Understanding Meiosis: The Process of Genetic Variation

2. The most important fact of mitosis is that each daughter cell has the exact same genetic make-up as the original cell. Gregor Mendel – The Father of Genetics - didn’t know where genes were located - described in detail how genes must move in the formation of gametes and subsequent fertilization - each organism must inherit a single copy of every gene from both of its parents - each offspring therefore has two copies of each gene - these two copies must be separated to form the gametes of this organism

3. Chromosome Number • Normal body cells contain two copies of each chromosome • one received from each of the two parents • Homologous chromosomes • Same shape, size, and contain the same genes in same order • Diploid – term used to describe a cell with homologous chromosomes • Symbol – 2N • Found in all normal body (somatic) cells • Haploid – term describing a cell with a single copy of each chromosome • Symbol – N • Found in gametes (sex cells)

4. Phases of Meiosis Meiosis is a process of reduction division in which the chromosome number per cell is cut in half through the separation of homologous chromosomes in a diploid cell. (Diploid  Haploid) (2N  N) - requires two distinct divisions – Meiosis I and Meiosis II - allows organisms to reproduce sexually and maintain the normal diploid number in the offspring Meiosis I = Reduction Division (Figure 11.15, pg 276) - prior to meiosis I the chromosomes replicate

5. A. Prophase I - nucleolus, nuclear membrane break down - centrioles migrate to poles - homologous chromosomes pair up to form tetrads – 4 chromatids - crossing-over occurs - homologous chromosomes exchange portions of themselves which results in a mixing of genes between the two chromosomes

6. Crossing-Over Section 11-4

7. B. Metaphase I - tetrads line-up at the equator of the cell C. Anaphase I - homologous pairs separate - sister chromatids stay connected at their centromeres D. Telophase I - nuclear membranes reform around the chromosomes and cytyokinesis takes place - each cell is now haploid

8. ** No DNA replication takes place before Meiosis II 2. Meiosis II (Figure 11.15, page 277) - each cell’s chromosomes consist of two chromatids connected at the centromere A) Prophase II - just like prophase in mitosis B) Metaphase II - chromosomes align at center of cell C) Anaphase II - sister chromatids are separated at the centromere and are pulled to opposite poles D) Telophase II - new nuclear envelopes appear - cytokinesis occurs

9. Figure 11-17 Meiosis II Section 11-4 Meiosis II Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells.

10. Results of Meiosis = Four haploid cells which are genetically unique Gamete formation (Figure 11.17, page 278) - in male animals and the pollen grains of plants the haploid gametes are called sperm cells - in female animals generally only one of the cells formed by meiosis develops into an egg - in female animals uneven cytokinesis at the end of Meiosis I and Meiosis II result in a large egg and 3 polar bodies