Transcript Chapter 12 - Laurel County Schools

THE CELL CYCLE IS PRECISELY REGULATED

The timing and rate of cell division is crucial to
normal growth, development, and maintenance
of multicellular organisms.
CELL CYCLE
FREQUENCY OF CELL DIVISION

Frequency of cell division varies with cell type
 skin
cells sustain a lot of damage - divide
frequently throughout life
 liver cells retain ability to divide, but keep it in
reserve
 mature nerve & muscle cells are highly specialized
- do not divide at all after maturity
HOW DO CELLS KNOW WHEN TO DIVIDE?

cell communication = signals
chemical signals in cytoplasm give cue
 signals usually mean proteins



activators
inhibitors
experimental evidence: Can
you explain this?http://bio.kimunity.com
HOW DO CELLS KNOW WHEN TO DIVIDE?
Evidence of cytoplasmic signals
 Fuse M phase with G1 cell – cell immediately
condenses DNA and builds spindle
 Fuse S phase with G1 cell – cell enters S phase
CHECKPOINT CONTROL SYSTEM
 Checkpoints
 Control
points where stop
and go signals regulate
the cycle
 Check if key cellular
processes have been
completed correctly
 Signals
may be internal or
external
 Generally STOP that must
be over ridden by GO
CHECKPOINT CONTROL SYSTEM
3
major checkpoints:
 G1
can
DNA synthesis begin?
 G2
has
M
DNA synthesis been completed correctly?
phase
can
sister chromatids separate correctly?
G1 CHECKPOINT

G1 checkpoint is most
critical
 restriction
point
 if cells receives go-ahead
signal, completes cell cycle
& divides
 if does not receive go-ahead
signal, cell exits cycle &
switches to a non-dividing
state called G0 phase
 Most cells are in G0 phase
HOW DOES THE CELL “TELL TIME”?
Rhythmic fluctuations of key molecules
function as the cell cycle “clock”
 2 main types: kinases and cyclins

 Kinases
- enzymes that activate or deactivate
other proteins by phosphorylating them
 Cyclins are proteins that combine with kinases
HOW DOES THE CELL “TELL TIME”?



cyclin and Cdks combine to form MPF, M-phase
promoting factor; MPF phosphorylates a variety of
proteins and initiates many events of mitosis
such as destruction of the nuclear envelope and
condensation of chromosomes
Cdk levels are constant throughout the cell cycle
but cyclin synthesis rises during S and G2 phases
MPF activity peaks at metaphase – MPF activates
a molecule that destroys cyclin
HOW DOES THE CELL “TELL TIME”?

cyclin and Cdks combine to form MPF,




M-phase promoting factor
MPF phosphorylates a variety of proteins which
initiates many events of mitosis , ex. destruction of
nuclear envelope and condensation of chromosomes
Cdk levels are constant throughout the cell cycle
but cyclin synthesis rises during S and G2 phases
MPF activity peaks at metaphase – MPF activates
a molecule that destroys cyclin
CDK ALWAYS PRESENT – RISE IN CYCLIN MEANS
RISE IN MPF – TRIGGERS MITOSIS
DESTRUCTION OF CYCLIN ENDS MITOSIS
“GO-AHEAD” SIGNALS
 Signals
that promote cell growth &
division
 proteins
 internal
signals
“promoting
 external
factors”
signals
“growth
factors”
PROTEIN SIGNALS
 Promoting
factors
 Cyclins
 proteins
whose concentrations fluctuate in the cell
 CDKs
 cyclin-dependent
kinases
 MPF
 maturation
(mitosis) promoting factor
 APC
 anaphase
promoting complex
PROTEIN SIGNALS
 Growth
factors
 external
signals
 protein signals released by body cells that
stimulate other cells to divide
density-dependent
 crowded
anchorage
 to
inhibition
cells stop dividing
dependence
divide cells must be attached to a substrate
GROWTH FACTORS

EXAMPLE: Platelet derived growth factor (PDGF)


made by platelets (blood cells)
binding of PDGF to cell receptors stimulates division and
wound begins to heal
CANCER CELLS

Cancer cells have escaped cell cycle controls


cancer cells are free of both density-dependent
inhibition & anchorage dependence
Noncancerous cells grown in culture stop dividing
when they touch each other and the sides of the
container
CANCER OVERRIDES CHECKPOINTS

Cancer cells divide excessively & invade other
tissues
 free
of body’s control mechanisms
 breakdown in cell cycle control system
 breakdown in signaling pathway
 cancer cells manufacture their own growth factors
 stimulate
cell division
 stimulate blood vessel growth
CANCER CELLS DIVIDE INDEFINITELY

Cancer cells divide indefinitely if have continual
supply of nutrients
 nearly
all normal mammalian cells divide 20-50
times under culture conditions before they stop,
age & die
 cancer cells may be “immortal”
 HeLa
cells from a tumor removed from a woman
(Henrietta Lacks) in 1951 are still reproducing in culture
CANCER

The abnormal behavior of cancer cells begins
when a single cell in a tissue undergoes a
transformation that converts it from a normal
cell to a cancer cell
 usually
immune system recognizes & destroys
transformed cells
 cells that evade destruction proliferate to form a
tumor, a mass of abnormal cells
BENIGN TUMORS

Benign tumor
 abnormal
cells remain at originating site as a lump
 most do not cause serious problems and can be
removed by surgery
MALIGNANT TUMORS

Malignant tumors
 cells
leave the original site
 impair the functions of one or more organs
 chromosomal & metabolic abnormalities
 lose attachment to nearby cells & are carried by the
blood & lymph system to other tissues
 start
more tumors = metastasis
TREATMENT FOR CANCERS
 Treatments
target rapidly dividing
cells
high-energy
radiation &
chemotherapy with toxic drugs
WHAT CAN CAUSES THE GENETIC
CHANGES THAT LEAD TO LOSS OF
CONTROL OF THE CELL CYCLE
 CHEMICALS
 HIGH
ENERGY RADIATION
 VIRUSES
 INHERITED DEFECTS