MEIOSIS

1. MEIOSIS • CHROMOSOME NUMBER (FRUIT FLIES) • Homologous: corresponding chromosomes from male & female • Homologs: the actual chromosomes of the set • Diploid: A cell that contains both sets of homologs • 2N: the diploid number; Drosophila 2N=8 • So, a diploid cell contains 2 complete sets of chromosomes and complete sets of genes. • Gametes have only a single set of genes due to segregation • Haploid: cells that contain a single set of chromosomes • N: the haploid number; Drosophila N=4 • In order for gametes to be produced there must be a process that divides the diploid number of chromosomes in half

2. PHASES OF MEIOSIS • Meiosis: Process of reduction division in which the number of chromosomes per cell is cut in half and the homologous chromosomes that exist in a diploid cell are separated. • The names of the phases are the same for mitosis/meiosis • There are two stages in meiosis division (See fig. 9-17) • MEIOSIS I • DNA replication in reproductive organs (CAREFUL) • Prophase I: each chromosome seeks out its corresponding homologous chromosome for form a TETRAD. • Metaphase I: the tetrads line up in the center of the cell • Crossing Over: The exchange of chromatid pieces between homologous chromo., resulting in new combinations of gene info. This results in an exchange of genes between homologous chromo.

4. What happens next? • The homologous chromo separate and two new cells form • Each cell now has 8 chromatids like mitosis w/ a difference • The two cells no longer have two complete sets of 4 chromo. • Instead maternal and paternal chromo have been shuffled • The two cells have sets of chromo that are • MEIOSIS II • No DNA replication, so each cell’s chromo has 2 chromatids • Metaphase II: 4 chromo line up in the center of each cell • Anaphase II: the paired chromatids separate; each of the 4 daughter cells receives 4 chromatids w/ N-4 chromo each • The amount of genetic material has been reduced

5. REVIEW • PURPOSE: The production of haploid gametes • PROPHASE I: Homologous chromo pair off; forms tetrads • METAPHSE I: Tetrads line up in the center of the cell • ANAPHASE I: Cell divides the tetrads; it moves half the chromo to one end, the other half to the opposite end • TELOPHASE I: A membrane forms between the cell’s two ends; the result is 2 haploid cells • MEIOSIS II: It is identical to mitotic division of chpt. 8. The only difference is that in meiosis II, the parent cell is haploid. • THE END PRODUCT: 4 haploid gametes

6. MEIOSIS AND GENETICS • Chromosomes pair and separate during meiosis exactly as Mendel predicted for the structures that carry genes. • MEIOSIS I: results in segregation and independent assortment • During Anaphase I the homologous chromo separate and are segregated to different cells, which also segregates the homologous forms of the genes (alleles). • All this is completely random and a matter of chance • GAMETE FORMATION: • In males: animals-sperm, plants-pollen; contain haploid cells • In females: animals-egg, plants ovule; contain haploid cells • In female animals, the cells divisions at the end of Meiosis I and II are uneven, so that the egg receives most of the cytoplasm. The other 3 cells, known as polar bodies, usually don’t participate in reproduction

7. COMPARING MITOSIS AND MEIOSIS • MITOSIS: Results in the production of two genetically identical cells. • A diploid cell that divides by mitosis gives rise to two diploid daughter cells. • The daughter cells have sets of chromosomes and genes identical to each other and to the original parent cell. • MEIOSIS: Begins with a diploid cell, but produces four haploid cells which are genetically different from the diploid cell and from one another. • This is because the homologous chromo are separated during the first meiotic division. • This is also because of cross-over resulting in the production of new gene combinations on the chromosomes.