Chapter 10 Chromosomes, Mitosis, Meiosis PDF

Chromosomes, Mitosis and Meiosis Chapter 10  Introduction Pre-existing cells divide to form new cells. This remarkable process enables an organism to grow, repair damaged parts, and reproduce. Cells serve as the essential link between generations....

Chromosomes, Mitosis and Meiosis Chapter 10  Introduction Pre-existing cells divide to form new cells. This remarkable process enables an organism to grow, repair damaged parts, and reproduce. Cells serve as the essential link between generations. Even the simplest cell contains a large amount of precisely coded genetic information in the form of deoxyribonucleic acid (DNA).  Learning Objective 1 What is the significance of chromosomes in terms of information?  Chromosomes The major carriers of genetic information in eukaryotes are the chromosomes, which lie within the cell nucleus. Although chromosome means “colored body,” chromosomes are virtually colorless; the term refers to their ability to be stained by certain dyes.  Organization Genes cell’s informational units (provide information needed to carry out specific cell function) made of DNA Chromatin DNA and associated proteins makes up chromosomes (eukaryotes) Chromosomes allow DNA sorting into daughter cells  KEY CONCEPTS In eukaryotic cells, DNA is wound around specific proteins to form chromatin, which in turn is folded and packaged to make individual chromosomes  Learning Objective 2 How is DNA organized in prokaryotic and eukaryotic cells?  Prokaryotic Cells Contain circular DNA molecules Singular chromosome  Eukaryotic Chromosomes Nucleosome: 8 Histones +DNA wrapped around Histone: package DNA into coiled structures (chromosome) histone (protein) bead wrapped in DNA organized into coiled loops held together by nonhistone proteins called scaffolding proteins The wrapping of DNA into nucleosome represents the first level of chromsome structure Nucleosomes  Scaffolding Proteins Help maintain chromosome structure  Chromosome Organization First, DNA is wrapped around histone proteins to form nucleosomes. Then, the nucleosomes are compacted into chromatin fibers, which are coiled into looped domains. The looped domains are compacted, ultimately forming chromosomes.  1400 nm 700 nm 300 nm fiber (looped domains) 30 nm chromatin fiber DNA wound around a Condensed Condensed Scaffolding cluster of chromosome chromatin protein histone Extended chromatin molecules Condensin protein is required for chromosome compaction Packed nucleosomes Histone 10 nm 2 nm Nucleosomes DNA double helix Fig. 10-4, p. 214  Learning Objective 3 What are the stages in the eukaryotic cell cycle, and their principal events?  Eukaryotic Cell Cycle Cycle of cell division interphase M phase  Interphase First gap phase (G1 phase)-longest phase cell grows and prepares for S phase Toward the end of G1, the enzymes required for DNA synthesis become more active. Synthesis of these enzymes, along with proteins needed to initiate cell division, enable the cell to enter the S phase.  Interphase Synthesis phase (S phase) DNA replicates and histone proteins are synthesized so that the cell can make duplicate copies of its chromosomes.  Interphase Second gap phase (G2 phase) protein synthesis increases preparation for cell division Organelles replicate, chromosomes coil more tightly; centrioles replicate, tubulin synthesis  M Phase M Phase involves two processes: Mitosis begins at the end of the G2 phase nuclear division that produces two nuclei identical to parent nucleus Cytokinesis begins before mitosis is complete cytoplasm divides to form two daughter cells  KEY CONCEPTS Cell division is an important part of the cell cycle, which consists of the successive stages through which a cell passes  Learning Objective 4 What is the structure of a duplicated chromosome, including the sister chromatids, centromeres, and kinetochores?  A Duplicated Chromosome Consists of a pair of sister chromatids containing identical DNA sequences Centromere constricted region joins sister chromatids Kinetochore protein to which microtubules bind attached to centromere Sister Chromatids  Sister chromatids are physically linked by a ringshaped protein complex called cohesin.  Cohesins extend along the length of the sister chromatid arms and are particularly concentrated at the centromere.  Learning Objective 5 What is the process and significance of mitosis? Interphase  Mitosis Preserves chromosome number in eukaryotic cell division Identical chromosomes are distributed to each pole of the cell nuclear envelope forms around each set  M Phase Mitosis is divided into 5 phases: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase INTERPHASE PROPHASE PROMETAPHASE Nucleolus Sister chromatids Kinetochore Chromatin Nucleus of duplicated chromosome Pieces of nuclear envelope Spindle Nuclear envelope microtubule Centrioles Developing mitotic Plasma membrane spindle Fig. 10-6a, p. 216 METAPHASE ANAPHASE TELOPHASE 25 μm Spindle Cleavage furrow Centriole pair at spindle pole Daughter Reforming Cell’s midplane nuclear envelope chromosomes (metaphase plate) Fig. 10-6b, p. 217  Prophase Chromatin condenses into duplicated chromosomes (pair of sister chromatids) Duplicated chromosomes first become visible with the light microscope Nuclear envelope begins to disappear units of the disassembled nuclear envelope are sequestered in vesicles to be used later, to assemble nuclear envelopes for the daughter cells. Mitotic spindle begins to form Mitotic Spindle Metaphase plate (cell’s midplane) Kinetochore microtubule (spindle microtubule) Centrioles Astral microtubules Pericentriolar material Polar (non- kinetochore) microtubule Sister chromatids Fig. 10-9a, p. 219 10 μm Fig. 10-9b, p. 219 Prophase - Chromatin condenses into duplicated chromosomes (pair of sister chromatids) - Nuclear envelope begins to disappear -units of the disassembled nuclear envelope are sequestered in vesicles to be used later, to assemble nuclear envelopes for the daughter cells. -Prophase begins with chromosome compaction, long chromain fibers begin with coiling process that makes them shorter thicker. After compaction, the chromtin is referred to as chromosomes  Prometaphase Spindle microtubules attach to kinetochores of chromosomes Chromosomes begin to move toward cell’s midplane Prometaphase  Metaphase Chromosomes align on cell’s midplane (metaphase plate) Mitotic spindle is complete Microtubules attach kinetochores of sister chromatids to opposite poles of cell Metaphase  Anaphase Sister chromatids separate move to opposite poles Each former chromatid is now a chromosome Anaphase  Telophase Nuclear envelope re-forms Nucleoli appear Chromosomes uncoil Spindle disappears Cytokinesis begins Telophase  KEY CONCEPTS In cell division by mitosis, duplicated chromosomes separate (split apart) and are evenly distributed into two daughter nuclei  Cytokinesis Division of the cytoplasm to yield two daughter cells In animals and fungi: Formation of actomysoin contractile ring (composed of actin and myosin filaments) The ring contracts and produces a cleavage furrow that separate the cytoplasm into two daughter cells In plants: formation of cell plate Cytokinesis  Cleavage furrow Actomyosin contractile ring 10 μm Fig. 10-10a, p. 220  Small Cell plate vesicles Nucleus forming Vesicles fuse, Eventually New cell gather on forming one large walls (from cell’s larger vesicle vesicle midplane vesicles exists contents) Plasma Cell Cell plate New plasma membrane wall forming membranes (from vesicle 5 μm membranes) Fig. 10-10b, p. 220  Learning Objective 6 How is the cell cycle controlled?  Cell-Cycle Control Cell cycle checkpoints: ensure that all events of a particular stage of cell cycle have occurred correctly before moving to the next stage Cyclin-dependent kinases (Cdks) protein kinases that control cell cycle active only when bound to cyclins Cyclins regulatory proteins levels fluctuate during cell cycle form cyclin-Cdk complexes: Cyclins  1 1 Cyclin is synthesized and accumulates. 1 2 2 Cdk associates with cyclin, forming a cyclin–Cdk complex, M-Cdk. 3 3 M-Cdk phosphorylates proteins, Cdk activating those that facilitate mitosis 5 G1 S and inactivating those that inhibit mitosis. 4 4 An activated enzyme complex recognizes a specific amino acid sequence in cyclin and targets it for M G2 destruction. When cyclin is degraded, M-Cdk activity is terminated, and the cells formed by mitosis enter G1. Cyclin 5 5 Cdk is not degraded but is recycled and reused. 2 4 Degraded M-Cdk Cdk cyclin 3 (triggers M phase) Fig. 10-12, p. 222  Regulation of the cell cycle How cell division (and thus tissue growth) is controlled is very complex. The following terms are some of the features that are important in regulation, and places where errors can lead to cancer. Cancer is a disease where regulation of the cell cycle goes awry and normal cell growth and behavior is lost.Cdk (cyclin dependent kinase, adds phosphate to a protein), along with cyclins, are major control switches for the cell cycle, causing the cell to move from G1 to S or G2 to M. MPF (Maturation Promoting Factor) includes the CdK and cyclins that triggers progression through the cell cycle. p53 is a protein that functions to block the cell cycle if the DNA is damaged. If the damage is severe this protein can cause apoptosis (cell death). - p53 levels are increased in damaged cells. This allows time to repair DNA by blocking the cell cycle. - A p53 mutation is the most frequent mutation leading to cancer. An extreme case of this is Li Fraumeni syndrome, where a genetic a defect in p53 leads to a high frequency of cancer in affected individuals. p27 is a protein that binds to cyclin and cdk blocking entry into S phase. Research (Nature Medicine 3, 152 (1997)) suggests that breast cancer prognosis is determined by p27 levels. Reduced levels of p27 predict a poor outcome for breast cancer patients.  KEY CONCEPTS An internal genetic program interacts with external signals to regulate the cell cycle  Learning Objective 7 What is the difference between asexual and sexual reproduction?  Asexual Reproduction Single parent offspring have identical hereditary traits Mitosis basis for eukaryotic asexual reproduction Binary Fission  Prokaryotic cell Plasma 1 DNA replication begins at membrane Bacterial single site on bacterial Cell wall DNA DNA. Origin of replication 2 Replication continues, as Two copies replication enzymes work of bacterial in both directions from DNA site where replication began. 3 Replication is completed. Cell begins to divide, as plasma membrane grows inward. 4 Binary fission is complete. Two identical prokaryotic cells result. Two identical prokaryotic cells Fig. 10-11, p. 221  Sexual Reproduction Two haploid sex cells (gametes) fuse to form a single diploid zygote Meiosis produces gametes  Learning Objective 8 What is the difference between haploid and diploid cells? What are homologous chromosomes?  Diploid Cell Chromosomes are paired, 2n (homologous chromosomes) similar in length, shape, other features carry genes affecting the same traits  Haploid Cell Contains only one member of each homologous chromosome pair n Sexual Life Cycle 62 Mitosis is used by single-celled organisms to reproduce; it is also used for the organic growth of tissues, fibers, and membranes. Meiosis is found in sexual reproduction of organisms. The male and female sex cells (i.e., egg and sperm) combine to create new, genetically different offspring.  MITOSIS PROPHASE No synapsis of homologous chromosomes ANAPHASE Sister chromatids move to opposite poles DAUGHTER CELLS Two 2n cells with unduplicated chromosomes Fig. 10-16a, p. 229  MEIOSIS PROPHASE I Synapsis of homologous chromosomes to form tetrads ANAPHASE I Homologous chromosomes move to opposite poles PROPHASE II Two n cells with duplicated chromosomes ANAPHASE II Sister chromatids move to opposite poles HAPLOID CELLS Four n cells with unduplicated chromosomes Fig. 10-16b, p. 229  Learning Objective 9 What is the process and significance of meiosis?  Meiosis One diploid cell divides two times, yielding four haploid cells Sexual life cycles in eukaryotes require meiosis each gamete contains half the number of chromosomes in parent cell  Meiosis I Prophase I - homologous chromosomes find each other and become closely associated, a process called pairing, or synapsis - Includes formation of synaptonemal complexes Formation also called tetrad or bivalents Crossing-over between homologous (nonsister) chromatids Remain attached at chiasmata exchanges segments of DNA strands Results in genetic recombination Synapsis  Maternal sister chromatids Paternal sister chromatids Synaptonemal complex Chromatin Protein Chromatin Maternal sister chromatids Fig. 10-14a, p. 228  Chromosome Synaptonemal complex Chromosome 0.5 μm Fig. 10-14b, p. 228 Tetrads and Chiasmata Chiasmata Sister chromatids Kinetochores Sister chromatids 1 μm Fig. 10-15a, p. 228  Sister Chiasmata chromatids Kinetochores Fig. 10-15b, p. 228  Meiosis I Metaphase I tetrads (homologous chromosomes joined by chiasmata) line up on metaphase plate Anaphase I homologous chromosomes separate distributed to different nuclei Telophase I: Each nucleus contains haploid number of chromosome but each chromosome is a duplicated chromosome (has 2 chromatids)  Meiosis II Two chromatids of each chromosome separate one distributed to each daughter cell Each former chromatid is now a chromosome Meiosis Meiosis INTERPHASE MEIOSIS I Mid-prophase I Late prophase I Nucleolus Nuclear envelope Homologous Chromatin chromosomes Developing Centrioles meiotic spindle Interphase Homologous chromosomes preceding meiosis; synapse, forming tetrads; DNA replicates. nuclear envelope breaks down. Fig. 10-13a (1), p. 226  Metaphase I Anaphase I Telophase I Microtubule Cleavage furrow attached to kinetochore Separation of Sister homologous chromatids chromosomes Tetrads line up on cell's Homologous chromosomes One of each pair of midplane. Tetrads held separate and move to homologous together at chiasmata opposite poles. Note that chromosomes is at (sites of prior crossing- sister chromatids remain each pole. over). attached at their Cytokinesis occurs. centromeres. Fig. 10-13b (1), p. 227 MEIOSIS II Prophase II Metaphase II Anaphase II Daughter chromosomes Chromosomes condense Chromosomes line up Sister chromatids separate, again following brief along cell's midplane. and chromosomes move to period of interkinesis. opposite poles. DNA does not replicate again. Fig. 10-13a (2), p. 226  Telophase II Four haploid cells 25 μm Nuclei form at opposite poles of Four gametes (animal) or four each cell. Cytokinesis occurs. spores (plant) are produced. Fig. 10-13b (2), p. 227  Learning Objective 10 What are the different processes and outcomes of mitosis and meiosis?  Mitosis Single nuclear division 2 daughter cells genetically identical to each other and to original cell No synapsis of homologous chromosomes Mitosis  Meiosis Two successive nuclear divisions form four haploid cells Synapsis of homologous chromosomes occurs during prophase I Meiosis  KEY CONCEPTS Meiosis, which reduces the number of chromosome sets from diploid to haploid, is necessary to maintain the normal chromosome number when two cells join during sexual reproduction  KEY CONCEPTS Meiosis helps to increase genetic variation among offspring  Learning Objective 11 Compare the roles of mitosis and meiosis in various generalized life cycles  Animals Somatic cells are diploid produced by mitosis A somatic cell is any cell of the body except sperm and egg cells. Gametes are haploid produced by meiosis (gametogenesis) Animal Life Cycle  Gametes (n) Meiosis Fertilization Zygote (2n) Mitosis Multicellular diploid organism (2n) Animals Fig. 10-17a, p. 230  Simple Eukaryotes May be haploid produced by mitosis Only diploid stage is the zygote which undergoes meiosis to restore the haploid state Simple Eukaryote Life Cycle  Unicellular or multicellular haploid organism (n) Mitosis Mitosis Gametes (n) Meiosis Fertilization Zygote (2n) Simple eukaryotes Fig. 10-17b, p. 230