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End Show Slide 1 of 35 Copyright Pearson Prentice Hall 11-4 Meiosis.

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Presentation on theme: "End Show Slide 1 of 35 Copyright Pearson Prentice Hall 11-4 Meiosis."— Presentation transcript:

1 End Show Slide 1 of 35 Copyright Pearson Prentice Hall 11-4 Meiosis

2 End Show 11-4 Meiosis Slide 2 of 35 Copyright Pearson Prentice Hall Each organism must inherit a single copy (or allele) of every gene from each of its “parents.” That means that gametes must be formed by a process that separates the two alleles of each gene so that each gamete ends up with just one allele.

3 End Show 11-4 Meiosis Slide 3 of 35 Lesson Overview Lesson OverviewMeiosis Chromosome Number Review: Chromosomes—strands of DNA and protein inside the cell nucleus—are the carriers of genes. The genes are located in specific positions on chromosomes.

4 End Show 11-4 Meiosis Slide 4 of 35 Copyright Pearson Prentice Hall Chromosome Number All organisms 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.

5 End Show 11-4 Meiosis Slide 5 of 35 Copyright Pearson Prentice Hall Chromosome Number The chromosomes can be arranged in pairs. Each of the 4 chromosomes that came from the male parent has a corresponding chromosome (carrying the genes for the same traits) from the female parent. These two sets chromosomes are homologous.

6 End Show 11-4 Meiosis Slide 6 of 35 Copyright Pearson Prentice Hall 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. The diploid cells of most adult organisms contain two complete sets of inherited chromosomes and two complete sets of genes.

7 End Show 11-4 Meiosis Slide 7 of 35 Copyright Pearson Prentice Hall 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 End Show 11-4 Meiosis Slide 8 of 35 Copyright Pearson Prentice Hall Phases of Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell (2N) is cut in half through the separation of homologous chromosomes in a diploid cell. Gametes are produced in meiosis.

9 End Show Slide 9 of Meiosis Copyright Pearson Prentice Hall Meiosis involves two divisions, meiosis I and meiosis II. At the end of meiosis I, there are 2 haploid cells. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells. Phases of Meiosis

10 End Show Slide 10 of Meiosis Copyright Pearson Prentice Hall Phases of Meiosis Meiosis I Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Interphase I Meiosis I

11 End Show 11-4 Meiosis Slide 11 of 35 Copyright Pearson Prentice Hall Phases of Meiosis Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase I

12 End Show 11-4 Meiosis Slide 12 of 35 Copyright Pearson Prentice Hall Phases of Meiosis Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromatids in a tetrad. MEIOSIS I I Prophase I

13 End Show Slide 13 of Meiosis Copyright Pearson Prentice Hall 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.

14 End Show 11-4 Meiosis Slide 14 of 35 Copyright Pearson Prentice Hall Phases of Meiosis Spindle fibers attach to the chromosomes. MEIOSIS I Metaphase I

15 End Show 11-4 Meiosis Slide 15 of 35 Copyright Pearson Prentice Hall Phases of Meiosis MEIOSIS I Anaphase I The fibers pull the homologous chromosomes toward opposite ends of the cell.

16 End Show 11-4 Meiosis Slide 16 of 35 Copyright Pearson Prentice Hall Phases of Meiosis MEIOSIS I Telophase I and Cytokinesis 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.

17 End Show 11-4 Meiosis Slide 17 of 35 Copyright Pearson Prentice Hall Phases of Meiosis Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. Because each pair of homologous chromosomes was separated, neither daughter cell has the two complete sets of chromosomes.

18 End Show Slide 18 of Meiosis Copyright Pearson Prentice Hall Phases of Meiosis Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, neither cell goes through chromosome replication. Remember: Each of the cell’s chromosomes has 2 chromatids (from replication that occurred back at the beginning of meiosis.)

19 End Show Slide 19 of Meiosis Copyright Pearson Prentice Hall Phases of Meiosis Meiosis II Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Telophase I and Cytokinesis I Meiosis II

20 End Show 11-4 Meiosis Slide 20 of 35 Copyright Pearson Prentice Hall Phases of Meiosis A new spindle forms. The chromosomes line up in the center of cell. MEIOSIS II Prophase II Metaphase II

21 End Show 11-4 Meiosis Slide 21 of 35 Copyright Pearson Prentice Hall Phases of Meiosis The sister chromatids separate and move toward opposite ends of the cell. MEIOSIS II Anaphase II

22 End Show Slide 22 of Meiosis Copyright Pearson Prentice Hall Phases of Meiosis Meiosis II results in four haploid (N) daughter cells. MEIOSIS II Telophase II and Cytokinesis

23 End Show 11-4 Meiosis Slide 23 of 35 Copyright Pearson Prentice Hall Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm.

24 End Show 11-4 Meiosis Slide 24 of 35 Copyright Pearson Prentice Hall 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.

25 End Show 11-4 Meiosis Slide 25 of 35 Lesson Overview Lesson OverviewMeiosis Gametes to Zygotes Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote. The zygote undergoes cell division by mitosis and eventually forms a new organism.

26 End Show 11-4 Meiosis Slide 26 of 35 Lesson Overview Lesson OverviewMeiosis Comparing Mitosis and Meiosis In mitosis, when the two sets of genetic material separate, each daughter cell receives one complete set of chromosomes. In meiosis, homologous chromosomes line up and then move to separate daughter cells. Mitosis does not normally change the chromosome number of the original cell. This is not the case for meiosis, which reduces the chromosome number by half.

27 End Show 11-4 Meiosis Slide 27 of 35 Copyright Pearson Prentice Hall Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.

28 End Show 11-4 Meiosis Slide 28 of 35 Copyright Pearson Prentice Hall Comparing Mitosis and Meiosis Mitosis Cells produced by mitosis have the same number of chromosomes and alleles as the original cell. Mitosis is a single cell division. Mitosis allows an organism to grow and replace cells. Some organisms reproduce asexually by mitosis.

29 End Show 11-4 Meiosis Slide 29 of 35 Copyright Pearson Prentice Hall Comparing Mitosis and Meiosis Meiosis Cells produced by meiosis have half the number of chromosomes as the parent cell. Meiosis requires two rounds of cell division. These cells are genetically different from the diploid cell and from each other. Meiosis is how sexually-reproducing organisms produce gametes.

30 End Show 11-4 Meiosis Slide 30 of 35 Copyright Pearson Prentice Hall Gene Linkage Thomas Hunt Morgan’s research on fruit flies led him to the principle of linkage. Morgan discovered that many of the more than 50 Drosophila genes he had identified appeared to be “linked” (inherited) together. They seemed to violate the principle of independent assortment.

31 End Show 11-4 Meiosis Slide 31 of 35 Lesson Overview Lesson OverviewMeiosis Gene Linkage For example, Morgan used a fly with reddish-orange eyes and miniature wings in a series of test crosses. His results showed that the genes for those two traits were almost always inherited together. Only rarely did the genes separate from each other.

32 End Show 11-4 Meiosis Slide 32 of 35 Copyright Pearson Prentice Hall Morgan and his associates grouped all of the fly’s genes into four linkage groups. Each group assorted independently from the other groups BUT all the genes in one group were inherited together. As it turns out, Drosophila has four linkage groups and four pairs of chromosomes. Gene Linkage

33 End Show 11-4 Meiosis Slide 33 of 35 Lesson Overview Lesson OverviewMeiosis Gene Linkage Morgan’s findings led to two remarkable conclusions: First, each chromosome is actually a group of linked genes. Second, it is the chromosomes that assort independently, not individual genes. Alleles of different genes tend to be inherited together when those genes are located on the same chromosome.

34 End Show 11-4 Meiosis Slide 34 of 35 Copyright Pearson Prentice Hall Gene Maps Gene Mapping Remember that: Crossing-over during meiosis sometimes separates genes that had been on the same chromosomes onto the homologous chromosome. Crossover events occasionally separate linked genes and produce new combinations of alleles.

35 End Show 11-4 Meiosis Slide 35 of 35 Copyright Pearson Prentice Hall Alfred Sturtevant, a student of Morgan, reasoned that the farther apart two genes were on a chromosome, the more likely it would be that a crossover event would occur between them. If two genes are close together, then crossovers between them should be rare. If two genes are far apart, then crossovers between them should be more common. Gene Maps Gene Mapping

36 End Show 11-4 Meiosis Slide 36 of 35 Lesson Overview Lesson OverviewMeiosis Gene Mapping By this reasoning, he could use the frequency of crossing-over between genes to determine their distances from each other. Sturtevant gathered lab data and presented a gene map showing the relative locations of each known gene on one of the Drosophila chromosomes. Sturtevant’s method has been used to construct gene maps ever since this discovery.


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