NEET Biology - Unit 3- B Cell division- Study Notes - New Syllabus

Cell Division: Cell Cycle, Mitosis, Meiosis and Their Significance

📌 Introduction

• Cell division is the process by which a parent cell divides to produce daughter cells.
• It ensures growth, repair, and reproduction in living organisms.
• Types: Mitosis (for growth & repair) and Meiosis (for gamete formation).

🔹 Cell Cycle

Defined as the series of events that a cell undergoes leading to division.

Duration: ~1 hour for fast-dividing cells; variable in others.

Phases:

• Interphase (preparation for division)
• M-phase (Mitosis) (nuclear division)

🟢 Interphase

The cell grows and prepares for division. DNA is replicated.

Sub-phases:

• G1 Phase (First Gap) – Rapid biosynthesis: proteins, mitochondria, ribosomes increase.
• S Phase (Synthesis) – DNA replication occurs → each chromosome has 2 sister chromatids. DNA content doubles; ploidy remains same.
• G2 Phase (Second Gap) – Preparation for mitosis: enzymes & organelles synthesized. Some cells enter G0 phase: exit the cycle, do not divide (e.g., neurons).

🔹 Mitosis (Equational Division) 

Purpose: Produce two genetically identical daughter cells.

Process of nuclear division = karyokinesis.

Phases:

• Prophase – Chromatin condenses into visible chromosomes (2 sister chromatids per chromosome). Nuclear membrane and nucleolus disintegrate. Centrosomes move to opposite poles; spindle fibers form.
• Metaphase – Chromosomes align at the equatorial/metaphase plate. Used in cytogenetics to study chromosome abnormalities.
• Anaphase – Sister chromatids separate → move to opposite poles. Shortest phase; chromosomes appear Y-shaped.
• Telophase – Chromosomes decondense → chromatin. Nuclear membrane and nucleolus reappear. Marks the end of karyokinesis.
• Cytokinesis – Division of cytoplasm and organelles → forms two daughter cells identical to parent.

🔹 Meiosis (Reductional Division)

Purpose: Produce gametes with half the chromosome number (haploid).

Two divisions: Meiosis I and Meiosis II.

🟢 Meiosis I (Reductional Division)

Homologous chromosomes separate, reducing chromosome number by half.

Phases of Prophase I (Longest stage)

• Leptotene: Chromosomes condense; synaptonemal complex starts forming.
• Zygotene: Homologous chromosomes pair (synapsis); forms bivalents/tetrads.
• Pachytene: Crossing over occurs at chiasmata → genetic recombination.
• Diplotene: Homologous chromosomes start to separate; remain connected at chiasmata.
• Diakinesis: Chromosomes condense further; nuclear envelope disappears; spindle forms.

• Metaphase I – Homologous chromosomes align at metaphase plate; sister chromatids remain together.
• Anaphase I – Homologous chromosomes move to opposite poles; sister chromatids still attached.
• Telophase I – Chromosomes decondense; spindle disappears; two haploid daughter cells formed.

🟢 Meiosis II (Equational Division)

Similar to mitosis, separates sister chromatids. Produces four haploid cells from two haploid cells.

Phases:

• Prophase II: Nuclear envelope disappears; spindle forms.
• Metaphase II: Chromosomes align at metaphase plate (perpendicular to Meiosis I).
• Anaphase II: Sister chromatids separate → move to opposite poles.
• Telophase II: Chromosomes decondense; nuclear envelopes form; cytokinesis → four haploid gametes.

🔹 Significance of Cell Division

• Mitosis – Growth and development of multicellular organisms; repair and regeneration; maintains chromosome number.
• Meiosis – Formation of gametes for sexual reproduction; genetic variation via crossing over and independent assortment; reduces chromosome number by half → maintains species chromosome number after fertilization.

📝 Quick Recap
Cell cycle = Interphase + Mitosis.
Interphase: G1 (growth), S (DNA replication), G2 (preparation), G0 (resting).
Mitosis: Prophase → Metaphase → Anaphase → Telophase + Cytokinesis.
Meiosis I: Homologous chromosomes separate → crossing over occurs in Prophase I.
Meiosis II: Sister chromatids separate → produces 4 haploid gametes.
Significance: Mitosis = identical cells; Meiosis = genetic diversity & gamete formation.