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Divide and Conquer.docx

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Western University
Biology 1201A
Richard Gardiner

1 Divide and Conquer What do all cells require to survive? o A complete set of genetic instruction  Genetic instruction is coded in the DNA of the cell. o Produce required molecules o Direct life process Why do cells divide? o Growth o Repair o Development Cell cycle: Activities of a cell from one cell division to the next. o Cell grow, adding more cytoplasmic constituents o DNA is replicated o Cell divides into 2 identical daughter cells. Essential Features of Cell Division: Transmit a complete copy of genetic information (DNA) Transmit materials necessary for cell to survive and use genetic information Prokaryotic Cell: o Bacteria No nucleus- genetic material located in cytoplasm No membrane- bound organelles Cell division is call binary fission Prokaryotic cell cycle: Prokaryotic chromosome a circular loop Chromosomes attach to one plasma membrane Chromosomes are replicated o Replicated chromosomes attach to plasma membrane at a different nearby point Cell elongates o New plasma membrane is added between chromosomes, pushing them towards opposite ends of the cell Plasma membrane grows inward at middle of cell Parent Cell is divided into 2 identical daughter cells 2 Eukaryotic Cells: o Fungi, protists, plants Membrane- Bound organelles including nucleus o DNA contained in nucleus Cell division of somatic cells is called mitotic cell division. Eukaryotic Chromosomes: Contains almost all genetic information o Mitosis only deals with nuclear chromosomes Mitochondria and Chloroplast also have DNA o DNA replication here is handled differently Chromosomes Chromosomes= long threads like structure o Highly condensed during mitosis o DNA + protein o Contains most of the organisms genetic information o Numbers vary with species Eukaryotic Chromosome structure: Strands of linear DNA Human Cells: o 46 strands (46 chromosomes) o Average length 4cm o Human cell approximately 3 meters of DNA Chromatin: Many proteins are bound for DNA o Protect o Duplication o Regulation o Modification o Transcription o Packaging DNA and bound protein= chromatin o Chromatin is only around 50 percent DNA During non- division phase of cell cycle: DNA molecules are extended, uncondensed form called chromatin Cell can only use DNA to produce molecules when in extended state. 3 During division phase of cell cycle: DNA molecules condense to form chromosomes prior to division Each chromosome is a single molecule of DNA o Easier to sort and organize into daughter cells What is mitotic cell division? Division of somatic cells in eukaryotic organisms o Somatic cell= non- sex cells A single cell divides into 2 identical daughter cells Organisms have a specific number of sets in haploid- diploid Mitosis and meiosis lead to different ploidy outcomes Ploidy- Refers to the number of pairs of chromosomes in a cell Haploid- one copy of each chromosome “n” Diploid- 2 copies of each chromosomes= pairs “2n” Triploid- 3 sets of chromosomes “3n” Polyploid- more than 2 complete sets o Common in plants, not animals Eukaryotic Cell Cycle: 2 major phases: 1. Interphase o DNA uncondensed (chromatin) o 3 phases 2. Mitotic Cell division o DNA Condensed (Chromosomes) o 5 phases G1 phase= First Gap o Size of cell increases o Organelles may replicated o Normal Growth and development S phase= DNA synthesis o DNA is replicated o Synthesis of proteins associated with mitosis o Cell committed to divide Interphase: Occurs before mitosis Genetic material called chromatin DNA replicated occurs during this phase 4 Nucleus well defined o Nucleoli present o Centrosomes replicated  In animals only Microtubules extend from centrosomes Chromosomes have duplicated but not condensed 5 Stages of Mitosis: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase  As well with cytokinesis Prophase: Chromatin fibers become tightly coiled chromosomes Nucleus disappears Mitotic spindles begin to form Centrioles move away from each other Prometaphase: Nuclear envelope fragments Microtubules connect to chromosomes Kinetochores have formed Some microtubules connect with those from the opposite pole Metaphase: Centrosomes now at opposite poles Chromosomes at metaphase plate Centromeres of the chromosomes are on the metaphase plate Kinetochores of each chromatid connected to microtubules from different pole Anaphase: Begins when paired centromeres separate Chromosomes move to opposite poles  ½ of each sister chromatid pair Move centromere first Poles of cell move further apart at same time 5 Mitotic Spindles: Arises from two microtubules organizing centers o Centrosomes Microtubules spontaneously arise from tubulin dimmers Microtubules arising from M O centers have opposite polarity They meet at equatorial plane Some microtubules are bound together by proteins Chromatids attach to microtubules via kinetochores o Kinetochores of duplicated chromosomes attach to microtubules of opposite polarity They walk down microtubules to microtubules organizing centers Pull chromatids apart after centromere break down o This is the beginning of anaphase Telophase: Daughter nuclei form at the two poles Nuclear envelope reforms Chromatin fibers become less condensed Nucleus reforms Mitosis complete Cytokinesis well underway Cytokinesis: Cytokinesis is the process of splitting daughter cells apart Mitosis is cell division of nucleus o Cytokinesis is division of everything else Cytokinesis is optional. o Does not often occur in muscle, some protists and plant cells Cytokinesis in animals: 1. The furrow begins as an indentation running completely around the cell in the plane of the former spindle midpoint 2. The furrow deepens by contraction of the microfilaments like a drawstring tightening around the cell 3. Furrowing continues until the daughter nuclei are enclose in separate cell Cytokinesis in plants: 1. A layer of vesicle containing wall material collects in the plane of the former midpoint 2. More vesicles are added to the layer until it extends across the cell 3. The vesicles fuse together, dumping their contents into a gradually expanding wall between the daughter cells 4. Vesicles fusion continues until the daughter cells are separated by a continuous new wall, the cell plate 6 Regulation of the cell cycle: Timing and rate of cell division in different parts of a plant or animal are critical to normal development o Skin cells divide regularly o Nerve and muscle do not o Liver cells if needed o These cell to cell difference are the result of regulation of the cell cycle The distinct events of the cell cycle are directed by a distinct cell cycle control system o These molecules trigger and coordinate key events in the cell cycle o The control cycle, has a built in clock but it is also regulated by external adjustments and internal controls G1 Check point: G1- restrict point- most important If cell receives go ahead it will complete cycle and divide If it does not it will exit the cell cycle and switch into a non-dividing state called G0 phase G0 (permanent G1 arrest) Specialized cells stop reproduction, to perform specialized functions Example: nerve cells, liver cells, some muscle cells. Internal and External Clues Kinetochores signal when they are all connected so Anaphase does not begin to early and leave a chromosomes Growth factors in media may be needed Cells crowded together stop dividing Most animal cells must be anchored to divide Haploid: Haploid=n= cells with one complete set of chromosomes o Unfertilized egg and sperm cells Haploid sperm fuses with haploid egg to get diploid zygote Male parent provides 23 chromosomes (1 set) and female parents 23 chromosomes (1 set) Diploid: Diploid=2n= cells with 2 complete sets of chromosomes Almost all body cells of human are diploid o Muscles, brain, bone, skin etc. Diploid human cells have 46 chromosomes Diploid cells do have pairs of homologous chromosomes 7 Sexual vs. Asexual Reproduction Sexual Asexual  Each new individual gets ½ genetic  Individuals inherit all its genes from information from one parents and one parents ½ from other parent  Offspring are genetically identical to  Offspring are genetically different parents from both parents 1. One celled organisms divide  Sexual reproduction produces much 2. Hydra bonding greater variation 3. Self pollinating flowers o Evaluation requires pre-existing 4. Some lizards genetic variation 5. Aphids o Sexual reproduction provides 6. Aspen shouts up from roots much of the pre- existing  Genetic variations come from variation
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