Sexual reproduction and meiosis Chapter 11 Genes and Development

Fig. 11.1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Haploid sperm Haploid egg Diploid zygote Fertilization Paternal homologue Maternal homologue

Fig. 11.2 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. n 2n FERTILIZATION MEIOSIS Sperm (haploid) n Egg (haploid) n Zygote (diploid) 2n Somatic cells Germ-line cells Adult male (diploid) 2n Adult female (diploid) 2n MITOSIS Germ-line cells

Fig. 11.3b c. Diploid cell Chromosome duplication Meiosis I Meiosis II Haploid cells Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fig. 11.3a-1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Sister chromatids Homologues Kinetochore Centromere Synaptonemal complex

Fig. 11.3a Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. b. Sister chromatids Homologues Kinetochore Centromere Synaptonemal complex Synaptonemal complex Homologous chromosomes 138 nm b: Reprinted, with permission, from the Annual Review of Genetics, Volume 6 1972 by Annual Reviews, www.annualreviews.org

Fig. 11.4 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Site of crossover = Chiasmata

Fig. 11.7left-a Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS I Prophase I Chromosome (replicated) Spindle Chiasmata In prophase I of meiosis I, the chromosomes begin to condense, and the spindle of microtubules begins to form. The DN A has been replicated, and each chromosome consists of two sister chromatids attached at the centromere. In the cell illustrated here, there are four chromosomes, or two pairs of homologues. Homologous chromosomes pair up and become closely associated during synapsis. Crossing over occurs, forming chiasmata, which hold homologous chromosomes together. Paired homologous chromosomes Sister chromatids Clare A. Hasenkampf/Biological Photo Service 40 m

Fig. 11.7left-b Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metaphase I Kinetochore microtubule In metaphase I, the pairs of homologous chromosomes align along the metaphase plate. Chiasmata help keep the pairs together and produce tension when microtubules from opposite poles attach to sister kinetochores of each homologue. A kinetochore microtubule from one pole of the cell attaches to one homologue of a chromosome, while a kinetochore microtubule from the other cell pole attaches to the other homologue of a pair. Homologue pair on metaphase plate Clare A. Hasenkampf/Biological Photo Service 40 m MEIOSIS I

Fig. 11.7left-c Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Anaphase I Homologous chromosomes Sister chromatids In anaphase I, kinetochore microtubules shorten, and homologous pairs are pulled apart. One duplicated homologue goes to one pole of the cell, while the other duplicated homologue goes to the other pole. Sister chromatids do not separate.This is in contrast to mitosis, where duplicated homologues line up individually on the metaphase plate, kinetochore microtubules from opposite poles of the cell attach to opposite sides of one homologue's centromere, and sister chromatids are pulled apart in anaphase. Clare A. Hasenkampf/Biological Photo Service 40 m MEIOSIS I

Fig. 11.7left-d Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Telophase I Chromosome Nonidentical sister chromatids In telophase I, the separated homologues form a cluster at each pole of the cell, and the nuclear envelope re-forms around each daughter cell nucleus. Cytokinesis may occur. The resulting two cells have half the number of chromosomes as the original cell: In this example, each nucleus contains two chromosomes (versus four in the original cell). Each chromosome is still in the duplicated state and consists of two sister chromatids, but sister chromatids are not identical because crossing over has occurred. Homologous chromosomes Clare A. Hasenkampf/Biological Photo Service 40 m MEIOSIS I

Fig. 11.7left Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS I Prophase IMetaphase IAnaphase I Telophase I Chromosome (replicated) Spindle Chiasmata Kinetochore microtubule Homologous chromosomes Sister chromatids Chromosome Nonidentical sister chromatids In prophase I of meiosis I, the chromosomes begin to condense, and the spindle of microtubules begins to form. The DN A has been replicated, and each chromosome consists of two sister chromatids attached at the centromere. In the cell illustrated here, there are four chromosomes, or two pairs of homologues. Homologous chromosomes pair up and become closely associated during synapsis. Crossing over occurs, forming chiasmata, which hold homologous chromosomes together. In metaphase I, the pairs of homologous chromosomes align along the metaphase plate. Chiasmata help keep the pairs together and produce tension when microtubules from opposite poles attach to sister kinetochores of each homologue. A kinetochore microtubule from one pole of the cell attaches to one homologue of a chromosome, while a kinetochore microtubule from the other cell pole attaches to the other homologue of a pair. In anaphase I, kinetochore microtubules shorten, and homologous pairs are pulled apart. One duplicated homologue goes to one pole of the cell, while the other duplicated homologue goes to the other pole. Sister chromatids do not separate.This is in contrast to mitosis, where duplicated homologues line up individually on the metaphase plate, kinetochore microtubules from opposite poles of the cell attach to opposite sides of one homologue's centromere, and sister chromatids are pulled apart in anaphase. In telophase I, the separated homologues form a cluster at each pole of the cell, and the nuclear envelope re-forms around each daughter cell nucleus. Cytokinesis may occur. The resulting two cells have half the number of chromosomes as the original cell: In this example, each nucleus contains two chromosomes (versus four in the original cell). Each chromosome is still in the duplicated state and consists of two sister chromatids, but sister chromatids are not identical because crossing over has occurred. Paired homologous chromosomes Homologue pair on metaphase plate Homologous chromosomes Sister chromatids Clare A. Hasenkampf/Biological Photo Service 40 m

Fig. 11.7right-e Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS II Prophase II Spindle Nuclear membrane breaking down 40 m Following a typically brief interphase, with no S phase, meiosis II begins. During prophase II, a new spindle apparatus forms in each cell, and the nuclear envelope breaks down. In some species the nuclear envelope does not re-form in telophase I removing the need for nuclear envelope breakdown. Clare A. Hasenkampf/Biological Photo Service

Fig. 11.7right-f Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metaphase II Sister chromatids Chromosome 40 m In metaphase II, a completed spindle apparatus is in place in each cell. Chromosomes consisting of sister chromatids joined at the centromere align along the metaphase plate in each cell. No w, kinetochore microtubules from opposite poles attach to kinetochores of sister chromatids. Clare A. Hasenkampf/Biological Photo Service MEIOSIS II

Fig. 11.7right-g Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Anaphase II Sister chromatids 40 m When microtubules shorten in anaphase II, the centromeres split, and sister chromatids are pulled to opposite poles of the cells. Kinetochore microtubule Clare A. Hasenkampf/Biological Photo Service MEIOSIS II

Fig. 11.7right-h Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Telophase II 40 m In telophase II, the nuclear membranes re-form around four di f ferent clusters of chromosomes. After cytokinesis, four haploid cells result. No two cells are alike due to the random alignment of homologous pairs at metaphase I and crossing over during prophase I. Nuclear membrane re-forming Clare A. Hasenkampf/Biological Photo Service MEIOSIS II

Fig. 11.7right Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS II Prophase IIMetaphase IIAnaphase II Telophase II Sister chromatids Spindle Nuclear membrane breaking down Chromosome 40 m Following a typically brief interphase, with no S phase, meiosis II begins. During prophase II, a new spindle apparatus forms in each cell, and the nuclear envelope breaks down. In some species the nuclear envelope does not re-form in telophase I removing the need for nuclear envelope breakdown. In metaphase II, a completed spindle apparatus is in place in each cell. Chromosomes consisting of sister chromatids joined at the centromere align along the metaphase plate in each cell. No w, kinetochore microtubules from opposite poles attach to kinetochores of sister chromatids. When microtubules shorten in anaphase II, the centromeres split, and sister chromatids are pulled to opposite poles of the cells. In telophase II, the nuclear membranes re-form around four di f ferent clusters of chromosomes. After cytokinesis, four haploid cells result. No two cells are alike due to the random alignment of homologous pairs at metaphase I and crossing over during prophase I. Nuclear membrane re-forming Kinetochore microtubule Clare A. Hasenkampf/Biological Photo Service

Parent cell (2n) MEIOSIS I Prophase IMetaphase IAnaphase ITelophase I ProphaseMetaphaseAnaphase T elophase Homologous chromosomes do not pair. Individual homologues align on metaphase plate. Paternal homologue Homologous chromosomes Chromosome replication Chromosome replication Homologous chromosomes pair; synapsis and crossing over occur. Paired homologous chromosomes align on metaphase plate. Maternal homologue MITOSIS Fig. 11.8left-a Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sister chromatids separate, cytokinesis occurs, and two cellsresult, each containing theoriginal number of homologues. Two daughter cells (each 2n) Homologous chromosomes separate; sister chromatids remain together.

Fig. 11.8right-b Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS II Prophase IIMetaphase IIAnaphase IITelophase II Chromosomes align, sister chromatids separate, and four haploid cells result, each containing half the original number of homologues. Four daughter cells (each n)

Fig. 11.8right Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS II Prophase IIMetaphase IIAnaphase IITelophase II Chromosomes align, sister chromatids separate, and four haploid cells result, each containing half the original number of homologues. Four daughter cells (each n)

Fig. 11.5-1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Meiosis I Mitosis Metaphase I Metaphase Chiasmata hold homologues together. The kinetochores of sister chromatids fuse and function as one. Microtubules can attach to only one side of each centromere. Homologues do not pair; kinetochores of sister chromatids remain separate; microtubules attach to both kinetochores on opposite sides of the centromere.

Fig. 11.5 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Meiosis I Mitosis Metaphase I Anaphase I Metaphase Anaphase Chiasmata hold homologues together. The kinetochores of sister chromatids fuse and function as one. Microtubules can attach to only one side of each centromere. Microtubules pull the homologous chromosomes apart, but sister chromatids are held together. Homologues do not pair; kinetochores of sister chromatids remain separate; microtubules attach to both kinetochores on opposite sides of the centromere. Microtubules pull sister chromatids apart.

Fig. 11.6 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fig. 11.9-1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING Question: Why are cohesin proteins at the centromeres of sister chromatids not destroyed at anaphase I of meiosis?

Fig. 11.9-2 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Question: Why are cohesin proteins at the centromeres of sister chromatids not destroyed at anaphase I of meiosis? Hypothesis: Meiosis-specific cohesin component Rec8 is protected by another protein at centromeres. SCIENTIFIC THINKING

Fig. 11.9-3 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING Question: Why are cohesin proteins at the centromeres of sister chromatids not destroyed at anaphase I of meiosis? Hypothesis: Meiosis-specific cohesin component Rec8 is protected by another protein at centromeres. Prediction: If Rec8 and the centromere protecting protein are both expressed in mitotic cells, chromosome separation will be prevented. This is lethal to a dividing cell.

Fig. 11.9-4 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING Red colony = dead cells Expresses Rec8 alone Question: Why are cohesin proteins at the centromeres of sister chromatids not destroyed at anaphase I of meiosis? Hypothesis: Meiosis-specific cohesin component Rec8 is protected by another protein at centromeres. Prediction: If Rec8 and the centromere protecting protein are both expressed in mitotic cells, chromosome separation will be prevented. This is lethal to a dividing cell. Test: Fission yeast strain is designed to produce Rec8 instead of normal mitotic cohesin. These cells are transformed with a cDNA library that expresses all cellular proteins. Transformed cells are duplicated onto media containing dye for dead cells (allows expression of Rec8 and cDNA), and media that will result in loss of plasmid cDNA (expresses only Rec8). Cells containing cDNA for protecting protein will be dead in presence of Rec8. cDNA library that expresses all proteins Strain that expresses Rec8 in mitosis Extract plasmid containing cDNA Expresses cDNA + Rec8

Fig. 11.9-5 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING Red colony = dead cells Expresses Rec8 alone Question: Why are cohesin proteins at the centromeres of sister chromatids not destroyed at anaphase I of meiosis? Hypothesis: Meiosis-specific cohesin component Rec8 is protected by another protein at centromeres. Prediction: If Rec8 and the centromere protecting protein are both expressed in mitotic cells, chromosome separation will be prevented. This is lethal to a dividing cell. Test: Fission yeast strain is designed to produce Rec8 instead of normal mitotic cohesin. These cells are transformed with a cDNA library that expresses all cellular proteins. Transformed cells are duplicated onto media containing dye for dead cells (allows expression of Rec8 and cDNA), and media that will result in loss of plasmid cDNA (expresses only Rec8). Cells containing cDNA for protecting protein will be dead in presence of Rec8. Result: Transformed cells that die on the plates where Rec8 is coexpressed with cDNA identify the protecting protein. When the cDNA is extracted and analyzed, the encoded protein localizes to the centromeres of meiotic cells. cDNA library that expresses all proteins Strain that expresses Rec8 in mitosis Extract plasmid containing cDNA Expresses cDNA + Rec8

Fig. 11.9-6 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING Red colony = dead cells Expresses Rec8 alone Question: Why are cohesin proteins at the centromeres of sister chromatids not destroyed at anaphase I of meiosis? Hypothesis: Meiosis-specific cohesin component Rec8 is protected by another protein at centromeres. Prediction: If Rec8 and the centromere protecting protein are both expressed in mitotic cells, chromosome separation will be prevented. This is lethal to a dividing cell. Test: Fission yeast strain is designed to produce Rec8 instead of normal mitotic cohesin. These cells are transformed with a cDNA library that expresses all cellular proteins. Transformed cells are duplicated onto media containing dye for dead cells (allows expression of Rec8 and cDNA), and media that will result in loss of plasmid cDNA (expresses only Rec8). Cells containing cDNA for protecting protein will be dead in presence of Rec8. Result: Transformed cells that die on the plates where Rec8 is coexpressed with cDNA identify the protecting protein. When the cDNA is extracted and analyzed, the encoded protein localizes to the centromeres of meiotic cells. Conclusion: This screen identifies a protein with Rec8 protecting activity. cDNA library that expresses all proteins Strain that expresses Rec8 in mitosis Extract plasmid containing cDNA Expresses cDNA + Rec8

Fig. 11.9 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING Red colony = dead cells Expresses Rec8 alone Question: Why are cohesin proteins at the centromeres of sister chromatids not destroyed at anaphase I of meiosis? Hypothesis: Meiosis-specific cohesin component Rec8 is protected by another protein at centromeres. Prediction: If Rec8 and the centromere protecting protein are both expressed in mitotic cells, chromosome separation will be prevented. This is lethal to a dividing cell. Test: Fission yeast strain is designed to produce Rec8 instead of normal mitotic cohesin. These cells are transformed with a cDNA library that expresses all cellular proteins. Transformed cells are duplicated onto media containing dye for dead cells (allows expression of Rec8 and cDNA), and media that will result in loss of plasmid cDNA (expresses only Rec8). Cells containing cDNA for protecting protein will be dead in presence of Rec8. Result: Transformed cells that die on the plates where Rec8 is coexpressed with cDNA identify the protecting protein. When the cDNA is extracted and analyzed, the encoded protein localizes to the centromeres of meiotic cells. Conclusion: This screen identifies a protein with Rec8 protecting activity. Further Experiments: If the gene encoding the protecting protein is deleted from cells, what would be the expected phenotype? In mitotic cells? In meiotic cells? cDNA library that expresses all proteins Strain that expresses Rec8 in mitosis Extract plasmid containing cDNA Expresses cDNA + Rec8

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Sexual reproduction and meiosis Chapter 11 Genes and Development