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The Cell Cycle

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The Cell Cycle Overview of the Control of the Cell Cycle Pages 1053-1067 The Cell Cycle Phases of the Cell Cycle Stages of Mitosis Control Points of the Cell Cycle ... – PowerPoint PPT presentation

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Title: The Cell Cycle


1
The Cell Cycle
Overview of the Control of the Cell Cycle Pages
1053-1067
2
The Cell Cycle
-Cell cycle control system a network of
regulatory proteins -The cell monitors 1.
Events inside the cell 2. Conditions outside
the cell
3
Phases of the Cell Cycle
G1 Cell growth and doubling of organelles S
Duplication of DNA G2 Preparation of
division M Mitosis, Separation of
chromosomes Cytokinesis Division into 2
daughter cells
Interphase G1 S G2
4
Stages of Mitosis
Defined as structure and location of DNA
5
Control Points of the Cell Cycle
-Commitment points in the cell cycle are known as
Start (in yeast) or restriction point (in
mammalian cells) or checkpoints -The cell
monitors internal and external conditions -Main
Checkpoints are before S and M phases
6
Cell Cycle Control in Eucaryotes
-Time to complete the cell cycle can vary greatly
between cell types within the same organism
-All eucaryotes used similar machinery and
control mechanisms in the cell cycle Model
systems used to study the cell cycle 1.
Yeast 2. Animal embryos 3. Cell-free
systems 4. Cell culture
7
Yeast Model System
Advantages include -Reproduce rapidly
-Small genome, 1 of a mammal -Can proliferate
in a haploid state
-Nuclear envelope remains intact -Budding yeast
does not have a normal G2 pahse
8
Temperature-Sensitive Mutants
Cell-division-cell (Cdc) genes genes important
in controling the cell cycle A mutant that
cannot complete the cell cycle cannot grow and
divide
9
Temperature-Sensitive Mutants
Normal Population Cdc15 Ts mutant (high temp)
10
Animal Embryo Model System
Xenopus -Fertilized eggs are large (1mm) -Large
amounts of cell division proteins -Rapid
sequence of cell divisions after
fertilization -1st division takes 90 min -Next
11 divisions takes 30 min each
11
Xenopus Animal Model
During early embryonic cycles the basic processes
are occurring DNA duplication and separation
Very little cell cycle control
Mid-blastula transition
12
Cell-Free Model System
-Cell-free of in vitro system -Advantage
-Manipulate cell-cycle events under
controlled conditions
13
Cell Culture Model System
-Normal cells can be cultured for 25-40 divisions
before undergoing replicative cell senescence
-Immortalized cells can proliferate
indefinitely Advantage -Unlimited supply of
genetically homogeneous cells
14
Studying the Cell Cycle
  • Staining cells
  • -DNA-binding fluorescent dyes
  • -Antibodies to specific molecules
  • 2. BrdU (bromo-deoxyuridine) incorporation
  • 3. Tritiated thymidine incorporation
  • 4. Flow cytometer

15
BrdU Labeling
16
Flow Cytometry
-Measures DNA content -Thousands of cells per
minute
17
Control of the Cell Cycle
-The proteins that control the cell cycle are
different from the proteins that are involved in
the process -Series of checkpoints -Each
checkpoint serves as a biochemical switch
18
Control of the Cell Cycle
The control system 1. Swithes are binary On
or Off and once started the process continues to
completion 2. Reliable due to back-up
systems 3. Adaptable so it can be modified to
different cell types or conditions
19
Key Control Components
1. Cyclin-dependent kinases (Cdk)
-Activities vary throughout the cell cycle
-The targets of phosphorylation then varies 2.
Cyclins -Control the activity of Cdks
-Undergo synthesis and degradation in each
cell cycle Cycling of the cyclins creates the
cycles of the cyclin/Cdk complex allowing for the
varying activities resulting in progression
through the cell cycle
20
Cyclin-Cdk Complexes
G1-cyclin
4 classes of cyclins -All eucaryotes require 3
cyclins (G1/S, S, M)
21
Yeast 1 Cdk Vertebrates 4 Cdks
Cyclin activates the Cdk and helps direct it to
the specific target
22
Cdk Activation
Cdk fullly active 1. Cyclin bound 2.
Phosphorylated at the active site
23
Regulation of Cdk Activity
-Cdk activity can be fine tuned through 2
phosphorylations at the top of the active
site -Important in control of M-Cdk activity
24
Inhibition of Cyclin-Cdk Complexes
-Binding of inhibitors alters the structure of
the active site -Cdk inhibitors are utilized
more early in the cell cycle
25
Cyclin-Cdk Complexes
G1-cyclin
APC/C Anaphase-Promoting Complex, or
Cyclosome -Regulated by protein destruction
26
Marking Proteins by Ubiquitin
27
Ubiquitination Process I
28
Ubiquitination Process II
There are about 300 different E2-E3 each
recognizing a different degradation signal.
Therefore subsets of proteins can be regulated as
a group.
29
Control of Proteolysis of APC/C
-Degrading S and M-cyclins stops the Cdk activity
and the Cdks targets become dephosphorylated and
inactive
or Cdh1
or S-cyclin
-APC/C is active in G1 keeping Cdks inactive
30
Control of Proteolysis by SCF
-Degrades Cdk Inhibitors of S-Cdks and allows S
phase to occur -F-box protein is constant
through the cell cycle and is used to recognize
the target
31
Activation of a Ubiquitin Ligase
32
Activation of a Degradation Signal
33
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34
Cell-Cycle Control Overview
10 of yeast genes encode mRNAs which oscillate
in the cell cycle
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