1.  Normal human body cells each contain 46 chromosomes.  The cell division process that body cells undergo is called mitosis and produces daughter cells that are virtually identical to the parent cell. How Many Chromosomes?

2. 1.How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis? 2.If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo? 3.In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have? Working with a partner, discuss and answer the questions that follow.

3. Chromosome Number  Human somatic cells, or body cells, contain 46 chromosomes – 23 chromosomes from each parent.  The pairs of chromosomes are homologous. That is, each has a corresponding chromosome from the opposite-sex parent.  Cells that contain both sets of homologous chromosomes are diploid (2N).  Cells that contain only a single set of chromosomes are haploid (1N). These cells are gametes of sexually reproducing organisms.

4. Meiosis  The process of cell division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.  Two distinct cell divisions take place – meiosis I and meiosis II.  Meiosis I begins with one diploid cell and results in four haploid cells being produced by the end of meiosis II.

5. Crossing-Over 1. Homologous chromosomes form a tetrad.

6. 2. Chromatids cross over one another. Crossing-Over

7. 3. The crossed sections of the chromatids are exchanged. Crossing-Over

8. Crossing-over  As homologous chromosomes pair up and form tetrads in meiosis I, they exchange portions of their chromatids = crossing-over.  This results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles.  Alleles are different forms of the same gene.

9. Meiosis I

10. Crossing over occurs.

11. Meiosis I

14. Meiosis II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase IIMetaphase IIAnaphase IITelophase II 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.

15. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase IIMetaphase IIAnaphase IITelophase II 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. Meiosis II

16. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase IIMetaphase IIAnaphase IITelophase II 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. Meiosis II

17. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase IIMetaphase IIAnaphase IITelophase II 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. Meiosis II

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

19. Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Three mechanisms contribute to genetic variation: –Independent assortment of chromosomes –Crossing over –Random fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

20. Spermatogenesis  Sperm = the haploid gametes produced in male animals by meiosis. In some plants, pollen grains contain haploid sperm. Gamete Formation

21. Oogenesis  Egg = the haploid gamete produced in female animals by meiosis. In some plants, an egg cell is produced by meiosis.  In many female animals, the cell divisions at the end of meiosis I and II are uneven so that a single cell receives most of the cytoplasm. This cell becomes the egg. The other three cells produced are called polar bodies. Gamete Formation