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The Cell Cycle

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Theory of Biogenesis. The Roles of Cellular Division: organism reproduction ... Cells can't simply pinch in half. Involves duplicating DNA and distributing ... – PowerPoint PPT presentation

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Title: The Cell Cycle


1
Chapter 12
  • The Cell Cycle

2
  • Unifying characteristic of life - ability to
    reproduce
  • cellular basis
  • Theory of Biogenesis
  • The Roles of Cellular Division
  • organism reproduction
  • growth and development
  • replacement of damaged or dead cells

3
  • Cell cycle
  • Interphase
  • Mitotic phase
  • Cells cant simply pinch in half
  • Involves duplicating DNA and distributing
    identical sets to daughter cells

4
  • Genome
  • Chromosomes
  • Chromatid
  • Centromere
  • Chromatin
  • Genes

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  • Somatic cell
  • diploid number
  • Humans have 46 chromosomes (23 pairs)
  • Gamates
  • haploid number
  • Humans have 23 chromosomes

8
  • The mitotic (M) phase
  • includes mitosis and cytokinesis
  • shortest part of the cell cycle
  • Interphase
  • accounts for about 90 of the cycle
  • cell growth
  • cell differentiation
  • DNA synthesis

9
  • Interphase is divided into three sub-phases
  • Gap 1 (G1) phase cell growth and differentiation
    occurs
  • S phase DNA synthesis
  • Gap 2 (G2) phase cell growth and maturation
    completion of preparations for mitosis

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  • Mitosis is a continuous process, divided into
    five stages
  • Prophase
  • Prometaphase
  • Metaphase
  • Anaphase
  • Telophase

12
  • 1. Prophase
  • nucleolus disappears
  • chromatin begins to coil
  • centrioles migrate to the poles
  • spindle apparatus forms
  • 2.Prometaphase
  • nuclear envelope breaks down
  • microtubules of the spindle apparatus attach to
    the chromosomes at kinetochores

13
  • 3. Metaphase
  • chromosomes move to the metaphase plate
  • centromeres align on the metaphase plate
  • 4. Anaphase
  • Sister chromatids separate
  • Daughter chromatids move to opposite poles
  • Cell elongates

14
  • 5. Telophase
  • cell continues to elongate
  • daughter nuclei form at the poles
  • nuclear envelopes form around each nucleus
  • nucleoli reappear
  • chromosomes begin to uncoil

15
  • Cytokinesis division of the cytoplasm
  • in animal cells cleavage furrow forms near the
    old metaphase plate, microfilaments pinch the
    parent cell in two
  • in plant cells a new cell wall is "built" across
    the cell at the old metaphase plate dividing the
    cell into two
  • cell plate

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  • Mitotic spindle
  • begins to form during prophase
  • fibers made of microtubules and associated
    proteins
  • microtubules of the cytoskeleton partially
    disassemble providing the material used to
    construct the spindle
  • The spindle microtubules elongate by
    incorporating more subunits of tubulin protein

21
  • During prometaphase, some spindle microtubules
    attach to kinetochores
  • Kinetochores are equipped with motor proteins
    that "walk" a chromosome along the attached
    microtubules toward the nearest pole
  • Microtubules shorten by depolymerizing at their
    kinetochore ends
  • Release tubulin protein subunits

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  • In prokaryotes, cell division is by binary
    fission
  • the single circular DNA molecule is replicated
  • the cell membrane grows in the region between the
    copies of the chromosomes
  • the cell grows to about twice its initial size
    and the plasma membrane pinches inward
  • a cell wall forms across the cell, dividing the
    cell in two

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  • The cell cycle is genetically regulated
  • Certain genes are expressed to initiate or stall
    the cell cycle
  • Cell cycle is driven by specific chemical signals
    present in the cytoplasm
  • cell cycle control system
  • Checkpoints in the cell cycle give stop and
    go-ahead signals

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  • Fluctuations in control molecules pace the events
    of the cell cycle
  • Proteins
  • Kinases
  • Give the go-ahead signals at the G1 and G2
    checkpoints
  • Kinases that drive the cell cycle are present at
    a constant concentration in inactive form

28
  • Kinases are activated by attaching to a cyclin
    protein
  • cyclin-dependent kinases, or Cdks
  • Activity rises and falls with changes in the
    concentration of cyclin
  • Cyclin level rises throughout interphase, then
    falls during mitosis

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  • Internal Signals Messages from the Kinetochores
  • M phase checkpoint
  • External Signals Growth Factors
  • both chemical and physical
  • growth factors
  • protein released by certain body cells that
    stimulates other cells to divide

31
  • Density-dependent inhibition of cell division
  • crowded cells stop dividing
  • when a cell population reaches a certain density,
    the amount of required growth factors and
    nutrients available to each cell becomes
    insufficient to allow continued cell growth
  • Cancer cells lack this growth control mechanism

32
  • Cancer cells do not respond to control mechanisms
  • They divide excessively and invade other tissues
  • Do not exhibit density-dependent inhibition
  • Current hypothesis
  • cancer cells do not need growth factors in media
  • They may make the required growth factor
  • May have an abnormality in the signaling pathway
  • Cell cycle control system itself may be abnormal

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  • If and when they stop dividing, cancer cells do
    so at random points in the cycle, rather than at
    the normal checkpoints
  • In culture, cancer cells can go on dividing
    indefinitely if they are given a continual supply
    of nutrients
  • Benign tumor
  • Most do not cause serious problems and can be
    completely removed by surgery
  • Malignant tumor becomes invasive enough to impair
    the functions of one or more organs
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