Cell Division

 

CELL DIVISION (NEET COMPLETE NOTES)

What is Cell Division?

Cell division is the process by which a parent cell divides into daughter cells. It is essential for:

• Growth
• Repair and regeneration
• Reproduction

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TYPES OF CELL DIVISION

1. Amitosis (Direct Division)

• Simple division without spindle formation
• No chromosome condensation
• Seen in:
  • Prokaryotes (like Bacteria)
  • Some protozoa
• Not common in higher organisms

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2. Mitosis (Equational Division)

Definition:

Division of a somatic cell producing two identical daughter cells with same chromosome number.

Occurs in:

• Body (somatic) cells

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Cell Cycle (Important for NEET)

Phases of Cell Cycle:

1. Interphase (95% time)
   • G₁ phase: Growth
   • S phase: DNA replication
   • G₂ phase: Preparation for division
2. M Phase (Mitotic phase)

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Stages of Mitosis

1. Prophase

• Chromosomes condense
• Nuclear membrane disappears
• Spindle fibers form

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2. Metaphase (Most important stage)

• Chromosomes align at equatorial plate
• Best stage to study chromosome structure

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3. Anaphase

• Sister chromatids separate
• Move to opposite poles

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4. Telophase

• Nuclear membrane reappears
• Chromosomes decondense

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Cytokinesis

• Division of cytoplasm
• Forms two daughter cells

In plants: Cell plate formation
In animals: Cleavage furrow

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Significance of Mitosis

• Maintains chromosome number
• Growth and repair
• Asexual reproduction

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3. Meiosis (Reduction Division)

Definition:

Special type of division producing four haploid cells from one diploid cell.

Occurs in:

• Germ cells (gamete formation)

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Two Stages:

• Meiosis I (Reductional division)
• Meiosis II (Equational division)

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MEIOSIS I (Most Important for NEET)

Prophase I (Sub-stages VERY IMPORTANT )

1. Leptotene
   • Chromosomes become visible
2. Zygotene
   • Homologous chromosomes pair (Synapsis)
   • Formation of bivalent
3. Pachytene
   • Crossing over occurs 
   • Exchange of genetic material
4. Diplotene
   • Homologous chromosomes separate
   • Chiasmata visible
5. Diakinesis
   • Terminalization of chiasmata
   • Nuclear membrane disappears

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Metaphase I

• Bivalents align at equator

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Anaphase I

• Homologous chromosomes separate
• Chromosome number reduces (2n → n)

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Telophase I

• Two haploid cells formed

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MEIOSIS II (Like Mitosis)

• Prophase II
• Metaphase II
• Anaphase II (chromatids separate)
• Telophase II

Final result: 4 haploid daughter cells

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KEY TERMS (VERY IMPORTANT)

• Chromosome – DNA + protein structure carrying genes
• Chromatid – One half of chromosome
• Centromere – Joins chromatids
• Synapsis – Pairing of homologous chromosomes
• Crossing over – Exchange of genetic material
• Chiasmata – Points of crossing over

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MITOSIS vs MEIOSIS (NEET FAVORITE)

Feature	Mitosis	Meiosis
Type	Equational	Reductional
Cells formed	2	4
Chromosome number	Same	Half
Genetic variation	No	Yes
Occurs in	Somatic cells	Germ cells

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IMPORTANT NEET POINTS

• Crossing over occurs in Pachytene
• Synapsis occurs in Zygotene
• Chiasmata visible in Diplotene
• Metaphase = best stage for chromosome study
• Anaphase I → reduction division
• Meiosis increases genetic variation

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QUICK MEMORY TRICK

Mitosis:

PMAT (Prophase → Metaphase → Anaphase → Telophase)

Prophase I:

L Z P D D
(Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis)

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SIGNIFICANCE OF MEIOSIS

• Maintains chromosome number across generations
• Produces genetic variation
• Essential for sexual reproduction