Lecture #12 – 10/1 – Dr. Mike Wormington
Chromosomes, Cell Cycle, & Cell Division
The BIG Picture
• Regulation
•
• Lead – Proliferate
• Follow – Quiescent aka Stationary
Get out of the Way – Apoptosis aka Programmed Cell Death
• Fidelity/Checkpoints
•
•
Replicate Entire Genome 1X and only 1X
Segregate Entire Genome to Progeny Cells
• Mechanics
•
•
• Mitosis – Maintain Ploidy (2n or n) & Genetic Constancy
Meiosis – Reduce Ploidy (2n to n) & Enhance Genetic Diversity
Aberrant Cell Cycle Control is the Molecular Basis for Cancer
The Cell Cycle
Is it really a cycle?
Cycle (Webster's 9th New Collegiate Dictionary)
1.
An interval of time during which a sequence of a recurring
succession of events or phenomena is completed.
2.
A course or series of events or operations that recur
regularly and usually lead back to the starting point.
Best Description of the Cell Cycle
"You can check out any time you like but you can never leave"
The Eagles (1976) - Hotel California
Bacterial Cell Division = Simple Fission
Nutrient pool triggers
DNA replication
Replication of circular
chromosome initiates
at single origin = ori
Proceeds bidirectionally
to termination site = ter
Cytokinesis triggered by
doubling of cell mass specifically # ribosomes
Process is rapid
optimal doubling time
20'-30'
DNA compacted
>103 to fit in cell
Duplicated oris attach
DNA to membrane ensures
segregation to daughters
Symmetrical Cytokinesis
Fission Yeast
Asymmetrical Cytokinesis
Budding Yeast
This Bud's for you
Bud Lite
Reductive or Cleavage Divisions
During Amphibian Embryogenesis
Cell # Increases but Cell Mass decreases
Cell #
Hrs.
Post-Fert
Cell #
Hrs.
Post-Fert
1
0
32
4,096
~104
3
7
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10
~2X104
~5X104
~105
~106
17
22
24
48
How do 1st 12 Cell Divisions Occur So Rapidly?
G2-M Boundary
Decision Point
Checkpoint
M Phase
~1 hr
G2 Phase
~4-6 hr
G1 Phase
~8-10 hr
Checkpoint
G0 = Nonproliferative
Arrest Phase
Indefinite duration
S Phase
~6 hr
G1-S Boundary
Decision Point
What "Drives" The Cell Cycle?
Early Cell Divisions in Frog Embryos Lack G1 & G2 Phases
Egg Stockpiles Components Required for DNA Synthesis & Mitosis
1 Egg Generates ~10,000 Cells in 10 hrs
Doubling Time ~48X Faster than typical proliferative mammalian cell
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X
X
X
G0
Biochemical
Approach to
Discover
G2-M
Factor using
Frog eggs
Genetic
Approach to
Discover G1-S
Factor using
yeast budding
mutants
X
Frog Maturation Promoting Factor = Yeast Mitosis Promoting Factor
Identified As Cyclin-Dependent Kinases
An Evolutionarily Conserved Family of Cell-Cycle Regulated
Serine/Threonine Protein Kinases
Context of Amino Acids
Flanking Ser, Thr, Tyr Residues
Provide Recognition Sites for
Different Protein Kinases
Thereby Providing Specificity
e.g., SPSQR
(or Inactive)
R=Ser, Thr, Tyr
Generally Active
Throughout Cell
Cycle
e.g., Cdks
Phosphorylation
Of Target Protein
Is Rapidly Removed
If Cdk is Inactive
(or Active)
Cyclin-Dependent Kinases
Cdk = Catalytic Subunit
Cyclin = Regulatory Subunit
Cdk Monomer Inactive
Cdk-Cyclin Heterodimer Active
WHY?
Inactive Cdk Monomer
Active Cdk-Cyclin Heterodimer
Catalytic Site
ATP
Catalytic Site
ATP
T loop
T Loop Blocks Catalytic Site
from Phosphorylating
Target Proteins
Cyclin Subunit Binds T Loop on Cdk
Conformational Change
Allows Catalytic Site to
Phosphorylate Target Proteins
The Molecular "Logic" of the Cell Cycle
1. Cdks generally present throughout cell cycle
but are inactive w/o cyclin subunits.
2. Cyclin subunits synthesized in
discrete cell cycle phases
G1 phase Cyclins D & E
G2 phase Cyclins A & B
3. Cdks phosphorylate
relevant target proteins to alter
their activities at discrete times.
Note that Cdk activity rises
gradually as phase progresses.
4. Cyclins are degraded at end of
each cell cycle phase.
Hence name "cyclin" as levels
"cycle". Degradation necessary to
proceed to next phase of cell cycle.
5. Enables unidirectional progression
through cell cycle.
WHY HAVE MULTIPLE CYCLINS & CDKS?