Meiosis a form of cell division that halves

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

Meiosis I Diploid 2 n Crossing Over Independent Assortment Haploid n

Prophase I: n chromosomes condense; nuclear envelope breaks down; n homologous chromosomes pair up forming a tetrad (4 chromatids); n crossing over can occur here

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. This leads to genetic variation.

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

Anaphase I n homologous pairs separate

Telophase I n nuclear membrane reforms n two cells are produced each containing 1 set of the homologous pairs n cytokinesis begins (cytoplasm divides)

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

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

Prophase II n a new spindle fibers form around the chromosomes

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

Anaphase II n centromeres divide n sister chromatids separate

Telophase II n nuclear membrane forms around each set of chromosomes n cytokinesis begins (cytoplasm divides) n four haploid cells are produced

Diploid (2 n) Meiosis: -reduces the centromere Crossing-over occurs in Prophase I. Haploid (n) number of chromosomes by half to form gamete cells

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 !