Cell Cycle and Cell Division – Class 11 (NCERT Biology)

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The cell cycle and cell division is one of the most fundamental concepts in biology because it explains how new cells are formed, how organisms grow, and how genetic material is passed on to the next generation.


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What is Cell Cycle?

The cell cycle is the sequence of events by which a cell grows, prepares for division, and divides into two daughter cells.

It consists of two main phases:

1. Interphase (95% of the cycle) – The stage between two successive cell divisions.


2. M Phase (Mitotic phase) – The stage of actual cell division.




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Phases of the Cell Cycle

1. Interphase

Also called the resting phase, but it is highly active metabolically. It has three sub-stages:

G1 phase (Gap 1):

Cell growth occurs.

Proteins and RNA are synthesized.

Organelles duplicate.

Cell is metabolically most active.


S phase (Synthesis):

DNA replication occurs → chromosomes duplicate.

Centrosome also duplicates.


G2 phase (Gap 2):

Further growth of the cell.

Proteins for mitosis are synthesized.

Cell prepares for division.



2. Mitotic Phase (M phase)

It includes karyokinesis (nuclear division) and cytokinesis (cytoplasmic division).


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Mitosis – Equational Division

Takes place in somatic cells.

Produces two daughter cells identical to the parent cell (same chromosome number).

Helps in growth, repair, and asexual reproduction.


Stages of Mitosis:

1. Prophase: Chromosomes condense, spindle forms, nuclear membrane disappears.


2. Metaphase: Chromosomes align at the equatorial plate.


3. Anaphase: Sister chromatids separate and move to opposite poles.


4. Telophase: Chromatids reach poles, nuclear envelope reforms, spindle disappears.


5. Cytokinesis: Division of cytoplasm → two daughter cells.




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Meiosis – Reduction Division

Occurs in germ cells during gamete formation.

Produces four daughter cells, each with half the chromosome number (haploid).

Ensures genetic variation and sexual reproduction.


Meiosis I (Reductional division):

Homologous chromosomes pair and segregate.

Crossing over occurs in prophase I → genetic recombination.


Meiosis II (Equational division):

Similar to mitosis.

Sister chromatids separate.

Final result: 4 haploid cells.



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Significance of Cell Division

Mitosis: Growth, repair, maintenance, and asexual reproduction.

Meiosis: Formation of gametes, maintenance of chromosome number across generations, and genetic variation.



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Cell Cycle Regulation

Controlled by cyclins and cyclin-dependent kinases (CDKs).

Checkpoints ensure proper DNA replication and error correction.

Loss of regulation → uncontrolled cell division → cancer.



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Quick Revision Points

Cell cycle = Interphase + M Phase.

G1, S, G2 → growth and preparation phases.

Mitosis → somatic cells, identical daughter cells.

Meiosis → germ cells, haploid cells, variation.

Checkpoints & CDKs regulate the cycle.



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✨ Conclusion:
The cell cycle and cell division are the basis of life itself. While mitosis ensures continuity and growth, meiosis ensures diversity and evolution. Understanding these processes helps us appreciate how organisms develop, reproduce, and adapt.

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