Transcript cell cycle control system

Cell Growth and Reproduction
Cell Cycle
Growth in Organisms
For an organism to grow, its cells divide instead
of getting larger.
Cell Size Limitations
• 1) Diffusion:
• If organelles are too far from the cell membrane,
they would have to wait a long time to get
molecules that are diffusing through (ex.
lysosomes, mitochondria).
Cell Size Limitations
• DNA:
• The nucleus cannot provide DNA fast enough to
act as the blue print for the high amounts of
proteins and enzymes that a large cell would
need. (ex. cytoskeleton).
Cell Size Limitations
• Surface area-to-volume ratio:
• The volume of the cell increases a lot faster than
the surface area. Therefore if the cell gets too
big, the cell membrane will not have enough
surface area to diffuse the amount of nutrients
the cell needs or get rid of the waste the cell
produces
4mm
2mm
4mm
1mm
1mm
2mm
4mm
1mm
2mm
Surface area =6mm2 Surface area= 24mm2
Volume= 1mm3
Volume = 8mm3
Surface area= 96mm2
Volume= 64mm3
Cell Cycle
• Cell cycle – stages that the cell goes through from growth to
repair to division.
Interphase
• Interphase- period where the cell is preparing to divide
through growth and maintenance. (90% of a cells life).
– G1 phase (“first gap”) growth
– S phase (“synthesis”) DNA duplication
– G2 phase (“second gap”) Maintenance
Synthesis
• Synthesis – portion of interphase where cells
duplicate their DNA.
• DNA is packaged into sets of chromosomes.
• Humans have 46 chromosomes (23 from each
parent)
Synthesis cont’d
• Somatic cells normal body cells, have two sets
of chromosomes (diploid 2n)
• Gametes reproductive cells (sperm and eggs)
only have one set of chromosomes (haploid n)
Synthesis cont’d
• In synthesis each chromosome gets copied and is
attached to its copy.
• Sister chromatids –
identical copies of
a chromosome
• Centromere – holds
sister chromatids
together.
- At this point their
are double the number
of chromosomes
Mitosis
• Mitosis - Cell division where 1 diploid parent cell makes 2
identical diploid daughter cells. **
• Technically mitosis id the division of one nucleus into two.
chromosomes
Parent cell
Sister chromatids
identical daughter cells
Prophase
• the chromosomes coil up and become visible while the
nuclear envelope disappears. Sister chromatids are present.
• A centriole forms at each pole and spindle fibers made of
microtubules grow out.
• Short microtubules stick out from the centriole in a star
shape known as an aster.
Metaphase
• Early in metaphase (prometaphase) spindle fibers
attach to each of the chromatids at a sight called the
kinetochore.
• The spindle fibers line the chromatids up at the
equator of the cell called the metaphase plate.
Kinetochore
Fig. 12-7
Aster
Centrosome
Sister
chromatids
Microtubules
Chromosomes
Metaphase
plate
Kinetochores
Centrosome
1 µm
Overlapping
nonkinetochore
microtubules
Kinetochore
microtubules
0.5 µm
Anaphase
• Anaphase- During this phase, the kinetochores “reel
in” or “gobble up” the spindle fibers to pull the sister
chromatids apart by splitting their centromere.
• The split chromatids are pulled towards opposite
poles of the cell
Telophase
• Once the chromatids have reached the opposite sides
the spindle fibers disappear, the chromosomes
unravel, and the nuclear envelope reappears on the
two new nuclei.
• In cytokinesis, the cytoplasm then forms a cleavage
furrow at the equator to split the cytoplasm. (in plants
a cell plate forms at the equator.)**
Two identical daughter cells are formed
Mitosis Vid
Mitosis
**
Fig. 12-UN2
Binary Fission
• Prokaryotes (bacteria and archaea) reproduce by a
type of cell division called binary fission. (believed that
mitosis may have evolved from binary fission)
• In binary fission, the chromosome replicates
(beginning at the origin of replication), and the two
daughter chromosomes actively move apart
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Fig. 12-11-4
Cell wall
Origin of
replication
E. coli cell
Two copies
of origin
Origin
Plasma
membrane
Bacterial
chromosome
Origin
Cytoplasmic Signals
• The cell cycle is controlled by chemical signals
in the cytoplasm
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Fig. 12-13
EXPERIMENT
Experiment 1
S
G1
Experiment 2
M
G1
RESULTS
S
S
When a cell in the
S phase was fused
with a cell in G1, the G1
nucleus immediately
entered the S
phase—DNA was
synthesized.
M
M
When a cell in the
M phase was fused with
a cell in G1, the G1
nucleus immediately
began mitosis—a
spindle formed and
chromatin condensed,
even though the
chromosome had not
been duplicated.
The Cell Cycle Control System
• cell cycle control system- a set of events that
occurs in a sequence to regulate the cell cycle.
• The clock has specific checkpoints where the
cell cycle stops until a go-ahead signal is
received
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The Cell Cycle Control System
• the G1 checkpoint is the most important
• Most cells that make it past G1 go on to divide.
• If the cell does not receive the go-ahead signal, it will
go into a non-dividing state called the G0 phase. (ex.
Kinetochores didn’t attach correctly)
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•
Cyclin
and
Cdks
cyclin – a regulatory protein that builds up in the G
2
phase.
• Cyclin dependent kinases (Cdks) – a regulatory
protein that is always present waiting for the arrival of
cyclin.
• MPF (maturation-promoting factor) - cyclin and Cdks
combined to trigger a passage through G2 and Mitosis.
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External Signals
• growth factors - proteins released by certain cells that
stimulate other cells to divide
• platelet-derived growth factor (PDGF) stimulates the
division of human fibroblast cells (connective tissue).
This is released when cut from blood platelets so
healing can begin.
External Signals cont’d
• density-dependent inhibition - crowded cells stop
dividing
• anchorage dependence – cells must be attached to
something in order to divide
Cancer Cells
• Cancer cells do not respond normally to the cells cycle
check points or external signals.
• Cancer cells may not need growth factors to grow and
divide:
– They may make their own growth factor
– They may convey a growth factor’s signal without
the presence of the growth factor
– They may have an abnormal cell cycle control
system
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Transformation
• Transformation - A normal cell is converted to a
cancerous cell
• If the cancer stays at the original site, the lump is
called a benign tumor
• Malignant tumors spread to surrounding tissues
through metastasis (transport by blood vessels.)
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You should now be able to:
1. Describe the organization eukaryotic genome.
2. List the phases of the cell cycle; describe the
sequence of events during each phase
3. List the phases of mitosis and describe the
events characteristic of each phase
4. Draw the phases of mitosis and describe the
structures and events in each phase.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
5. Compare and contrast cytokinesis in
animals and plants
6. Describe the process of binary fission in
bacteria.
7. Describe cytoplasmic signaling and
checkpoints as well as external signaling of
cells.
8. Explain how the abnormal cell division of
cancerous cells escapes normal cell cycle
controls
9. Distinguish between benign and malignant
tumors.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings