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1Copyright Pearson Prentice Hall 11-4 Meiosis11-4 MeiosisCopyright Pearson Prentice Hall
2Copyright 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.Copyright Pearson Prentice Hall
3Chromosome NumberReview:Chromosomes—strands of DNA and protein inside the cell nucleus—are the carriers of genes.The genes are located in specific positions on chromosomes.
4Copyright Pearson Prentice Hall Chromosome NumberChromosome NumberAll 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.These chromosomes are from a fruit fly. Each of the fruit fly’s body cells has 8 chromosomes.Copyright Pearson Prentice Hall
5Copyright Pearson Prentice Hall Chromosome NumberThe 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.Copyright Pearson Prentice Hall
6Copyright Pearson Prentice Hall Chromosome NumberA 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.Copyright Pearson Prentice Hall
7Copyright Pearson Prentice Hall Chromosome NumberThe 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.Copyright Pearson Prentice Hall
8Copyright Pearson Prentice Hall Phases of MeiosisPhases of MeiosisMeiosis 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.Copyright Pearson Prentice Hall
9Copyright Pearson Prentice Hall Phases of MeiosisMeiosis 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.Copyright Pearson Prentice Hall
10Telophase I and Cytokinesis Phases of MeiosisMeiosis IMeiosis IInterphase IDuring meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.Prophase IMetaphase IAnaphase ITelophase I and CytokinesisCopyright Pearson Prentice Hall
11Copyright Pearson Prentice Hall Phases of MeiosisCells undergo a round of DNA replication, forming duplicate chromosomes.Interphase IInterphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.Copyright Pearson Prentice Hall
12Copyright Pearson Prentice Hall Phases of MeiosisEach chromosome pairs with its corresponding homologous chromosome to form a tetrad.There are 4 chromatids in a tetrad.MEIOSIS IProphase IMEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.Copyright Pearson Prentice Hall
13Copyright Pearson Prentice Hall Phases of MeiosisWhen 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.Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.Copyright Pearson Prentice Hall
14Copyright Pearson Prentice Hall Phases of MeiosisSpindle fibers attach to the chromosomes.MEIOSIS IMetaphase IMEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes.Copyright Pearson Prentice Hall
15Copyright Pearson Prentice Hall Phases of MeiosisMEIOSIS IAnaphase IThe fibers pull the homologous chromosomes toward opposite ends of the cell.MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell.Copyright Pearson Prentice Hall
16Copyright Pearson Prentice Hall Phases of MeiosisMEIOSIS ITelophase I andCytokinesisNuclear 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 - Nuclear membranes form. The cell separates into two cells.Copyright Pearson Prentice Hall
17Copyright Pearson Prentice Hall Phases of MeiosisMeiosis 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.MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.Copyright Pearson Prentice Hall
18Copyright Pearson Prentice Hall Phases of MeiosisMeiosis IIThe 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.)Copyright Pearson Prentice Hall
19Telophase II and Cytokinesis Phases of MeiosisMeiosis IIDuring meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.Meiosis IITelophase I and Cytokinesis IMetaphase IIAnaphase IITelophase II and CytokinesisProphase IICopyright Pearson Prentice Hall
20Copyright Pearson Prentice Hall Phases of MeiosisMEIOSIS IIProphase IIMetaphase IIA new spindle forms.The chromosomes line up in the center of cell.MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis.Copyright Pearson Prentice Hall
21Copyright Pearson Prentice Hall Phases of MeiosisMEIOSIS IIAnaphase IIThe sister chromatids separate and move toward opposite ends of the cell.MEIOSIS II Anaphase II - The sister chromatids separate and move toward opposite ends of the cell.Copyright Pearson Prentice Hall
22Copyright Pearson Prentice Hall Phases of MeiosisMEIOSIS IITelophase II and CytokinesisMeiosis II results in four haploid (N) daughter cells.MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells.Copyright Pearson Prentice Hall
23Copyright Pearson Prentice Hall Gamete FormationGamete FormationIn male animals, meiosis results in four equal-sized gametes called sperm.Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm.Copyright Pearson Prentice Hall
24Copyright Pearson Prentice Hall Gamete FormationIn many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction.Copyright Pearson Prentice Hall
25Gametes to ZygotesFertilization—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.
26Comparing 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.
27Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.Copyright Pearson Prentice Hall
28Comparing Mitosis and Meiosis 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.Copyright Pearson Prentice Hall
29Comparing Mitosis and 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.Copyright Pearson Prentice Hall
30Copyright Pearson Prentice Hall Gene LinkageGene LinkageThomas 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.Copyright Pearson Prentice Hall
31Gene LinkageFor 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.
32Copyright Pearson Prentice Hall Gene LinkageMorgan 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.Copyright Pearson Prentice Hall
33Gene 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.
34Copyright Pearson Prentice Hall Gene MapsGene MappingRemember 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.Copyright Pearson Prentice Hall
35Copyright Pearson Prentice Hall Gene MapsGene MappingAlfred 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.Copyright Pearson Prentice Hall
36Gene MappingBy 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.