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अध्याय 10: कोशिका चक्र और कोशिका विभाजन

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CBSE • कक्षा XI • Biology • अध्याय 10

अनुमानित समय: 45 Mins

प्रगति: अध्ययनरत

कोशिका चक्र और कोशिका विभाजन

In Class 11 Biology, "Cell Cycle and Cell Division" provides an authoritative, curriculum-verified master resource aligned with the 2026–27 NCERT syllabus.

अध्याय CBT ऑनलाइन टेस्ट दें सभी अध्याय देखें

प्रस्तावना अध्याय अवधारणाएँ अध्याय सारांश स्व-मूल्यांकन अभ्यास टेस्ट दें

🧬 Have You Ever Wondered?

When you scrape your knee on the pavement, how do your skin cells know to divide rapidly to patch the wound and then stop dividing the exact millisecond the skin is restored? The tightly regulated cell cycle and mitosis/meiosis checkpoints preserve genetic stability.

यह अध्याय क्यों महत्वपूर्ण है

In Class 11 Biology, "Cell Cycle and Cell Division" provides an authoritative, curriculum-verified master resource aligned with the 2026–27 NCERT syllabus.

अध्ययन से पूर्व (आवश्यक ज्ञान)

Cell theory from Chapter 8.
Chromosomes and DNA.
Gametes and fertilization.

इस अध्याय के लक्ष्य

Explain the phases of the Eukaryotic Cell Cycle: Interphase ($G_1, S, G_2$) and M-Phase (Mitosis).
Describe DNA replication and centrosome duplication during S-phase ($2C \to 4C$, chromosome count remains $2n$!).
Trace the stages of Mitosis: Prophase, Metaphase (chromosomes align at equator, equatorial plate), Anaphase (centromeres split, chromatids move to opposite poles), Telophase, and Cytokinesis.
Analyze Meiosis-I: Prophase-I substages (Leptotene, Zygotene/synapsis/synaptonemal complex, Pachytene/crossing over/recombination nodules, Diplotene/chiasmata, Diakinesis/terminalization).
Contrast Mitosis (equational division, $2n \to 2n$) and Meiosis (reductional division, $2n \to n$, source of genetic variation).

अध्याय रूपरेखा एवं प्रगति

1 1. Interphase & The S-Phase Rule

2 2. Mitosis: Equational Division

3 3. Meiosis-I & Crossing Over

सम्पूर्ण सैद्धांतिक एवं वैचारिक अध्ययन

1. Interphase & The S-Phase Rule

A 24-hour human cell cycle spends 23 hours in Interphase (preparation) and barely 1 hour in M-phase:
• $G_1$ Phase (Gap 1): Cell metabolically active, continuous growth.
• S Phase (Synthesis): DNA replication occurs! DNA content doubles from $\mathbf{2C \to 4C}$. Critical Rule: The chromosome number remains strictly unchanged ($2n$)! Centriole duplicates in cytoplasm.
• $G_2$ Phase: Proteins synthesized for mitosis.
• $G_0$ Phase (Quiescent): Metabolically active cells that exit the cycle without dividing (e.g. Heart muscle, neurons).

2. Mitosis: Equational Division

Nuclear division (Karyokinesis) across 4 continuous stages:
• Prophase: Chromatin condenses into distinct chromosomes; nucleolus and nuclear envelope disintegrate.
• Metaphase: Spindle fibers attach to Kinetochores; all chromosomes align at the cell center forming the Metaphase Plate (best stage to study chromosome morphology!).
• Anaphase: Centromeres split simultaneously; sister chromatids pull apart to opposite poles ($46 \to 46$ on each side).
• Telophase & Cytokinesis: Two identical diploid daughter cells emerge!

3. Meiosis-I & Crossing Over

Reduction division ($2n \to n$). Prophase-I has 5 crucial sub-stages:
(1) Leptotene: Chromosomes become visible.
(2) Zygotene: Homologous chromosomes pair up (Synapsis), forming the Synaptonemal Complex (Bivalents/Tetrads).
(3) Pachytene: Crossing over occurs between non-sister chromatids of homologous chromosomes, mediated by the enzyme Recombinase (generates new genetic combinations!).
(4) Diplotene: Synaptonemal complex dissolves; homologous chromosomes remain attached only at X-shaped crossover junctions called Chiasmata.
(5) Diakinesis: Terminalization of chiasmata.

Cell Cycle and Cell Division - Key Biological & Anatomical Model

अध्याय का सार संक्षेप एवं 10 मुख्य निष्कर्ष

मुख्य बिंदु 1

S-Phase DNA Doubling: Nuclear DNA doubles ($2C \to 4C$) while chromosome count remains invariant ($2n$).

मुख्य बिंदु 2

Kinetochore Attachment: Protein discs at the centromere docking spindle microtubules in metaphase.

मुख्य बिंदु 3

Synaptonemal Complex: Protein zipper aligning homologous chromosomes during zygotene.

मुख्य बिंदु 4

Crossing Over: Reciprocal genetic exchange at pachytene driven by recombinase enzyme.

मुख्य बिंदु 5

Chiasmata: Visible cytological X-shaped crossover junctions unmasked during diplotene.

स्व-मूल्यांकन अभ्यास (Check Your Understanding)

मूल वैचारिक स्पष्टता की जांच के लिए नैदानिक प्रश्न। पहले स्वयं हल करें, फिर उत्तर देखें।

A cell has 24 chromosomes ($2n = 24$) and 2C DNA content at $G_1$ phase. What will be the chromosome number and DNA content of this cell: (i) after S phase, (ii) after Mitosis, (iii) after Meiosis-I?

उत्तर एवं व्याख्या देखें

उत्तर: (i) After S phase: Chromosome number remains $2n = 24$; DNA content doubles to $4C$. (ii) After Mitosis: Chromosome number $= 24$; DNA content $= 2C$. (iii) After Meiosis-I: Chromosome number is halved to $n = 12$; DNA content is $2C$.
(i) 24 chromosomes, 4C DNA; (ii) 24 chromosomes, 2C DNA; (iii) 12 chromosomes, 2C DNA.

Name the stage of the cell cycle at which each of the following events occurs: (i) Chromosomes align at equatorial plate, (ii) Centromeres split and chromatids separate, (iii) Crossing over takes place, (iv) Synapsis of homologous chromosomes, (v) Terminalization of chiasmata.

उत्तर एवं व्याख्या देखें

उत्तर: (i) Metaphase, (ii) Anaphase, (iii) Pachytene of Prophase-I, (iv) Zygotene of Prophase-I, (v) Diakinesis of Prophase-I.
Metaphase, Anaphase, Pachytene, Zygotene, Diakinesis.

What is the significance of Meiosis in sexually reproducing organisms?

उत्तर एवं व्याख्या देखें

उत्तर: (1) It conserves the specific chromosome number of each species across generations by reducing chromosome number to half ($2n \to n$) in gametes, which restores diploidy ($2n$) upon fertilization. (2) Crossing over and independent assortment generate novel genetic variations essential for evolution and natural selection.
Conserves chromosome number across generations and generates genetic variation.

Differentiate between Cytokinesis in an animal cell and a plant cell.

उत्तर एवं व्याख्या देखें

उत्तर: In animal cells, cytokinesis occurs through the formation of a peripheral cleavage furrow in the plasma membrane that deepens centripetally from outside to inside; in plant cells with rigid walls, cytokinesis occurs through the formation of a central Cell Plate (precursor: phragmoplast) that grows centrifugally from center to the outer lateral walls.
Cleavage furrow (centripetal in animals) vs Cell Plate (centrifugal in plants).

What is the $G_0$ (Quiescent) phase of the cell cycle?

उत्तर एवं व्याख्या देखें

उत्तर: It is an inactive non-dividing stage of the cell cycle where cells that do not divide further exit the $G_1$ phase. Cells in $G_0$ remain metabolically active and perform specialized functions (e.g. neurons and heart cells) but divide only when required to replace damaged or lost cells.
Non-dividing, metabolically active resting phase exiting G1.

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