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1 biology 1 of 35

2 Do Now: Turn in mitosis worksheet Write down your homework s/2013/chromosome#/chromosome -2 s/2013/chromosome#/chromosome -9 2 of 35

3 Chromosomes Class Discussion The pictures below are called karyotypes. Karyotypes are photographs of chromosomes. What is the difference between the two karyotypes below? 3 of 35

4 Chromosome Number Chromosome Number Species can have different numbers of chromosomes. A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent. 4 of 35

5 Chromosome Number These sets of chromosomes are homologous. Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent. 5 of 35

6 Chromosome Number A cell that contains both sets of homologous chromosomes is said to be diploid. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. For Drosophila, the diploid number is 8, which can be written as 2N=8. 6 of 35

7 Each organism must inherit a single copy of every gene from each of its parents. Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set. Diploid = 2 sets of genes (2n) somatic cells Haploid = 1 set of genes (1n or n) - gametes N represents a full set of genes 7 of 35

8 Chromosome Number The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes. These cells are haploid. Haploid cells are represented by the symbol N. For Drosophila, the haploid number is 4, which can be written as N=4. 8 of 35

9 Do Now Turn your homework (Chapter 11-4) into bin. Write down your homework. Draw an duplicated chromosomes and a duplicated chromosome. Label the duplicated chromosome with the following terms: centromere, sister chromatids. Use your notes or returned homework to help you. Turn into bin. 9 of 35

10 10 of 35

11 Phases of Meiosis Phases of Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. 11 of 35

12 Phases of Meiosis Meiosis involves two divisions, meiosis I and meiosis II. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells. How is this different from mitosis? 12 of 35

13 Phases of Meiosis Meiosis I Interphase I Meiosis I Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis 13 of 35

14 Phases of Meiosis Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase I 14 of 35

15 Phases of Meiosis Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. MEIOSIS I Prophase I There are 4 chromatids in a tetrad. 15 of 35

16 Phases of Meiosis When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over. Crossing-over produces new combinations of alleles which increases genetic variability. 16 of 35

17 Phases of Meiosis Spindle fibers attach to the chromosomes. MEIOSIS I Metaphase I 17 of 35

18 Phases of Meiosis The fibers pull the homologous chromosomes toward opposite ends of the cell. MEIOSIS I Anaphase I 18 of 35

19 Phases of Meiosis Nuclear membranes form. The cell separates into two cells. The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. MEIOSIS I Telophase I and Cytokinesis 19 of 35

20 Phases of Meiosis Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Why are these cells now considered HAPLOID cells? Unlike meiosis I, neither cell goes through chromosome replication. Why not? Each of the cell s chromosomes has 2 chromatids. 20 of 35

21 Phases of Meiosis Meiosis II Telophase I and Cytokinesis I Meiosis II Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis 21 of 35

22 Phases of Meiosis Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. MEIOSIS II Prophase II 22 of 35

23 Phases of Meiosis The chromosomes line up in the center of cell. MEIOSIS II Metaphase II 23 of 35

24 Phases of Meiosis The sister chromatids separate and move toward opposite ends of the cell. MEIOSIS II Anaphase II 24 of 35

25 Phases of Meiosis Meiosis II results in four haploid (N) daughter cells. MEIOSIS II Telophase II and Cytokinesis 25 of 35

26 Gamete Formation Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm. 26 of 35

27 Gamete Formation In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction. 27 of 35

28 Comparing Mitosis and Meiosis Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells. 28 of 35

29 Comparing Mitosis and Meiosis Mitosis Cells produced by mitosis have the same number of chromosomes and alleles as the original cell. Mitosis allows an organism to grow and replace cells. Some organisms reproduce asexually by mitosis. 29 of 35

30 Comparing Mitosis and Meiosis Meiosis Cells produced by meiosis have half the number of chromosomes as the parent cell. These cells are genetically different from the diploid cell and from each other. Meiosis is how sexually-reproducing organisms produce gametes. 30 of 35

31 11-4 Continue to: - or - Click to Launch: 31 of 35

32 11-4 If the body cells of humans contain 46 chromosomes, a single sperm cell should have a. 46 chromosomes. b. 23 chromosomes. c. 92 chromosomes. d. between 23 and 46 chromosomes. 32 of 35

33 11-4 During meiosis, the number of chromosomes per cell is cut in half through the separation of a. daughter cells. b. homologous chromosomes. c. gametes. d. chromatids. 33 of 35

34 11-4 The formation of a tetrad occurs during a. anaphase I. b. metaphase II. c. prophase I. d. prophase II. 34 of 35

35 11-4 In many female animals, meiosis results in the production of a. only 1 egg. b. 1 egg and 3 polar bodies. c. 4 eggs. d. 1 egg and 2 polar bodies. 35 of 35

36 11-4 Compared to egg cells formed during meiosis, daughter cells formed during mitosis are a. genetically different, while eggs are genetically identical. b. genetically different, just as egg cells are. c. genetically identical, just as egg cells are. d. genetically identical, while egg cells are genetically different. 36 of 35

37 END OF SECTION