Cell Division – Class 11 Maharashtra Board Biology Chapter 7

Cell division is a fundamental biological process through which cells replicate and reproduce. It plays a vital role in growth, development, repair, and reproduction in living organisms.

In multicellular organisms, it ensures proper distribution of genetic material, while in unicellular organisms, it serves as a mode of reproduction. The two main types of cell division are Mitosis and Meiosis.

Before a cell divides, it must pass through the cell cycle, which ensures the accurate duplication and distribution of its genetic material.

🔄 The Cell Cycle

The cell cycle is the series of events a cell goes through during its life. It consists of two main phases:

1. Interphase

This is the longest phase and is divided into three sub-phases:

• G₁ phase (Gap 1): Cell grows, performs normal functions.
• S phase (Synthesis): DNA replication occurs; chromosomes duplicate.
• G₂ phase (Gap 2): Cell prepares for division by synthesizing proteins and organelles.

2. M Phase (Mitotic Phase)

This includes Mitosis (division of the nucleus) and Cytokinesis (division of the cytoplasm).

Cells may also enter a resting phase known as G₀ phase, where they stop dividing (e.g., nerve cells).

🧬 Mitosis – Equational Division

Mitosis is the process by which a single cell divides to produce two genetically identical daughter cells. It occurs in somatic (body) cells and is essential for growth, repair, and asexual reproduction.

🔢 Phases of Mitosis:

1. Prophase:

• Chromosomes condense and become visible.
• Nuclear membrane dissolves.
• Spindle fibers start forming from centrioles.

1. Metaphase:

• Chromosomes align at the cell’s equator.
• Spindle fibers attach to centromeres.

1. Anaphase:

• Sister chromatids separate and move to opposite poles.

1. Telophase:

• Chromatids reach poles and de-condense.
• Nuclear envelope reforms.
• Spindle fibers disintegrate.

🌱 Cytokinesis:

• Division of the cytoplasm.
• In animal cells: Occurs by cleavage furrow.
• In plant cells: Occurs via cell plate formation
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🔍 Importance of Mitosis:

• Growth and development
• Tissue repair
• Replacement of old cells
• Asexual reproduction in some organisms

🌱 Meiosis – Reductional Division

Meiosis is a type of cell division that reduces the chromosome number by half. It occurs in germ cells and leads to the formation of gametes (sperm and egg).

It includes two successive divisions: Meiosis I and Meiosis II.

📘 Meiosis I (Reductional Division)

1. Prophase I: (Longest phase)

• Homologous chromosomes pair (synapsis) to form bivalents.
• Crossing over occurs between chromatids (genetic recombination).
• Nuclear membrane disappears; spindle fibers appear.

1. Metaphase I:

• Bivalents align at the equator.

1. Anaphase I:

• Homologous chromosomes separate to opposite poles.
• Sister chromatids remain attached.

1. Telophase I:

• Two haploid nuclei form.
• Cytokinesis follows.

📘 Meiosis II (Equational Division)

Similar to mitosis, separates sister chromatids.

1. Prophase II:

• New spindle forms.

1. Metaphase II:

• Chromosomes align at the equator.

1. Anaphase II:

• Sister chromatids separate.

1. Telophase II:

• Four haploid daughter cells form.

🔍 Significance of Meiosis

• Maintains chromosome number across generations
• Introduces genetic variation through crossing over and independent assortment
• Essential for sexual reproduction

🧾 Differences between Mitosis and Meiosis

FeatureMitosisMeiosisOccurs inSomatic cellsGerm cells (testes/ovaries)Number of divisionsOneTwoDaughter cellsTwo, diploid, identicalFour, haploid, genetically differentGenetic recombinationAbsentPresent (crossing over)RoleGrowth, repair, asexual reproductionSexual reproduction

⚠️ Errors in Cell Division

Mistakes during cell division can lead to severe consequences:

• Cancer: Uncontrolled mitotic division.
• Down syndrome: Caused by nondisjunction in meiosis leading to trisomy 21.
• Infertility: Errors in gamete formation.

📝 Conclusion

Cell division is a vital biological process that ensures continuity of life, genetic stability, and diversity. Mitosis helps in organismal growth and tissue maintenance, while meiosis plays a crucial role in sexual reproduction and evolution by ensuring genetic variation.

For students of Class 11 Maharashtra Board, understanding this chapter forms a strong base for topics in genetics, heredity, and advanced biology in Class 12 and beyond.

✅ Quick Revision Points

• Interphase = G₁ + S + G₂
• Mitosis = Equational division (2 diploid cells)
• Meiosis = Reductional + equational division (4 haploid cells)
• Crossing over = Prophase I
• Mitosis: No genetic variation; Meiosis: Genetic variation
• Plant vs animal cytokinesis: Cell plate vs cleavage furrow