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Announcements
• Monday, April 16: the cell cycle, pp.
586-598.
• Wednesday, April 18: protein synthesis,
pp. 684-695.
• Friday, April 20: protein targeting, pp.
695-711.
– Homework problems 19-10, 14 due.
Day 39 Outline/Objectives
Regulation of the
cell cycle
A. MPF
B. Cyclin
C. Cell cycle
checkpoints
1. G2
2. Spindle
assembly
3. G1
After reading the text, attending
lecture, and reviewing lecture
notes, you should be able to:
• Discuss the evidence for and
components of mitosis-promoting
factor.
• Discuss the three main cell cycle
checkpoints (G1, spindle
assembly, and G2) and how
passage is controlled through the
these checkpoints.
Regulation of the Cell Cycle:
Cell Cycle Checkpoints
E.g. Oocytes
Differentiating cells
Evidence that Cytoplasmic
Signals Control the Cell Cycle
Evidence for a Maturation (or Mitosis)
Promoting Factor (MPF) – Masui et al. 1971
The Discovery of Cyclin
• Tim Hunt (Nobel Prize
co-winner, 2001) and
MBL Physiology
class, Woods Hole,
“Cyclin”
MA, in 1982
• Fertilized sea urchin
All other
eggs in presence of
35S-methionine
proteins
• Ran SDS-PAGE gel
0
and autoradiography:
• Saw the following
results:
Mitosis
1
15 30 45 60
Mitosis
2
75 90 105 120 135
Minutes post-fertilization
Synthesis and Degradation of
Cyclin correlates with MPF
Activity
Cell Division Cycle (cdc) Mutants
in Yeast (late 1980’s)
• Hartwell (Nobel Prize, 2001) et al., working
with budding yeast S. cerevisiae
– Found temperature-sensitive mutants stuck in
some point of cell cycle
• Nurse (Nobel Prize, 2001) et al., working with
fission yeast S. pombe
– Found gene they called cdc2, essential for
passing G2 checkpoint
– cdc2 turned out to be a new protein kinase - cyclin
dependant kinase (Cdk) - with counterparts in all
eukaryotic cells.
G2 Checkpoint Control by
MPF
• Active MPF = Mitotic Cdk + mitotic cyclin
• Cdk is cyclin-dependant kinase
• MPF controls G2  M by phosphorylating
and activating proteins involving in:
– Chromosome condensation
– Nuclear envelope breakdown
– Spindle assembly
– It’s own self-destruction
G2
checkpoint
Spindle Assembly Checkpoint
Controls Metaphase  Anaphase
•
MPF (+) anaphase promoting
complex, which destroys:
1.
2.
•
Securin, which allows separin
protease to cleave cohesin.
Mitotic cyclin, which causes loss
of MPF activity, leading to
chromosome decondensation
and envelope reformation.
Mad2 signal stops coming from
kinetochore MT’s once attached,
also (+) APC.
(+)
G1
checkpoint
• Controlled by G1
Cdk-cyclin
• G1 cyclin levels
also vary with
the cell cycle
• Many additional
levels of
phosphorylation,
dephosphorylation regulate.
Growth Factor Signaling Through the Ras
Pathway  crossing of G1 checkpoint
Ras*, Raf*
MAPK
cascade
Activation of
nuclear TFs
Activation of
G1 Cdk cyclin
genes: G1  S
Summary: Cell Cycle Control
• A “clock” is running within the cell - of synthesis
and degradation of cyclins - which activate
cyclin-dependant kinases (Cdk’s), which activate
other proteins to cause checkpoint transitions.
• The “clock” is adjusted (cyclins and Cdk’s are
regulated) by other proteins, based on:
– External signals (growth factors)
– Internal signals (correct completion of previous step)