Meiosis

• Meiosis is a special type of cell division that occurs in reproductive cells (sperm and egg cells).

• The purpose of meiosis is to reduce the chromosome number by half, ensuring that when sperm and egg cells combine during fertilization, the resulting organism has the correct number of chromosomes.

Meiosis involves two rounds of division: 

• Meiosis I 

• Meiosis II. 

Each round has stages similar to mitosis, but the end result is four unique daughter cells, each with half the number of chromosomes of the original cell.

Cell division is like making copies of a book. Mitosis creates identical copies for regular growth and maintenance, while meiosis produces special copies (sperm and egg cells) with half the information for creating new life during reproduction.

Meiosis is a type of cell division that occurs in sexually reproducing organisms. It involves two successive divisions, known as meiosis I and meiosis II, resulting in the production of four non-identical haploid cells.

*Haploid cells are cells that have half the usual number of chromosomes. In humans, haploid cells are the sperm and egg cells, each containing one set of chromosomes (23 in total), and they fuse during fertilisation to form a diploid zygote with the full chromosome number.

It involves two consecutive divisions, meiosis I and meiosis II, resulting in the production of four non-identical haploid cells (gametes).

Meiosis I

Prophase I

Chromosomes condense and pair up (homologous chromosomes) through synapsis.

Crossing-over occurs, where genetic material is exchanged between homologous chromosomes.

The nuclear envelope breaks down, and spindle fibres form.

Metaphase I

Homologous chromosome pairs align along the metaphase plate (equator) of the cell.

Spindle fibres attach to each homologous chromosome.

Anaphase I

Homologous chromosomes are pulled apart to opposite poles of the cell.

Whole chromosomes (with both sister chromatids*) move, resulting in a reduction of chromosome number.

*Sister chromatids are two identical copies of a chromosome connected at the centre. Formed during DNA replication, they separate during cell division, ensuring each new cell receives a complete set of genetic information. After division, each daughter cell inherits chromosomes with a single chromatid.

Telophase I

Chromosomes reach the poles, and the nuclear envelope reforms around each set.

Cytokinesis occurs, resulting in two daughter cells, each now haploid (half the chromosome number).

Meiosis II

Prophase II

If chromosomes decondense in telophase I, they recondense.

The nuclear envelope breaks down again, and spindle fibres begin to form in each haploid daughter cell. 

Metaphase II

Chromosomes align along the metaphase plate in both haploid daughter cells.

Anaphase II

Sister chromatids are pulled apart to opposite poles of each daughter cell.

Telophase II

Chromatids reach the poles, and a nuclear envelope forms around each set.

Cytokinesis occurs, resulting in a total of four non-identical haploid daughter cells (gametes).

Uses of Meiosis

• Gamete Formation: 

Meiosis produces specialised reproductive cells called gametes (sperm and egg).

• Chromosome Number Reduction: 

Ensures offspring have the correct number of chromosomes after fertilization.

• Genetic Diversity: 

Introduces variety in offspring through processes like crossing-over and independent assortment.

• Adaptation and Evolution: 

Contributes to species’ ability to adapt and evolve over time.

• Preventing Inbreeding: 

Shuffling genetic material helps avoid inbreeding, maintaining population health.