Key%20Events%20in%20the%20Cell%20Cycle

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Key Events in the Cell Cycle
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Corrected Slide from Lecture 17- New Title
The Spindle Assembly Checkpoint Blocks
Activation of the Anaphase Promoting Complex (APC)
Check for Proper Chromosome Attachment to
Spindle Phosphorylation of APC- now
Activated Sister Chromatids Can
Separate If Chromosomes not Properly
Attached Metaphase Arrest
Will not Separate
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Cell DivisionLecture 18

• We will revisit and complete Apoptosis
• Lecture on Monday

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Cell Cycle Overview
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Preparation for the Onset of M-phase

• Duplication of Chromosomes
• Member of pair referred to as Sister Chromatid
• Held together by Proteins Complexes called
  Cohesions
• Duplication of Centrosomes
• Centriole Pair and other Components Duplicated
• Remain Together as a Single Complex in Nucleus

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M phase in Animal Cells Depends upon Chromosome
and Centrosome Duplication in the Preceding
Interphase
MT IF

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Replicated Chromosomes Are Composed of Two Sister
Chromatids Held Together by Cohesion Protein
Complexes
Sister Chromatids
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M-phase Requires Formation of a New Apparatus
Called the Mitotic Spindle

• A Symmetrical, Bipolar Structure composed of
  Microtubules- with associated Motors and Other
  Proteins
• Each pole has a Centrosome
• Functions in M-phase
• Responsible for Separating the Replicated
  Chromosomes into Daughter Nuclei during Division
  of the Nucleus
• Directs the Process of Dividing the Cytoplasm

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Centrosome Duplication is Required for the
Formation of a Bipolar Mitotic Spindle
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M-CDK Activity is Required for Mitosis

• Key Ways to Regulate CDK Complex Activity
• Proteolysis of Cyclin Subunit
• Phosphorylation/ Dephosphorylation
• Binding by Inhibitors
• Subcellular Localization

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Mitosis Requires Active M-CDK
During G2- Cyclin Levels are Increasing Form
M-CDK Complexes But Held Inactive due to
Inhibitory Phosphate
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Active M-CDK Indirectly Activates More M-CDK
Leads to Sudden Explosive Increase in Active
M-CDK Levels
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MCDK Activity is Required for Triggering the
Mitotic Machinery

• 1) Phosphorylation and Activation of Various
  Kinases
• 2) Direct Phosphorylation of Targets
• Chromosome Condensation
• Assembly of Mitotic Spindle
• Nuclear Envelope Breakdown

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Stages of MitosisProphase

• Individual Replicated Chromosomes Condense
• Nucleoli Disperse (in Animal Cells)
• Two Centrosomes Separate From Each Other
• Assembly of the Mitotic Spindle Outside of the
  Nucleus

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Chromosome Condensation During ProphaseM-CDK
Phosphorylates Condensin Proteins
Cohesions Keep Sister Chromatids
Together Condensins bind DNA Phosphorylated by
M-CDK Results in Chromosome Condensation
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G2 to Prophase Transition
PROMETAPHASE
G2
PROPHASE
Centrosomes(with centriole pairs)
Aster
Chromatin(duplicated)
Early mitoticspindle
Kinetochore
Centromere
Nonkinetochoremicrotubules
Chromosome, consistingof two sister chromatids
Plasmamembrane
Nucleolus
Kinetochore microtubule
Nuclearenvelope
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Prometaphase

• Marked by
• 1) The Breakdown of Nuclear Envelope
• 2) Kinetochores Assembled on Chromosomes
• can now be attached to Spindle Microtubules
• 3) Chromosomes Attach to Mitotic Spindle Begin
  to move around

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Nuclear Envelope Breakdown Marks Transition into
Prometaphase
by M-CDK
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A Kinetochore Assembles on the Centromere Region
of Each Sister Chromatid
One Kinetochore Per Chromatid
Sister Chromatids
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Prophase to Prometaphase Transition
Interpolar Microtubules
Now MT Have Access To Chromosomes!
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Classification of Spindle Microtubules

• Kinetochore Microtubules- Attach to Kinetochores
• Polar Microtubules- Interact with Microtubules
  from Opposite Pole of the Cell
• Astral Microtubules- Nucleate from Centrosome
• Some Interact with Motor Proteins Associated with
  at Cell Cortex

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Organization of the Mitotic Spindle
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Metaphase
Maximal Chromosome Condensation Aligned at
Metaphase Plate -Not actually still- jerky
motions due to constant tension Forces Acting on
Chromosomes to Align them At Metaphase Plate
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Anaphase

• Abrupt Separation of Sister Chromatids
• Movement to Chromosomes to Opposite Pole
• Cell Elongates
• For Anaphase to Occur
• 1) Activation of the Anaphase Promoting Complex
  (APC) Mediated Indirectly through M-CDK
• 2) Must Pass SPINDLE ATTACHMENT CHECKPOINT
• All Chromosomes are properly attached to the
  spindle before Sister Chromatid Separation Occurs

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Activation of the Anaphase Promoting Complex
(APC), a Highly Regulated Ubiquitin Ligase

• APC Functions to Bring About
• Cleavage of Cohesion Complex to Separate Sister
  Chromatids (through proteolytic degradation of
  inhibitory protein)
• Proteolytic Cleavage and Inactivation of M-phase
  Cyclin thereby inactivating
• M-CDK

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Proteasome Degradation
(APC)
Key Targets Securin And Cyclin of M-CDK
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The Mechanism of Separation of Sister Chromatids
Requires APC Activity
Securin
Separase
Spindle Attachment Checkpoint If A Kinetochore
is Not Attached to The Spindle It Is
Sensed The APC is Kept Inactive
T
APC-Anaphase Promoting Complex
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Chromosome Separation During ANAPHASE
Anaphase A- Sister Chromatids are Pulled to
Opposite Poles Centromeres First Toward the
Spindle Poles as Kinetochore MT
Depolymerize Anaphase B- The Spindle Poles
Move Away from Each Other as Polar MT
Lengthen Anaphase A and B May Occur
Simultaneously or Shortly After One
Another Movements Rely on Motor Proteins that
Associate with the Various MT
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Movement Involved in Daughter Chromosome
Separation During Anaphase A
Anaphase A Movement Plus End of Kinetochore
Microtubules Dissassembles Minus End Directed
Motor (Dynein) Proteins Pull Toward Spindle
Pole
Pac Man Mechanism MT Depolymerize Dynein
Walks Toward Minus End
(-)
Taxol Inhibits
Grows to 2X the Metaphase Length
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Movement Involved in Daughter Chromosome
Separation During Anaphase B
Polar MT Assembly Occurs at Plus End Plus End
Directed Motor Proteins of Antiparallel Polar MT
Slide Filaments 2.Astral MT Motor
Proteins- Cytoplasmic Dynein Attach to Cell
Cortex Moving toward Centrosome -Exert Outward
pull on the spindle- help separate spindle poles

1.
2.
Taxol Inhibits
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Cytokinesis Divides the Cytoplasm

• Usually starts in late Anaphase and continues
  through Telophase (once sister chromatids are
  separated)
• Occurs by a Process called Cleavage in Animal
  Cells
• Requires Formation of a Contractile Ring
• Contractile Ring
• A Dynamic Assembly of Actin, Bipolar Myosin II
  Filaments, and many structural and regulatory
  proteins
• Forms Under Surface of Plasma Membrane and
  Contracts to Pinch Two Daughter Cells Apart
• Cytokinesis Cannot Occur Until
• 1) M-CDK is Inactivated- M-cyclin Targeted by APC
  for destruction by proteasome
• 2) Sister Chromatids have migrated to opposite
  poles of the cell Remaining MT form Midbody
• - determines where Contractile Ring Formation
  will occur

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Cleavage Requires The Contractile Ring
Remaining Polar MT form Midbody Determines
where Contractile Ring Formation will occur
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Cleavage Furrow Formation in Animal Cells

• 1)Slight Indentation or Puckering on Cell Surface
• 2) Rapidly Deepens to Form Cleavage Furrow
• 3) Continues Until Opposite Surfaces Make Contact
• and Cell is Split in Two.

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Telophase

• 1)Sister Chromatids have arrived at poles of the
  spindle
• 2) Now M-cyclin Degraded by APC
• Reversal of M-CDK Effects Including
• -Nuclear Envelope Reforms around two groups of
  daughter chromosomes to form Two Interphase
  Nuclei
• -Chromosomes Decondense
• -Disassembly of Mitotic Spindle

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Nuclear Envelope and Portions of Endomembrane
System Reform During Telophase
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Completion of Mitotic Phase of Cell Cycle