Transcript Document

SC430 Molecular Cell Biology
• Welcome to Unit 8 Seminar with Dr Hall-Pogar
• Tonight we will discuss
– Cell Cycle
– I will be available at AIM:KaplanHallPogar
before and throughout the seminar if you
have any questions or issues.
• We will begin promptly at 8:00pmET
Unit Review
• DB
• No Project!
2
Overview: The Key Roles of
Cell Division
• The ability of organisms to reproduce best
distinguishes living things from non-living matter
• The continuity of life is based upon the reproduction
of cells, or cell division
• Cell division is integral part of cell cycle
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Types of cell division
• Prokaryotes
– Binary fission
• Eukaryotes
– Mitosis:
• Growth, development & repair
• Asexual reproduction (yields genetically identical cells)
• Occurs in somatic (body) cells
– Meiosis:
• Sexual reproduction (yields genetically different cells with
half the # of chromosomes)
• Occurs in specific reproductive cells
• Yields gametes (e.g., eggs & sperm) or spores
Cell Cycle
The Cell’s Time Clock
• Smallest unit of life
– all living things must
reproduce
• Cells replicate for
growth,
replacement, and
repair
• Cell division
requires Mitosis &
Cytokinesis
Cytokinesis
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Phases of the Cell Cycle
• The cell cycle consists of
– Mitotic (M) phase (mitosis and cytokinesis)
– Interphase (cell growth and copying of
chromosomes in preparation for cell division)
• Interphase (about 90% of the cell cycle) can
be divided into subphases:
– G1 phase (“first gap”)
– S phase (“synthesis”)
– G2 phase (“second gap”)
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INTERPHASE
G1
S
(DNA synthesis)
G2
The Cell Cycle Control
System
• The sequential events of the cell cycle are
directed by a distinct cell cycle control
system, which is similar to a clock
• The clock has specific checkpoints where the
cell cycle stops until a go-ahead signal is
received
• For many cells, the G1 checkpoint seems to
be the most important one
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G1 checkpoint
Control
system
G1
M
M checkpoint
G2 checkpoint
G2
S
G0
G1 checkpoint
G1
If a cell receives a go-ahead
signal at the G1 checkpoint,
the cell continues on in the
cell cycle.
G1
If a cell does not receive a
go-ahead signal at the G1
checkpoint, the cell exits the
cell cycle and goes into G0, a
nondividing state.
Cyclins & Cylin-Dependent
Proteins
• Cyclin is a
cytoplasmic protein
that fluctuates in
concentration
• Cyclin-Dependent
Proteins: activate
other proteins in the
cell by
phosphorylation
(protein kinases)
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What does MPF do?
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Cdk
Degraded
cyclin
G2
Cdk
checkpoint
Cyclin is
degraded
MPF
Cyclin
Molecular mechanisms that help regulate the cell cycle
Stop and Go Signs: Internal
and External Signals at the
Checkpoints
• An example of an internal signal is that
kinetochores not attached to spindle
microtubules send a molecular signal that
delays anaphase
• Some external signals are growth factors,
proteins released by certain cells that
stimulate other cells to divide
• For example, platelet-derived growth factor
(PDGF) stimulates the division of human
fibroblast cells in culture
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• Another example of external signals is
density-dependent inhibition, in which
crowded cells stop dividing
• Most animal cells also exhibit
anchorage dependence, in which they
must be attached to a substratum in
order to divide
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• Cancer cells exhibit neither densitydependent inhibition nor anchorage
dependence
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Cells anchor to dish surface and
divide (anchorage dependence).
When cells have formed a complete
single layer, they stop dividing
(density-dependent inhibition).
If some cells are scraped away, the
remaining cells divide to fill the gap and
then stop (density-dependent inhibition).
Normal mammalian cells
25 µm
Cancer cells do not exhibit
anchorage dependence
or density-dependent inhibition.
25 µm
Cancer cells