Objectives

At the end of this section, you will be able to:

•  outline the pattern of mitotic cell division, 
•  tell how mitosis   produces daughter cells as an exact copy of
• mother cell and point out the importance of mitosis
• outline the pattern of cell division by meiosis,
•  tell the importance of meiosis  and
•  point out the difference between mitosis and meiosis

Overview

Mitosis and meiosis are two types of cell division processes that play crucial roles in growth, development, and reproduction in living organisms. Understanding these processes is essential for comprehending how organisms develop, reproduce, and maintain their genetic integrity.

Mitosis

Brain storming Question

Dear learner! What is mitosis?

Definition

Mitosis is a type of cell division that results in two daughter cells, each with the same number and kind of chromosomes as the parent nucleus. It is essential for growth, tissue repair, and asexual reproduction.

Dear learner!

How does a cell divides by mitosis?

What happens in the different phases  while a cell undergoes mitosis?

Dear student, when you go through the following stages of mitosis you will certainly over come it.

Stages of Mitosis

1. Prophase:
   • Chromosomes condense and become visible.
   • The nuclear envelope begins to break down.
   • Spindle fibers start to form from the centrosomes.
2. Metaphase:
   • Chromosomes align at the cell’s equatorial plane, known as the metaphase plate.
   • Spindle fibers attach to the centromeres of the chromosomes.
3. Anaphase:
   • Sister chromatids are pulled apart by the spindle fibers and move toward opposite poles of the cell.
4. Telophase:
   • Chromatids reach the poles and begin to decondense.
   • The nuclear envelope re-forms around each set of chromosomes.
   • The spindle fibers disassemble.
5. Cytokinesis:
   • The cytoplasm divides, resulting in two genetically identical daughter cells.

Key Points

• Results in two identical daughter cells.
• Maintains chromosome number (diploid in most organisms).
• Involves one round of cell division.
• Essential for growth and repair.

Meiosis

Dear learner! What is meiosis?  How does it differ from mitosis?

Definition

Meiosis is a type of cell division that reduces the chromosome number by half, resulting in four daughter cells, each with half the number of chromosomes of the parent cell. It is essential for sexual reproduction and genetic diversity.

Stages of Meiosis

Meiosis consists of two successive divisions: Meiosis I and Meiosis II.

Meiosis I:

1. Prophase I:
   • Chromosomes condense and homologous chromosomes pair up (synapsis).
   • Crossing over occurs, where homologous chromosomes exchange genetic material.
   • The nuclear envelope breaks down and spindle fibers form.
2. Metaphase I:
   • Homologous chromosome pairs align at the metaphase plate.
   • Spindle fibers attach to the centromeres of homologous chromosomes.
3. Anaphase I:
   • Homologous chromosomes are pulled apart to opposite poles of the cell.
4. Telophase I and Cytokinesis:
   • Chromosomes reach the poles, and the cell divides into two haploid cells.

Meiosis II:

1. Prophase II:
   • Chromosomes condense, and the nuclear envelope breaks down (if it re-formed).
   • Spindle fibers form again.
2. Metaphase II:
   • Chromosomes align at the metaphase plate in each haploid cell.
   • Spindle fibers attach to the centromeres.
3. Anaphase II:
   • Sister chromatids are pulled apart and move toward opposite poles.
4. Telophase II and Cytokinesis:
   • Chromatids reach the poles, decondense, and the nuclear envelope re-forms.
   • The cytoplasm divides, resulting in four genetically diverse haploid daughter cells.

Key Points

• Results in four genetically diverse daughter cells.
• Reduces chromosome number by half (haploid).
• Involves two rounds of cell division.
• Essential for sexual reproduction and genetic variation.

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Purpose	Growth and repair	Sexual reproduction
Number of Divisions	One	Two
Number of Daughter Cells	Two	Four
Genetic Makeup	Identical to parent cell	Genetically diverse
Chromosome Number	Maintained (diploid)	Halved (haploid)
Occurrence	Somatic (body) cells	Germ (reproductive) cells

Dear learner! Why are mitosis and meiosis

Importance in Biology

• Mitosis: Ensures that each new cell has the same genetic information as the parent cell, which is crucial for growth, development, and tissue repair.
• Meiosis: Generates genetic diversity through recombination and independent assortment, which is vital for evolution and adaptation in sexually reproducing populations.

Understanding these processes helps explain fundamental biological concepts, such as inheritance, genetic variation, and the continuity of life.

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This note provides a detailed overview of mitosis and meiosis, their stages, differences, and significance in biology.

Dr. Melaku Worede: A Renowned Ethiopian Geneticist

Dr. Melaku Worede is a celebrated Ethiopian geneticist and agronomist known for his pioneering work in seed conservation and sustainable agriculture. He played a critical role in preserving Ethiopia’s genetic wealth by establishing one of the world’s finest seed conservation centers. Dr. Melaku was instrumental in founding the Plant Genetic Resource Centre in Addis Ababa, where he served as Director from 1979 until his retirement after more than 14 years.

His work extended beyond retirement, focusing on conservation and enhancement of native seeds, known as landraces. He demonstrated the potential of these seeds to thrive without commercial fertilizers or chemicals, achieving yields 10-15% higher than high-input counterparts and 20-25% higher than traditional farmers’ cultivars. For his efforts in safeguarding Africa’s agricultural biodiversity and supporting small-scale farmers, Dr. Melaku Worede was awarded the Right Livelihood Award in 1989.