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 are moved by the spindle to the equator of the cell; independent assortment occurs

Anaphase I homologous pairs separate

Telophase I individual chromosomes gather at the poles of the cell; 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 chromosomes line up at the equator and are attached at their centromeres to spindle fibers; independent assortment occurs

Anaphase II centromeres divide; sister chromatids separate & move to opposite poles of the cell

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