the daughter cells.

Without spindle fibres, kinetochores cannot attach to spindle fibres from the poles, so chromosomes cannot be moved to and aligned at the metaphase plate — metaphase will be arrested.
Even if cell division somehow proceeds, without spindle fibres the centromeres cannot be pulled to opposite poles during anaphase, so chromosomes cannot segregate.
The result would be that both sets of chromosomes remain in one cell, producing daughter cells with double the normal chromosome number (polyploidy/aneuploidy) or a cell that simply cannot complete division.
This demonstrates the essential role of the spindle apparatus (centrosomes, asters, kinetochore microtubules) in ensuring accurate and equal chromosome distribution to daughter cells during anaphase.

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Chapter 10: Cell Cycle and Cell Division | Application Questions

A cell has 2n = 16 chromosomes and 2C DNA content at the end of M phase. What will be the chromosome number and DNA content at G1, after S phase, and at G2? What change occurs in DNA content but NOT in chromosome number during S phase?

With reference to cytokinesis, explain why a plant cell cannot use the same mechanism as an animal cell to divide its cytoplasm. What alternative mechanism does it use?

A scientist observes cells under a microscope and notices some cells with chromosomes maximally condensed and all aligned at the equatorial plane. Some cells show chromosomes moving apart to two poles. Identify the stages and state one key event that distinguishes each stage.

A student cuts the root tip of an onion (which has 2n = 16 chromosomes) and prepares a slide. She expects to see all four stages of mitosis. In which region of the root tip will she find dividing cells, and why? Predict the chromosome number she will observe in a cell in metaphase.

With reference to the cell cycle, explain why skin cells and gut lining cells need to divide frequently, while heart cells in adults do not. What stage does each cell type enter in the cell cycle?

During prophase I of meiosis, a student observes X-shaped structures connecting paired chromosomes under an electron microscope. Name these structures, explain how they formed, and state their biological significance.

If a cell undergoes meiosis starting with 2n = 8 chromosomes and 2C DNA, what will be the chromosome number and DNA content in each of the four daughter cells at the end of Meiosis II?

A biology student claims: 'Since meiosis reduces chromosome number, it would eventually lead to extinction of a species over generations.' Evaluate this claim using your knowledge of the life cycle of sexually reproducing organisms.

With reference to the chapter, explain why the stage between the two meiotic divisions is called 'interkinesis' rather than 'interphase', and what crucial difference exists between the two.

A cell biologist treats dividing animal cells with a drug that prevents spindle fibre formation. Predict what will happen during mitosis and explain the consequence for the daughter cells.

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