The Cell Cycle
CONSISTING OF INTERPHASE ,MITOTIC PHASE, &
CYTOKINESIS
The Mitosis Puzzle
Lay blank sheet lengthwise .
Write Interphase, Prophase, Metaphase, Anaphase, &
Telophase across the top of the sheet.
Cut out the cell diagrams and tape/glue them across the
top under the appropriate phase label.
Arrange each description beneath the appropriate picture
to describe the cellular changes of that phase.
Why do cells divide?
• Bacteria cells & unicellular eukaryotic
organisms divide & produce an entire
organism.
• Multi-cellular organisms:
– Development
-Growth
-Repair
What do you get at the end
of the cell cycle?
2 genetically identical
daughter cells
Cellular Organization
PROKARYOTIC
EUKARYOTIC
Replication of so much DNA is manageable
because of how DNA is packaged.
chromatin
chromosomes
Does the number of chromosomes in
an organism determine how complex
the organism is?
Organize the following organisms in
order from complex to simple
Bat
Herring gull
Human
Crayfish
Fern
Reptiles
Dog
Parts of a chromosome
THIS CHROMOSOME IS A DUPLICATED CHROMOSOME WITH 4 CHROMOSOMAL ARMS
The Cell Cycle
• Interphase
– Accounts for 90% of the cycle
– Divided up into 3 subunits
The Cell Cycle
• Mitotic phase
– Includes both mitosis and cytokinesis
– Usually the shortest part of the cell cycle.
and Cytokinesis
Mitotic Spindle
• Begins to form during prophase & is complete during
metaphase
Form from the
breakdown of
microtubules of
the cytoskeleton
Not present in
plant cells
Starts here
What is the significance of the fact that chromosomes condense before they are moved?
WHICH OF THESE WOULD YOU RATHER ORGANIZE WITHOUT BREAKING?
CYTOKINESIS
ANIMAL CELLS
PLANT CELLS
BINARY FISSION
PROKARYOTES (BACTERIA & ARCHEA)
A protein is thought to
anchor the DNA to specific
spot on membrane
Evolution of
Mitosis
Timing is everything!
• The frequency of cell division varies with cell
type.
– EX: human skin cells vs liver cells
• Some cells do not divide at all in a mature
human.
– EX: nerve cells and muscle cells
What Drives The Cell Cycle?
Hypothesis:
-Each event in the cycle
triggers the next.
What was concluded?
Molecules present in the
cytoplasm of cells in the S
or M phase control the
progression of phases.
CELL CYCLE CONTROL SYSTEM
THE G1 CHECKPOINT
The Cell Cycle Clock
• Maturation/Mitosis Promoting Factor
(MPF) are regulatory molecules (mainly
proteins)
–Protein kinases and cyclins plus a
phosphate group
Cyclin and Kinases
•Kinases are enzymes
•Activate or inactivate other proteins by
phosphorylating them.
•Give the go ahead signals at the G1 and G2
checkpoints.
•Present in a constant concentration in a growing
cell, but are mostly inactive.
•To be active they must attach to a cyclin (a
protein) = cyclin-dependent kinases, or Cdks
Cyclin D triggers cells to move from G0 to G1 into S phase
Cyclin E prepares the cell for DNA replication in S phase
Cyclin A activates DNA replication inside the nucleus in S phase
Cyclin B promotes the assembly of the mitotic spindle & other
tasks in the cytoplasm to prepare for mitosis
Cyclins and cyclin-dependent kinases
control the cell cycle.
After the MPF
does its job the
cyclin degrades.
Why?
The cell would
continue to divide
even when not ready
In summary
• Internally:
– The fluctuation of cyclin & cyclin-dependent
kinases seems to control the cell cycle internally
using checkpoints to tell the cell to proceed or not
– The MPF complexes (cyclin + Cdk) initiates mitosis
& can then go on to produce a cascade of other
cell responses including phosphorylation of other
proteins which:
• Promotes fragmentation of nuclear envelope
• Chromosome condensation and spindle formation
What about external factors?
Example of a growth
factor is PDGF (plateletderived growth factor)
PDGF is required for the
division of fibroblasts (a
type of connective tissue)
Triggers a transduction pathway
allowing the cells to pass the G1
checkpoint & divide.
An injury can instigate this
growth factor to help heal a
wound.
MORE EXTERNAL
FACTORS ON CELL
DIVISION:
DENSITY-DEPENDENT
INHIBITION (in culture)
In summary
• Externally:
– Growth factors secreted from the endocrine
glands or blood cells are present
– A substrate to attach to is needed
– Density of neighboring cells are a factor
– Receptors (on outside of cell that’s dividing) are
needed to receive each of the cell signals
mentioned above.
When do cells fail to divide?
If essential nutrients are missing.
If growth factors ( protein released by certain cells
to stimulate other cells to divide) are missing.
Note: there are more than 50 growth factors
How does a cell divide “wrong”?
Let’s look at a cell gone wrong in the video Non disjunction or non
segregation in Mitosis
What is the definition of cancer?
• The unregulated cell division of an organism’s
cell
LOSS OF CELL CYCLE CONTROLS
Cancer cells do not follow the normal signals that regulate the cell cycle.
•They don’t stop dividing even when there are no growth factors present.
•Can continue dividing indefinitely in culture with ample nutrients.
•EX: HeLa cells of 1951
MALIGNANT VS BENIGN TUMOR
Have genetic & cellular changes that enable
the cells to spread to new tissues & impair
functions of organs = cancer
Have too few genetic & cellular
changes to survive elsewhere.
What does it mean if the cancer has metastasized?
Cancer most often results from
mutations in genes
• Proto-oncogenes: they often code for proteins
that stimulate cell division, prevent cell
differentiation or regulate programmed cell
death (apoptosis).
• Tumor suppressor genes- produce proteins
that signal cells when they are getting too
crowded.
How does someone “get cancer”?
• It can be triggered by:
– Carcinogens- Substances and environmental exposures that
can lead to cancer
• Teratogens-Any agent that can disturb the development of an
embryo or fetus. Teratogens may cause a birth defect in the child.
Or a teratogen may halt the pregnancy outright. The classes of
teratogens include radiation, maternal infections, chemicals, and
drugs.
– Viruses: ex: HPV (human papilloma virus) causes cervical cancer &
EBV (Epstein Barr virus) is associated with Hodgkin’s lymphoma, and
gastric cancer.
– Aging: ex: breast cancer increases the older you get
How can we kill cancer cells?
• Radiation
• Chemotherapy
– CDKs are considered a potential target for anticancer medication.
– If it is possible to selectively interrupt the cell
cycle regulation in cancer cells by interfering with
CDK action, the cell will die.
– Targets fast dividing cells
Differentiation of Human Cells
A zygote starts development by dividing
over and over until you get a few dozen
identical cells. These cells are
embryonic stem cells.
What are stem cells?
Embryonic Stem Cells
• Cells that start to take different development paths to become
specialized cells, such as blood stem cells, which means they
can no longer produce any other type of cell.
Can give rise to
any and all tissues
in the body
they can differentiate into some, but not all, cell types.
TOTIPOTENT VS PLURIPOTENT VS
MULTIPOTENT
Totipotent cells can form all the cell types in a body, plus the
extra-embryonic, or placental cells.
Embryonic cells within the first couple of cell divisions
after fertilization are the only cells that are totipotent.
Pluripotent cells can give rise to all of the cell types that make up
the body; embryonic stem cells are considered pluripotent.
Multipotent cells can develop into more than one cell type, but
are more limited than pluripotent cells; adult stem cells and cord
blood stem cells are considered multipotent
Stem Cells Video
To generate cultures of
specific types of
differentiated cells—heart
muscle cells, blood cells, or
nerve cells, for example—
scientists try to control the
differentiation of embryonic
stem cells. They change the
chemical composition of the
culture medium, alter the
surface of the culture dish,
or modify the cells by
inserting specific genes.
What side of stem cell research
do you fall?