Genetics Meiosis Agriscience Curriculum Meiosis A) Form of cell division that ultimately creates gametes – there are two types – 1. Sperm formation (called spermatogenesis) – 2. Ovum or Egg formation (called oogenesis) Meiosis – the overview • In meiosis, there are two separate divisions, Meiosis I and Meiosis II. • By the end of both processes, the original diploid cell (2n) cell has divided into four haploid cells (n). • These haploid cells are called gametes: sperm and eggs. When fertilized, the resulting cell is called a zygote, again with a diploid number of cells. Interphase – a step ahead of meiosis! • Prior to beginning meiosis I, the cell replicates its chromosomes in interphase. After replication, each chromosome consists of two identical sister chromatids, held together by a centromere. • Directions for story board: Take out a piece of yarn (each student). This is your original chromosome. Take out another piece of yarn identical to the first. This is your duplicated chromosome. Tie them together with twisty-tie. This twisty-tie is called the centromere. Glue them to the first block on the cardboard and label it “Interphase.” Your chromosome has just been duplicated! All Chromosomes Replicate in Interphase! Prior to meiosis, all chromosomes are duplicated in a process similar to chromosome duplication prior to mitosis. Next is Meiosis I ! Meiosis I contains four steps. 1.Prophase I 2.Metaphase I 3.Anaphase I 4.Telophase I Prophase I 1. In Prophase I: a) Chromosomes appear, coil up and a spindle forms. b) Each chromosome pair comes together with another, and it matched up gene to gene. This creates a fourpart structure called a tetrad. c) Nuclear envelope disappears. d) Crossing over occurs at site called chiasma. This is simply where genetic information is exchanged because the chromosome pairs are held so tightly together. This results in new allele combinations. Prophase I Directions for Storyboard • Have two students pair their duplicated chromosomes (done in interphase). • Connect these two chromosome pairs with another twisty-tie. • Glue these pairs to the second storyboard block and label the diagram as seen in slide #9. This step is called…. Prophase I Metaphase I 2) Metaphase I: Pairs of homologous chromosomes move to midline of the cell as the centromeres (holding the chromosomes together) attach to the spindle fiber of the cell. Metaphase I