The Cell Cycle

1
The Cell Cycle

• Chapter 12

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When do cells divide?

• Reproduction
• Replacement of damaged cells
• Growth of new cells
• In replacement and growth cell divisions, how
  should daughter cells compare to parent cell?
• The daughter cells should be identical copies of
  the parent cell

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Functions of Cell Division
4
How can identical daughter cells form?

• The genome must be copied and then divided such
  that each daughter cell gets one of the copies.
• Genome all the genes in an organism

5
Bacterial Reproduction

• How do bacterial
• cells reproduce?

6
Important terms in eukaryotic cell division

• Chromosome - threadlike structures that are
  composed of DNA and protein
• Replication - process whereby DNA is identically
  copied (before cell division)
• Mitosis - division of the nucleus
• Cytokinesis - division of the cytoplasm
• Chromatin - DNA and protein complex that is thin
  and fibrous it will condense into distinct
  chromosomes during cell division

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Important terms in eukaryotic cell division

• Chromatid - after replication the chromosome
  consists of 2 sister chromatids joined at the
  centromere
• Centromere - specialized region of the
  chromosome, where chromatids are joined. Each
  chromosome has one centromere

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The Cell Cycle
9
Interphase

• 90 of cell cycle is spent in this phase
• G1 first growth phase
• S synthesis phase, DNA synthesis (replication)
  occurs here
• G2 second growth phase

10
G2 phase of Interphase in animal cells

• Nuclear envelope is visible
• One or more nucleoli are present
• Centrioles are replicated and the 2 pairs are
  near nucleus
• Aster forms around each pair of centrioles
• Chromosomes are loosely packed into chromatin
  fiber, not distinguishable

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General Overview of Mitosis
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Prophase

• In the Nucleus
• Nucleoli disappear
• Chromosome fibers condense into discrete
  chromosomes
• Each chromosome consists of 2 sister chromatids
  joined at the centromere

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• In the Cytoplasm
• Mitotic spindle begins to form
• Spindle consists of microtubules arranged between
  the centrosomes
• Centrosomes move apart due to lengthening of
  microtubules

14
Prometaphase

• Nuclear envelope breaks apart
• Each chromatid has specialized structure called
  kinetochore located at the centromere region
• Kinetochore microtubules interact with
  chromosomes at the kinetochore region
• The kinetochore microtubles cause the chromosomes
  to move
• Nonkinetochore microtubules radiate from each pole

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Metaphase

• Chromosomes move to the metaphase plate and line
  up there
• The centromeres of the chromosomes are all
  aligned on the metaphase plate
• Each sister chromatid of one chromosome, has a
  kinetochore microtubule attached to it from
  opposite poles
• Kinetochore microtubules nonkinetochore
  microtubules spindle fiber

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Anaphase

• Kinetochore microtubules shorten and
  non-kinetochore microtubules lengthen
• Centromeres divide and each chromosome has no
  sister chromatid component
• The shape of the cell elongates into an ellipse
• Chromosomes are pulled to the opposite poles

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Telophase

• Nonkinetochore microtubules continue to elongate
  the cell
• New daughter nuclei form at the two poles
• New nuclear envelopes are formed around the
  chromosomes
• Nucleoli reappear
• Chromosomes uncoil into chromatin fiber
• Last phase of mitosis

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Cytokinesis

• Begins before telophase has completed
• Evidenced by cleavage furrow in animal cells and
  cell plate in plant cells

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Evolution of mitosis
Bacterial cells
Dinoflagelates chromosomes attach to nuclear
envelope.
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Diatoms nuclear envelope stays, microtubules
inside nucleus
Most other eukaryotes spindle forms outside of
nucleus, and nuclear envelope breaks apart
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Cytoplasmic Cell Signals in Regulation of Cell
Cycle
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Checkpoints in the cell cycle If it passes the
G1 checkpoint cell divides -if not, enters G0
phase and does not divide
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Cyclin protein levels fluctuate according to cell
cycle stage. When cyclin is high the Cdk
attaches and phosphorylation leads to breakdown
of nuclear envelope. Later MPF initiates cyclin
breakdown
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Cancer cells

• How does abnormal cell division of cancer cells
  differ from normal cell division?
• Cancer cells are not under density dependent
  inhibition
• Continue to grow until all nutrients are used up
• Cancer cells are immortal, do not shorten
  telomeres