johnson - the living world: 3rd ed. - all rights reserved - mcgraw hill companies how cells divide...
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Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
How Cells Divide
Chapter 6
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Outline
• Simple Cell Cycle• Complex Cell Cycle• Chromosomes• Mitosis
Cancer• Meiosis I• Meiosis II• Evolutionary Consequences of Sex
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Simple Cell Cycle
• Cell division in bacteria takes place in two stages (Simple Cell Cycle). DNA is copied Cell Splits (Binary Fission)
- Forms two daughter cells• Heredity information in bacteria encoded in
single circle of DNA.
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Complex Cell Cycle
• Eukaryotic DNA is contained in linear chromosomes. Long DNA molecules packaged with
proteins.• Mitosis - Mechanism of cell division occurring
in non-reproductive (somatic) cells.• Meiosis - Mechanism of cell division
occurring in reproductive (germ) cells.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Complex Cell Cycle
• G1 phase - Primary growth phase
• S phase - DNA replicates• G2 phase - Microtubule synthesis
• M phase - Chromosomes pull apart• C phase - Cytokinesis
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Complex Cell Cycle
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Chromosomes
• Human cells each have 23 nearly identical pairs of chromosomes, for a total of 46. Each chromosome contains thousands of
genes that play important roles in body development and function.
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Chromosome Structure
• DNA helix is wrapped around proteins with positive charges (histones) and negative charges counteracting each other.
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Chromosome Structure
• Homologues - Nearly identical copies of the same chromosomes.
• Diploid Cells - Two copies of chromosomes. Before cell division, each homologue
replicates, resulting in two identical copies (sister chromatids).
- Remain attached at centromere.Copyright © McGraw-Hill Companies
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Cell Division
• Interphase - Chromosomes replicate and begin to wind up tightly.
• Mitosis Prophase - Nuclear envelope breaks
down, DNA further condenses, and spindle fibers form.
Metaphase - Chromosomes align at center of cell and kinetochore fibers attach to centromeres.
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Mitosis
Anaphase - Centromeres replicate and sister chromatids separate and move towards opposite poles.
Telophase - Nuclear envelope reappears and chromosomes decondense.
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Cytokinesis
• Cytokinesis - Division of cytoplasm. Animal cells - Cell pinches in two with
contracting belt of microtubules, forming a cleavage furrow.
Plant cells - Membrane assembled at right angle to mitotic spindle. Cell plate grows outward until it reaches the interior surface of plasma membrane.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Controlling Cell Cycle
• At critical points, further cell progress depends on a central set of switches regulated by cell feedback. G1 – Cell growth assessed G2 DNA replication assessed M mitosis assessed
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Cancer
• Cancer - Unrestrained cell growth and division. Tumor - Cluster of cells.
- Benign - Encapsulated and noninvasive.- Malignant - Not encapsulated, invasive,
and shed cells. Metastases - Process of cells
shedding from a malignant tumor and spreading to distant parts of the body.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Cell Cycle Control
• Gene p53 plays a key role in G1 checkpoint of cell division. Gene’s product monitors integrity of DNA,
checking for successful replication.- If protein detects damaged DNA, it halts
cell division and stimulates repair enzymes.
Nonfunctional p53 genes allow cancer cells to repeatedly divide.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Curing Cancer
• Preventing cancer from starting by focusing on decision-making process to divide. Receiving the signal to divide Relay switch Amplifying the signal Releasing the brake Checking that everything is ready Stepping on the gas
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Curing Cancer
• Preventing the Spread of Cancer Tumor growth
- Angiogenesis inhibition Metastasis
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Molecular Cancer Therapies
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Meiosis
• Gamete formation must involve some mechanism to halve the number of chromosomes. Two sets of chromosomes are present in
somatic cells of adults (diploid), but only one set is present in gametes (haploid).
- Sexual Reproduction involves the alteration of meiosis and fertilization.
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Alteration of Generations
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Stages of Meiosis
• Meiosis I Prophase I - Chromosomes pair up and
exchange segments (Crossing Over). Metaphase I - Chromosomes align at cell’s
center (Independent Assortment). Anaphase I - Homologous pairs pulled
apart. Telophase I - Individual chromosomes
gather at each of the poles.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Crossing Over
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Meiosis II
• Mitotic division involving products of Meiosis I. Prophase II - Nuclear envelope breaks
down and spindle fibers form. Metaphase II - Spindle fibers bind to both
sides of centromeres. Anaphase II - Spindle fibers contract,
moving sister chromatids to opposite poles. Telophase II - Nuclear envelope reforms
around four sets of daughter cells.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Evolutionary Consequences of Sex
• Genetic Diversity Independent Assortment Crossing Over Random Fertilization
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Review
• Simple Cell Cycle• Complex Cell Cycle• Chromosomes• Mitosis
Cancer• Meiosis I• Meiosis II• Evolutionary Consequences of Sex
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
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