Ch 12 The Cell Cycle
What is the cell cycle in eukaryotes?
Just as organisms have a life cycle
(birth-->maturation-->reproduction->-death)
Each cell has an ordered sequence of activities.
Draw cell cycle on board.
Most cells spend
90 % of life span in interphase: growth & synthesis
Remaining ~10% in M phase: mitosis and cell division (aka
cytokinesis)
LE 12-5
INTERPHASE
G1
S
(DNA synthesis)
G2
Summary of Cell Cycle Phases
– Mitotic (M) phase: mitosis and cytokinesis
– Interphase
• G1 phase (“1st gap”):mRNA & protein synthesis
• S phase: DNA synthesis aka DNA replication,
DNA doubling
• G2 phase (“2nd gap”):mRNA & protein synthesis
Regulation of cell cycle
Two families of proteins control when the cell moves to
different phases
1. Cyclins: regulatory subunits
2. Cyclin-dependent kinases (Cdks): catalytic
subunits What is a kinase?
One subunit of each complex together in a
heterodimer
Example: Cdk1/ cyclin B aka M-phase promoting factor (MPF)
Cdk: cyclin-dependent kinase
LE 12-16b
Cdk
Degraded
cyclin
G2
Cdk
checkpoint
Cyclin is
degraded
MPF
Cyclin
Molecular mechanisms that help regulate the cell cycle
LE 12-16a
M
G1
S
G2
M
G1
S
G2
M
MPF activity
Cyclin
Time
Fluctuation of MPF activity and cyclin concentration
during the cell cycle
Based on the data what correlates with the cell
Entering M- phase?
Exiting M-phase and entering interphase?
Cyclin is synthesized and complexed to Cdk1 to form
active MPF --> M phase induced.
Exit from M-phase correlated to cyclin degradation and
MPF inhibition.
Focus on M phase and cell division
Purpose of cell division
• For unicellular organisms
- Division
of one cell reproduces the entire organism
• For multicellular organisms
- Embryonic and fetal growth
- Growth
- Repair
LE 12-2
100 µm
Reproduction
200 µm
Growth and development
20 µm
Tissue renewal
Focus on M-phase
Mitosis
Cytokinesis
• In preparation
• During S phase
cell duplicates genetic material (DNA)
M-phase
• One DNA copy each moves to opposite ends of the cell
• Mother cell undergoes cytokinesis --> two genetically
identical daughter cells.
Cellular Organization of the Genetic Material
Genome
– A cell’s endowment of DNA
All its genetic information
Chromosomes
DNA packaged with associated proteins
LE 12-3
Staining of all the chromosomes or the genome in a cell
Orange: DNA
Yellow/green:MT
25 µm
LE 12-4
0.5 µm
Single chromosome
Chromosome
duplication
(DNA synthesis)
Centromere
Sister
chromatids
Separation
of sister
chromatids
Centromeres
Sister chromatids
• All eukaryotic species
– Characteristic number of chromosomes located
in nucleus
• Somatic cells
- Nonreproductive e.g. muscle, epidermal cells
- Two sets of chromosomes (diploid)
- Undergo mitosis
• Germ cells or gametes
– Reproductive e.g. sperm and eggs
–Half as many chromosomes as somatic cells
(haploid)
–Undergo meiosis
• Mitosis is conventionally divided into five
phases:
–
–
–
–
–
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
• Cytokinesis is well underway by late telophase
[Animations and videos listed on slide following figure]
LE 12-6ca
Blue: Chromosomes
Yellow/green: MT
G2 OF INTERPHASE
PROPHASE
PROMETAPHASE
LE 12-6da
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Video: Animal Mitosis
Video: Sea Urchin (time lapse)
Animation: Mitosis (All Phases)
Animation: Mitosis Overview
Animation: Late Interphase
Animation: Prophase
Animation: Prometaphase
Animation: Metaphase
Animation: Anaphase
Animation: Telophase
The Mitotic Spindle: A Closer Look
• Consists of MT
– control chromosome movement during mitosis
• Assembly of spindle MT
– at centrosome, the microtubule organizing
center (MTOC)
• Centrosome replicates (late G2)
– migrate to opposite ends poles, as spindle
microtubules grow out from them
• Aster (a radial array of short microtubules)
extends from each centrosome
• Spindle
• includes the centrosomes, spindle MT, asters
• Some MT
• attach to the kinetochores of chromosomes and
move chromosomes to metaphase plate
LE 12-7
Aster
Microtubules
Sister
chromatids
Chromosomes
Centrosome
Metaphase
plate
Kinetochores
Overlapping
nonkinetochore
microtubules
Centrosome
1 µm
Kinetochore
microtubules
0.5 µm
LE 12-8b
Movement of xhromosomes toward poles
Chromosome
movement
Microtubule
Motor
protein
Kinetochore
Tubulin
subunits
Chromosome
MT shortens--> drags chromosome toward pole
• In anaphase
– sister chromatids separate
– move along the kinetochore microtubules toward
opposite ends of the cell
• MTs shorten by depolymerization of their
kinetochore ends
• Nonkinetochore MT
– overlap and push against each other from
opposite poles
– Cause cellular elongation
–In telophase
– nuclear envelopes reform around each
chromosome set copy
–genetically identical daughter nuclei form at
opposite ends of the cell
–Chromosomes decondense
–MT depolymerize
Cytokinesis: A Closer Look
• In animal cells
– cleavage furrow forms by action of contractile ring
(actinomyosin)
• In plant cells
– cell plate forms by fusion and elongation of
vesicles between daughter cells
Animation: Cytokinesis
LE 12-9a
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Daughter cells
Cleavage of an animal cell (SEM)
LE 12-9b
Vesicles
forming
cell plate
Wall of
parent cell
Cell plate
1 µm
New cell wall
Daughter cells
Cell plate formation in a plant cell (TEM)
LE 12-10
Description of each mitotic phase
Nucleus
Nucleolus
Chromatin
condensing
Prophase. The
chromatin is condensing.
The nucleolus is
beginning to disappear.
Although not yet visible
in the micrograph, the
mitotic spindle is starting
to form.
Chromosomes
Prometaphase. We
now see discrete
chromosomes; each
consists of two identical
sister chromatids. Later
in prometaphase, the
nuclear envelope will
fragment.
Cell plate
Metaphase. The spindle is
complete, and the
chromosomes, attached
to microtubules at their
kinetochores, are all at
the metaphase plate.
Anaphase. The
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends of
the cell as their
kinetochore microtubules shorten.
10 µm
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will divide
the cytoplasm in two, is
growing toward the
perimeter of the parent
cell.
Prokaryotic Cell Division
• Binary fission
– Usually one circular chromosome attached to
plasma membrane via origin of replication (ori)
– chromosome replicates
– Daughter chromosomes move apart as cell
elongates
– Cell division
– Mechanism distinct from mitosis.
LE 12-11_1
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Two copies
of origin
Bacterial
chromosome
LE 12-11_2
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Replication continues.
One copy of the origin
is now at each end of
the cell.
Bacterial
chromosome
Two copies
of origin
Origin
Origin
LE 12-11_3
Cell wall
Origin of
replication
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Replication continues.
One copy of the origin
is now at each end of
the cell.
Replication finishes.
The plasma membrane
grows inward, and
new cell wall is
deposited.
Two daughter
cells result.
Plasma
membrane
Bacterial
chromosome
Two copies
of origin
Origin
Origin
My genome hurts!
It’s time to ask some
questions.
Regulation of Eukaryotic Cell Cycle
Checkpoint controls
LE 12-14
G1 checkpoint (pauses if DNA is damaged)
Control
system
G1
M
(aka spindle checkpoint)
M checkpoint
(pauses if any kinetochores
are not attached to MT)
S
G2
(pauses if DNA is damaged or
G2 checkpoint incompletely replicated)
• For many cells
• G1 checkpoint most important one
• Go-ahead signal (no damaged DNA) at the G1
checkpoint -->
• cell completes the S, G2, and M phases and divides
• e.g. skin cells, cells of gut lining and mouth
•No go-ahead signal
•Cells exits G1 and enters quiescent state of G0
•No cell division
•e.g. nerve cells
LE 12-15
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.
Internal and External Signals at the Checkpoints
Examples:
• Internal
– Attachment of spindle to kinetochore (spindle checkpoint)
– What happens if no attachment occurs?
Arrest in metaphase; cell delays anaphase
• External
–Growth factors secreted by neighboring cells
–e.g. platelet-derived growth factor (PDGF) stimulates the
division of human fibroblast cells in culture
LE 12-17
Scalpels
Dissociate tissue into cells
Petri
plate
Without PDGF
Day 0
With PDGF
Without PDGF
Day 4
What did PDGF induce?
With PDGF
10 mm
• Other examples of external signals
– Density-dependent inhibition (or contact inhibition):
-crowded cells stop dividing
–Anchorage dependence:
-cells must be attached to a substratum in order to
divide
LE 12-18a
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).
Note: each cell is anchored
to plastic substratum.
Normal mammalian cells
25 µm
• Cancer cells are NOT:
– Density or anchorage dependent
LE 12-18b
Cancer cells do not exhibit
anchorage dependence
or density-dependent inhibition.
25 µm
Cancer cells
Loss of Cell Cycle Controls in
Cancer Cells
• Do not respond normally to the body’s
control mechanisms
• Form tumors, masses of abnormal cells
within otherwise normal tissue
- Benign if stay at original site
- Malignant if migrate into other parts of body
(metastisis)
LE 12-19
Lymph
vessel
Tumor
Blood
vessel
Glandular
tissue
Cancer cell
A tumor grows from a
single cancer cell.
Cancer cells invade
neighboring tissue.
Cancer cells spread
through lymph and
blood vessels to
other parts of the
body.
Metastatic
tumor
A small percentage
of cancer cells may
survive and establish
a new tumor in another
part of the body.
What triggers cancerous growth?
Chemicals e.g. in smoke and soot, ionizing radiation, certain
RNA- and DNA containing viruses e.g. HIV, hepatitis B,
papilloma virus; normal metabolic damage
Common property:
Mutagenic: induce changes in the genome
that cause:
- over-expression of oncogenes, which encode
proteins that push cell cycle forward
- under-expression of tumor-suppressor genes,
which encode proteins that normally slow cell cycle down
Tumor Treatments:
-Chemotherapy
-Radiation
-Surgery
-Inhibitors of oncogenes
-Gene therapy
-Immunotherapy
-Anti-angiogenesis therapy (block blood supply
to tumor)
How do these treatments stop cancer? Can they have
negative side-effects,too?