Cell Cycle
Cell Cycle
• Ordered sequence of events:
cell growth
• DNA replication
• division.
• Regulation of the cell cycle is
critical for the normal
development of multi-cellular
organisms.
Cell Cycle
Gap 1 (G1)- growth
normal cell metabolism
Protein synthesis
Synthesis of enzymes needed for
DNA replication.
S- synthesis- DNA replicatedsister chromatids
Gap 2 (G2)- metabolic activity,
production of microtubules
M- Mitosis- cell division
Go- Cell is quiescent
Some cells stay in “time
out”
Cells will not
differentiate.
Lymphocytes
Most neurons
Mature heart muscle cells
They exit the cell cycle and continue to carry
out their cell functions until they die.
Interphase
Inter = between
Between divisions
Mitosis
• Karyokinesisdivision of the
chromosomes
• Cytokinesis- division
of the cytoplasm.
mito= thread
kine= move
Cell cycle
Is it regulated?
• When do you turn off the
Cell Cycle?
• How does the cell know
when to leave G1  S
All the time- skin, nails, intestinal
lining
Rarely- brain and CNS cells
Only if damaged- liver
Checkpoints
Kinases- control movement
Stop and go signals
3 major checkpoints
G1 Checkpoint
G1 Checkpoint- most important“Green light”, go ahead- will usually complete the
cell cycle
If it does not get the Green light- it will move to
Go.
G2
Checkpoint – verifies integrity of DNA
M Checkpoint
Prior to anaphase
Checks the condition of the mitotic spindle
Factors that Regulate Cell Divison
External Factors:
Physical – contact inhibition
Cell will divide until they
touch another cell.
Chemical- Growth factors –
Usually a proteins or
steroids
(lipid)
Internal Factors:
• Kinases- proteins – control
cell cycle
• Cyclins- proteins that are
made to activate the
kinases.
cyclins and protesases
Cell Size is Limited
• Cells have to be large enough to carryout their activities
• If cells grow too large they will cease to function.
Why?
Surface area to Volume Ratio
Surface area- Lx Wx H x 6 (number of sides)
Volume- LxWxH3
Rate of Cell Division
Usually 12 hr. for human cells - stages S- M
Depends:
1. Cells divide more rapidly in embryos &
children less frequently than adults
2. Cell function- protection (skin)abrasion = replacement
Hayflick Limit
• 1961- Hayflick and Moorhead
Embryonic stem cells will divide approx. 50 times.
A cell that has reached its
Hayflick limit will no
longer divide.
A cell is senescent- when it
no longer divides.
Different cells different rates
All the time- skin, nails, intestinal lining
Rarely- brain and CNS cells
Only when damaged- liver
Telomeres and Aging
Telomers
Telomeres are the tips of the
chromosomes.
Everytime the cell divides it loses
a little bit of the telomere.
How are telomers analogous to
the aglet on your shoe laces?
Telomerase
Telomerase is the enzyme that
builds and maintains the
telomers.
For our cells, the gene that codes
for telomerase is turned off.
Are Cells Immortal?
Cancer cell are Immortal
Cancer results from the loss of control of the cell
cycle.
Mutation(s) in one or more of the proteins
involved in the checkpoint system.
Telomerase- the enzyme that puts the telomers
back on the chromosome is turned on.
Cancer
•
•
•
•
•
Disease of the cell cycle
90% of the cancers are in somatic cells
Cancer will occur in 1: 5 people
Disease of the aging –
Cancer cells divide constantly and invade other
tissues.
Why is cancer a genetic disorder of the
elderly?
Carcinogens
Cancer causing agent (mutagens)
1. Environmental exposure
2. Viruses (15 – 20 %)
3. Inheritance ( 5 -10 %)
Contact Inhibition
Most cells are attached to other
cells, bathed in extracellular
fluid and is nourished by
blood.
When signaled, cells will divide
until they come into contact
with other cells – then they
will stop.
Not Cancer
Tumors
A tumor is a mass of dividing cells.
• Benign- not cancer, not spreading
• Malignant- cancerous mass of dividing cells
• Metastasis - the spread of cancer cells via the
circulatory system.
Cancer Types
Named by the organ/tissue in which they
originate.
• Sarcoma- connective tissue – bones & muscles
Carcinoma- (external/internal coverings )- skin & lining of
intestines
Leukemia & lymphoma- (blood forming tissues)- bone
marrow, spleen, lymph nodes
Chemotherapy
Toxic Drugs that interfere with cell division.
Taxol- freezes the spindle stopping mitosis.
Vinblastin- stops the spindle from forming
Tumor Suppressor Genes
Genes that are involved cell cycle
regulation- they inhibit the growth of
tumors.
1. p21- slows or stops cell division
2. P53- triggers apoptosis when the cell
cannot be repaired.
Mutations to Tumor Suppressor Genes
1. Alcohol
1. tobacco
Nova Cancer Warrior- 54 minutes
Mitosis – Asexual Reproduction
• How many cells are you composed of?
• When an organism grows bigger do you
get more cells or just bigger cells or both?
• When do your cells divide the fastest?
Slowest?
The Process
DNA
Nucleic Acid- Genetic code
Codes for proteins
Prophase
pro= before
Chromatin condenses- visible chromosomes
Chromosomes appear as sister chromatids
joined at the centromere.
Nuclear envelope disappears
Centrioles migrate to the poles and spindle
fiber begin to form.
Metaphase
meta- middle
Chomosomes line up at the equator.
Sister chromatids are attached to the
spindle fibers.
Anaphase
ana- apart
Spindle fibers contract.
Sister chromatids separate at the
centromere.
Sister chromatids are pulled to the
poles.
Telophase
telo- last
The chromosomes reach the poles
The nuclear membrane forms around
the chromosomes
Chromosomes begin to relax chromatin
Cytokinesis
Cytoplasm divides equally
Plant cells builds a cell plate from the inside out
Animal cells – a cleavage furrow forms from the
outside in.
Asexual Reproduction
a= without
sex= the exchange of genetic info
Occurs in somatic cells - body cells
Produces genetically identical offspring- clones
Mitosis
C
L
O
N
E
S
Asexual Reproduction
Prokaryotes- 1 circular DNA
Binary fission
Fungi
Budding
Asexual Reproduction- Plants
Vegetative Propagation
Leaflets
stolons/ runners
leaf/ stem
Animals
Hydra
Parthenogenesis
“virgin birth”
Budding is asexual
Organization of Life
Cells to Organ Systems
Plants- Cells to Organ Systems
Chapter 5.5
Multicellular Life
Stem Cells
Cells that have not yet
become specialized
All cells have the same genome
Stem cells 15 min
Differentiation
The process where cells become specialized
Stem cells and differentiation
The normal process by which a less specialized
cell develops, or matures into a more distinct
form or function.
Totipotent Cells- ability to be any of the
210 cells in the body.
Pluripotent- isolated from the
blastocyst- turns into cells of the 3
germ layers
Three Germ Layers
Multipotent- stem cells
Grow into closely
related family of
cells
Differentiation
The normal process by which a less
specialized cell develops, or matures into
a more distinct form or function.