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

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.

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

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

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

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.