Meiosis and Mitosis

Meiosis

Meiosis is the process of cell division that produces gametes (sex cells like sperm and eggs) in sexually reproducing organisms.

• It reduces the chromosome number by half, from diploid (2n) to haploid (n).

• This ensures that when two gametes unite during fertilization, the resulting organism will have the correct number of chromosomes.

• It creates four gametes, each of which is genetically different from one another and from the parent cell.

Key Characteristics:

• Diploid to haploid: Starts with a diploid cell (two sets of chromosomes) and results in haploid cells (one set of chromosomes).

• Two divisions: Meiosis involves two rounds of division, called Meiosis I and Meiosis II.

• Genetic diversity: Crossing over and independent assortment during meiosis lead to genetically unique offspring.

Stages of Meiosis:

Meiosis I: The Reduction Division

1. Prophase I:

• Chromosomes condense, and homologous chromosomes (one from each parent) pair up.

• Crossing over occurs, where homologous chromosomes exchange segments, increasing genetic diversity.

• The nuclear membrane dissolves, and spindle fibers begin to form.

2. Metaphase I:

• Homologous chromosome pairs line up at the metaphase plate (the middle of the cell).

• The arrangement of these pairs is random, leading to independent assortment, which increases genetic variation.

3. Anaphase I:

• The spindle fibers pull the homologous chromosomes apart, moving them to opposite poles of the cell.

• Unlike in mitosis, sister chromatids remain attached at this stage.

4. Telophase I & Cytokinesis:

• The cell divides into two haploid cells, each with half the original number of chromosomes.

• The nuclear membrane may reform, and the cell prepares for the second division.

Meiosis II: The Division of Sister Chromatids

• This division is similar to mitosis, but it starts with haploid cells.

1. Prophase II:

• Chromosomes condense again in each of the two new cells, and the spindle fibers form.

2. Metaphase II:

• Chromosomes (each made up of two sister chromatids) align at the metaphase plate.

3. Anaphase II:

• The spindle fibers pull the sister chromatids apart, moving them to opposite poles of the cell.

4. Telophase II & Cytokinesis:

• The cells divide again, resulting in four genetically distinct haploid gametes.

Mitosis

Purpose:

• Mitosis is the process of cell division used for growth, repair, and asexual reproduction in somatic (body) cells.

• It produces two identical daughter cells from one parent cell, both having the same number of chromosomes as the parent (diploid).

Key Characteristics:

• Diploid to diploid: Both the parent cell and the daughter cells have the same number of chromosomes (diploid).

• One division: Unlike meiosis, mitosis involves only one round of division.

• No genetic variation: The daughter cells are genetically identical to the parent cell and to each other.

Stages of Mitosis:

1. Prophase:

• Chromosomes condense and become visible under a microscope.

• The nuclear membrane starts to break down, and spindle fibers form, connecting to the chromosomes at the centromere.

2. Metaphase:

• Chromosomes line up in the middle of the cell along the metaphase plate.

• The spindle fibers are fully formed, ensuring that each sister chromatid will be pulled apart.

3. Anaphase:

• The sister chromatids are pulled apart to opposite poles of the cell.

• Each chromatid is now considered an individual chromosome.

4. Telophase:

• Chromosomes arrive at the poles, and the nuclear membrane reforms around each set of chromosomes.

• The chromosomes begin to de-condense, returning to their stringy form.

5. Cytokinesis:

• The cytoplasm divides, producing two separate daughter cells.

• Each daughter cell has the same number of chromosomes as the parent cell (diploid).

Comparison Between Meiosis and Mitosis:

Simplified Summary:

• Meiosis: Creates sex cells (sperm and eggs), reduces chromosome number by half, generates genetic diversity, and involves two divisions to produce four unique gametes.

• Mitosis: Produces two identical cells for growth and repair, maintains chromosome number, and involves one division.