11-4 Meiosis11-4 MeiosisI. Chromosome Number I. Chromosome Number

A. A. HomologousHomologous- corresponding - corresponding chromosomes, one from the male and chromosomes, one from the male and one from the female.one from the female.

B. B. DiploidDiploid - A cell that contains both sets - A cell that contains both sets of homologous chromosomes. of homologous chromosomes. 2N2N

C. C. HaploidHaploid - a cell that contains only a - a cell that contains only a single set of genes. single set of genes. NN

11-4 Meiosis11-4 MeiosisII. Phases of MeiosisII. Phases of Meiosis A. Meiosis is a process of reduction A. Meiosis is a process of reduction

division in which the number of division in which the number of chromosomes per cell is cut in half chromosomes per cell is cut in half through the separation of through the separation of homologous chromosomes in a homologous chromosomes in a diploid cell.diploid cell.

11-4 Meiosis11-4 MeiosisA. Meiosis I- A. Meiosis I- 1. Prior to meiosis I the 1. Prior to meiosis I the

chromosomeschromosomes replicate. replicate. 2. Interphase I- Cells undergo a 2. Interphase I- Cells undergo a

round of DNA replication, round of DNA replication, forming duplicate forming duplicate chromosomes.chromosomes.

11-4 Meiosis11-4 Meiosis3. In Prophase of meiosis I each 3. In Prophase of meiosis I each

chromosome pairs with its corresponding chromosome pairs with its corresponding homologous homologous chromosome to form a chromosome to form a structure called a structure called a ttetradetrad.. There are There are four four chromatidschromatids in a tetrad. As the in a tetrad. As the homologous chromosomes line up they homologous chromosomes line up they exchange portions of their exchange portions of their chromatids chromatids in in a process called a process called crossing overcrossing over. The . The exchange produces new exchange produces new formsforms of the of the alleles.alleles.

Crossing Over

11-4 Meiosis11-4 Meiosis4. 4. Metaphase IMetaphase I- Spindle fibers - Spindle fibers

attach to the chromosomes.attach to the chromosomes.5. 5. Anaphase IAnaphase I- The fibers pull the - The fibers pull the

chromosomes toward the chromosomes toward the opposite ends of the cell.opposite ends of the cell.

11-4 Meiosis11-4 Meiosis6. Telophase and Cytokinesis- Nuclear 6. Telophase and Cytokinesis- Nuclear

membrane form. The cell separates membrane form. The cell separates into into two two cells. cells.

7. At the end of Meiosis I there are two 7. At the end of Meiosis I there are two cells formed. But, unlike mitosis each cells formed. But, unlike mitosis each daughter cell has a set of daughter cell has a set of chromosomes that is chromosomes that is unique unique to that to that cell. The alleles have been shuffled cell. The alleles have been shuffled like a like a deck of cards.deck of cards.

Meiosis I

11-4 Meiosis11-4 MeiosisB. Meiosis IIB. Meiosis II

1. 1. Prophase II-Prophase II- Meiosis I results in two Meiosis I results in two haploid (N) daughter cells, each with haploid (N) daughter cells, each with half the number of chromosomes as half the number of chromosomes as the original cell.the original cell.

2. During 2. During metaphase IImetaphase II of meiosis, of meiosis, chromosomes line up at the center chromosomes line up at the center of each cell.of each cell.

11-4 Meiosis11-4 Meiosis3. In a3. In anaphase IInaphase II the paired chromatids the paired chromatids

separate and move towards opposite separate and move towards opposite ends of the poles.ends of the poles.

4. Telophase II and Cytokinesis- Each 4. Telophase II and Cytokinesis- Each of the four daughter cells produced of the four daughter cells produced only contain the only contain the haploid haploid number (N).number (N).

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe 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.

Meiosis II

11-4 Meiosis11-4 MeiosisC. Gamete FormationC. Gamete Formation

1. In males the haploid gametes that 1. In males the haploid gametes that are formed are called are formed are called spermsperm..

2. In females generally only 2. In females generally only one one of the of the cells produced forms an cells produced forms an eggegg, and the , and the other three form polar bodies.other three form polar bodies.

Spermatogenesis

OOGENESIS

11-4 Meiosis11-4 MeiosisIII. Comparing Mitosis and MeiosisIII. Comparing Mitosis and Meiosis A. A. Mitosis Mitosis results in the results in the

production of two genetically production of two genetically identical diploid cells, whereas identical diploid cells, whereas meiosismeiosis produces four produces four genetically different haploid genetically different haploid cells.cells.