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Unit 3.3: Cell Division: Mitosis

Unit 3.3: Cell Division: Mitosis. Vocabulary:. Chromatin : uncondensed, loosely-coiled DNA Chromosome : condensed, tightly-coiled DNA Histones : proteins around which DNA coils to become more tightly coiled

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Unit 3.3: Cell Division: Mitosis

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  1. Unit 3.3:Cell Division: Mitosis

  2. Vocabulary: Chromatin: uncondensed, loosely-coiled DNA Chromosome: condensed, tightly-coiled DNA Histones: proteins around which DNA coils to become more tightly coiled Chromatid: half of a duplicated chromosome, includes a short arm (p arm) and a long arm (q arm) Centromere: constriction that joins two sister chromatids of a chromosome at the vertical “center” Mitosis: phases of the cell cycle during which the chromosomes replicate and the nucleus divides into two daughter nuclei; sometimes called karyokinesis. Cytokinesis: phase of the cell cycle in which the cell divides into two daughter cells, each with its own nucleus.

  3. I. DNA Packaging • DNA is “packaged” in two different forms: • Chromatin: uncondensed, loosely-coiled DNA • “Open” to allow access to DNA for protein synthesis • Heterochromatin: region of DNA that is tightly coiled (“closed”) b/c that cell does not use proteins encoded in that length of DNA; stains darker • Euchromatin: region of DNA that is loosely coiled (“open”) b/c cell needs instructions encoded by that length of DNA; stains lighter

  4. Chromosomes: condensed, tightly-coiled DNA • Just before cell division, coil into linear, rod-shaped bodies: • Why? Allows for easier distribution within the cell • Packaging of DNA into chromosomes requires histones: • Histones: proteins around which DNA coils to become more condensed • As histone/DNA complex becomes more tightly coiled, becomes a nucleosome

  5. II. Chromosome Structure • Parts of a chromosome: • Chromatid: one linear half of a duplicated chromosome • Each linear half is called a sister chromatid • Centromere: constriction at the vertical center of a chromosome that holds two sister chromatids together

  6. III. Cell Cycle G1 Phase: Gap 1; cell growthoccurs S Phase: S = synthesis; during this phase, DNA is synthesized, (replicated) in preparation for mitosis G2 Phase: Gap 2; the cell goes through “checkpoints” to make sure cell is ready for mitosis/cell division • Interphase: inter=between; phase between cell divisions • A cell spends most of its “life” in this phase • DNA is in chromatin form for protein synthesis, etc… • Interphase has three sub-phases:

  7. Mitosis: division of the nucleus • Four stages of mitosis: • Prophase: (pro- = before, early) • DNA condenses into chromosomes • Mitotic spindle forms; built of microtubules • Like “fingers” that reach out to grab chromosomes, pull them apart • Nuclear membrane disappears DNA in chromosome form, for easier “sorting” into daughter cells

  8. Metaphase: meta- = middle • Chromosomes line up in “middle” of mitotic spindle • Homologous chromosomes are usually not next to each other • Homologous chromosomes have “same” structure, “same” genes • Anaphase: ana- = away from • Chromosomes are split at the centromere • Chromatids pull “away from” each other, move to opposite sides of the mitotic spindle

  9. Label each of the following phases of mitosis: • Telophase: telo- = the end • Nuclear envelope forms around each set of chromosomes • Chromosomes uncoil into uncondensed chromatin • Mitotic spindle disassembles (like taking TinkerToys apart!)

  10. Cytokinesis: division of the cell • Animals: cleavage furrow forms between daughter cells • Plants: cell plate forms between daughter cells, becomes cell wall • Each daughter cell is genetically identical to the parent. • Except for mutations

  11. IV. Purpose of Mitosis • Asexual reproduction: • Prokaryotes divide through binary fission, which is mitosis with a circular chromosome. • Growth: • A multicellular organism does not have larger cells, just many more of them. Mitosis is the mechanism by which large organisms grow and develop. • Repair: • When certain cells (ex: skin, liver, blood) of an organism are damaged, the body can make more of those cells through mitosis. • The body cannot make more of certain cells; ex: brain, nervous system cells

  12. V. Control of Cell Cycle • Cells do not grow continuously, signals control whether cell grows or stops • Internal regulators: proteins that determine if a cell will go through mitosis • Called “cyclins” because they control cell cycle • If DNA does not replicate properly, cell cycle stops… • If chromosomes do not attach to spindle properly, cell cycle stops… • External Regulators: growth factors that encourage cell division • Nearby cells will release factors to inhibit growth to prevent overcrowding • If cells are removed, remaining cells will divide until they touch other cells.

  13. Cancer: uncontrolled cell growth • Do not respond to internal or external regulators • Often caused by a defect in p53 gene • p53 stops cell cycle if DNA does not replicate properly • If p53 is defective, cell will divide even though DNA is not ready.

  14. Warm-up answers for this unit:

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