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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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vocabulary
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.

i dna packaging
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
slide4

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
ii chromosome structure
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
iii cell cycle
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:
mitosis division of the nucleus
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

slide10

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

slide11

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!)
slide12

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
iv purpose of mitosis
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
v control of cell cycle
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.
cancer uncontrolled cell growth
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.