Meiosis

1. Meiosis • animation

2. How are the Goals of MEIOSIS Different from the Goals of MITOSIS?

3. Homologous Chromosomes • are similar in shape and content • they carry genes for the same proteins. • This means that you have two copies for any given gene on a non-sex chromosome (not X or Y). • Remember, organisms with two copies of each gene (and/or chromosome) are called diploid • Homologous Chromosomes work in a similar way: • Chromosome 19 from your dad will have the same genes on it as Chromosome 19 from your mom • the information on these genes may be slightly different • Ex. On this chromosome, you might get a brown hair version of the gene from your dad and a blond hair version of the gene from your mom • Encylopedia Analogy- • Imagine you get a set of encylopedias from each parent. • Each has 23 volumes present similar information in a similar format • Ex. Each has a listing for “kangaroo", but may say slightly different things about “kangaroos". • You have inherited two versions of “kangaroo".

4. How does the process of meiosis accomplish the previously mentioned goals? • two meiotic divisions without DNA replication in between them. • Independent assortment and crossing over

5. MEIOSIS I • Prophase 1: • Homologous sister chromatids find each other. Crossing Over can occur during the latter part of this stage. • Metaphase 1: • Homologous chromosomes align at the equatorial plate. Independent assortment happens here. • Anaphase 1: • Homologous pairs separate with sister chromatids remaining together. • Telophase 1: • Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

6. Meiosis II • Prophase 2: • DNA does not replicate. Sister Chromatids prep for division • Metaphase 2: • Sister chromatids align at the equatorial plate. • Anaphase 2: • Centromeres divide and sister chromatids migrate separately to each pole. • Telophase 2: • Cell division is complete. Four haploid daughter cells are obtained.

7. Summary • One parent cell produces four daughter cells. • Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different. • Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.

8. Meiosis Square Dance • animation

9. Independent Assortment- • The process of random segregation and assortment of chromosomes during anaphase I of meiosis resulting in the production of genetically unique gametes. • Chiasmata- • A cross-shaped structure commonly observed between homologous chromosomes during meiosis; the site of crossing-over.

10. Random Fertilization-   • A mating system in which there is an equal opportunity for all male and female gametes to join in fertilization • This means that an egg cell representing one of eight million different genetic possibilities will be fertilized by a sperm cell representing one of eight million genetic possibilities and the resulting zygote can have any of the possible 64 trillion diploid combinations of genes.

11. Making Gametes • Spermatogenesis- • The process of sperm formation in male animals • Happens in the testes • Produces 4 haploid cells which go on to develop tails and are called “spermatozoa” • Oogenesis- •  The process of ovum formation in female animals • Happens in ovaries • Cytoplasm divides unequally after Meiosis 1 • This “ovum” grows into a mature egg cell and carries a rich supply of nutrients for the developing egg. • The other 3 cells become polar bodies and die