Meiosis
Stages of Meiosis:
Meiosis is a special type of cell division that makes sex cells (sperm and egg).
It reduces the chromosome number from diploid (2n) (two sets) to haploid (n) (one set) and creates variation.
Meiosis I – Reduction Division:
This stage reduces the chromosome number by half.
1. Prophase I
Chromosomes condense (become short and thick), so they can be seen clearly under a microscope.
Homologous chromosomes (same chromosomes from mother and father) come together and pair up.
This pairing is called synapsis (joining together).
Each pair forms a tetrad (four chromatids).
Crossing over occurs (exchange of DNA between chromatids), which creates new gene combinations.
Nuclear membrane breaks down and spindle fibres (structures that move chromosomes) appear.
2. Metaphase I
Homologous chromosome pairs line up at the equator (middle of the cell).
Spindle fibres attach to the centromere (region holding chromatids together).
The arrangement is random, which increases variation.
3. Anaphase I
Homologous chromosomes separate and move to opposite poles.
Sister chromatids remain attached.
This step reduces the chromosome number.
4. Telophase I
Chromosomes reach the poles.
Nuclear membrane may reform.
Cell divides into two haploid cells (half the chromosomes).
Meiosis II – Equational Division:
This stage is similar to mitosis.
5. Prophase II
• Chromosomes condense again and become visible.
Nuclear membrane disappears.
Spindle fibres form.
6. Metaphase II
Chromosomes line up at the equator.
Spindle fibres attach to centromeres.
7. Anaphase II
Sister chromatids separate and move to opposite poles.
Each chromatid is now a chromosome.
8. Telophase II
Chromosomes uncoil and become thin.
Nuclear membranes reform.
Cells divide again.