Cell cycle and division 📌 - Biology Class 11th - Handwritten Notes 📖 [PDF]📚

Introduction

• Growth and reproduction of cells occur through a cell cycle.
• Cell cycle = The sequence of events by which a cell grows, prepares, and divides into two daughter cells.
• Duration varies by cell type (in humans ~24 hours for dividing cells).

Phases of Cell Cycle

The cell cycle has two main phases:

1. Interphase (Preparation Phase, ~95% of cycle)

Cell is metabolically active and preparing for division. Divided into three subphases:

G1 Phase (Gap 1)

• Cell grows in size.
• Protein and RNA synthesis.
• Organelles double.
• Checkpoint ensures cell is ready for DNA replication.

S Phase (Synthesis)

• DNA replication (chromosomes duplicate).
• Histone proteins synthesized.
• Centrosome duplicates.

G2 Phase (Gap 2)

• Further growth.
• Synthesis of enzymes needed for mitosis.
• Cell checks for DNA damage and readiness for division.

2. M Phase (Mitosis/Meiosis Phase)

• Actual cell division.
• Two types: Mitosis (equational division) and Meiosis (reductional division).

Mitosis (Equational Division)

• Occurs in somatic cells.
• Produces two daughter cells identical to parent.
• Important for growth, repair, and asexual reproduction.

Phases of Mitosis:

1. Prophase

• Chromosomes condense, become visible.
• Spindle fibers start forming.
• Nuclear envelope breaks down (late prophase).

2. Metaphase

• Chromosomes align at equatorial plate.
• Spindle fibers attach to centromeres (kinetochores).

3. Anaphase

• Sister chromatids separate and move to opposite poles.

4. Telophase

• Chromosomes de-condense.
• Nuclear envelope reforms around each set of chromosomes.

5. Cytokinesis

• Division of cytoplasm.
• Animal cells → cleavage furrow.
• Plant cells → cell plate formation.

Meiosis (Reductional Division)

• Occurs in germ cells (gametes).
• Produces four haploid daughter cells.
• Maintains chromosome number across generations.
• Increases genetic variation due to crossing over.

Two divisions:

Meiosis I (Reductional Division)

• Homologous chromosomes separate → chromosome number halved.

Phases:

1. Prophase I (longest, divided into 5 stages):

• Leptotene → Chromosomes condense.
• Zygotene → Homologous chromosomes pair (synapsis).
• Pachytene → Crossing over occurs (exchange of genetic material).
• Diplotene → Homologous chromosomes begin to separate but remain attached at chiasmata.
• Diakinesis → Terminalization of chiasmata; spindle formed.

2. Metaphase I → Homologous pairs align at equator.

3. Anaphase I → Homologous chromosomes separate (sister chromatids remain attached).

4. Telophase I & Cytokinesis → Two haploid daughter cells form.

Meiosis II (Equational Division)

• Similar to mitosis; separates sister chromatids.
• Produces 4 haploid cells from 1 diploid parent cell.

Phases:

1. Prophase II → Chromosomes condense again.
2. Metaphase II → Chromosomes align at equator.
3. Anaphase II → Sister chromatids separate.
4. Telophase II → Nuclear envelope reforms.
5. Cytokinesis → 4 haploid cells produced.

Differences Between Mitosis and Meiosis

Feature	Mitosis	Meiosis
Occurs in	Somatic cells	Germ cells
Number of divisions	One	Two
Daughter cells	Two	Four
Chromosome number	Diploid → Diploid	Diploid → Haploid
Genetic identity	Identical to parent	Variation due to crossing over
Function	Growth, repair, asexual reproduction	Gamete formation, genetic diversity

Key Points to Remember

• Interphase = longest phase (preparation).
• Mitosis = equational division, conserves chromosome number.
• Meiosis = reductional division, maintains stability across generations.
• Crossing over = occurs in Pachytene of Prophase I (meiosis).
• Chiasmata = physical points of crossing over (Diplotene).
• Genetic variation arises due to crossing over + independent assortment.