Meiosis – a form of cell division that halves the number of chromosomes when forming reproductive cells, such as gametes (egg & sperm cells)

Stages of Meiosis: *Before meiosis begins, DNA in the original cell is replicated. In meiosis, the cell undergoes two cell divisions: Meiosis I and Meiosis II. The cell then goes through two cell divisions: Meiosis I and Meiosis II.

Meiosis I: Meiosis I: Homologous chromosomes separate; This stage consists of prophase I, metaphase I, anaphase I, and telophase I.

Prophase I: chromosomes condense; nuclear envelope breaks down; homologous chromosomes pair up forming a tetrad; crossing over can occur here

Metaphase I homologous chromosomes line up in the middle of the cell independent assortment occurs

Anaphase I homologous pairs separate

Telophase I nuclear membrane reforms; the cytoplasm divides two cells are produced each containing 1 set of the homologous pairs

Meiosis II: Meiosis II: The 2 chromatids (referred to as sister chromatids) separate; This stage consists of prophase II, metaphase II, anaphase II, and telophase II.

Prophase II a new spindle forms around the chromosomes

Metaphase II sister chromatids line up along the equator independent assortment occurs

Anaphase II centromeres divide sister chromatids separate

Telophase II nuclear envelope forms around each set of chromosomes the cytoplasm divides four haploid cells are produced

2n Meiosis: -reduces the number of chromosomes by half to form gamete cells centromere n n

Three Mechanisms that Contribute to Genetic Variation: 1. independent assortment – the random distribution of homologous chromosomes during meiosis 2. crossing over – when portions of a chromatid on one homologous chromosome is broken and exchanged with the corresponding chromatid portion of the other homologous chromosome 3. random fertilization – the random joining of 2 gametes

Importance of Genetic Variation: It allows organisms to change so they can better adapt to the changing environment !

Gametogenesis – the formation of gametes 1. spermatogenesis – the process by which sperm are produced in male animals; occurs in the testes 2. oogenesis – the process by which eggs are produced in female animals; occurs in the ovaries

Spermatogenesis vs. Oogenesis