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.

Meiosis I 2n Crossing Over Independent Assortment n

Crossing Over Occurs when portions of a chromatid on one homologous chromosome is broken and exchanged with the corresponding chromatid portion of the other homologous chromosome.

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.

* All 4 cells are genetically different!! Meiosis II n * All 4 cells are genetically different!! n

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

Three Mechanisms that Contribute to Genetic Variation: 1. 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 2. independent assortment – the random distribution of homologous chromosomes during meiosis 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 !