Cell Cycle Flashcards
Purpose of the cell cycle?
Enables cells to grow, replicate their DNA and proteins and then divide into two genetically identical daughter cells
How is the yeast cell cycle different from humans?
Since the human genome is much longer than that of yeast, the human cell cycle takes way longer
What is the M-phase of the cell cycle?
Mitosis
The process of nuclear division and cytokinesis(cell division)
Can the M-phase be seen under microscope?
Yes
The five stages of mitosis?
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
Cytokinesis
Longest part of the cell cycle and what occurs?
Interphase
Made up of the S-phase which is when DNA is replicated
Cells are continuously growing
Metaphase?
Chromosomes line up in the middle of the cell
-Cells can pause in metaphase if a chromosome is not attached
Anaphase?
Chromosomes are pulled apart by the mitotic spindles
What are the three phases of interphase?
G1: Gap between M-phase and S-phase
G2 : Gap between S-phase and M-phase
S-phase: DNA replication
Purpose of the G1 and G2 phases?
Slow down the growth of an organism
The primitive cell cycle?
Only consists of the S phase and M phase (no gap phases)
When is the primitive cell cycle found?
When we need raapid growth
Ex. Embryos
What occurs during mitosis?
- Chromosomes condense
-nucleus diappears
-Spindle duplicates + attaches to chromosomes
Fission Yeast cell cycle phases
-Consists of G1, G2, S and M phases
What happens to the nuclear envelope in the mitosis of fission yeast?
- The nuclear envelope does not breakdown during the M phase which causes the mitotic spindle to form inside the nucleus.
- The chromosomes are pulled apart within the nucleus via spindles and form a cell plate that splits in two
Budding yeast cell cycle phases
-Consists of G1, S and M phases (lacks G2 phase)
Desribe the spindle formation in budding yeast?
1.Microtubule spindles form inside the nucleus early in the cycle during S phase
2. Chromosomes are then pulled apart and then budding occurs to create two different cells
Fission vs Budding yeast?
Fission:
-condensed chromosomes are visible
-Formation of cell plate to divide the yeast cells
Budding:
-Budding to divide the eyast cells
-No G2 phase
Advantage of using yeast to study the cell cycle?
- Easy to grow (don’t need an incubator)
- Short genome
-Can freeze them
-Haploid state
-Can determine what stage of the life cycle they are in just by looking at their size
What is a temperature sensitive Cdc mutant?
Cell division cycle mutation that is only mutant at restrictive temperatures
Describe yeast with a temperature sensitive Cdc mutant at permissive temperatures, what would the yeast look like?
-The mutant Cdc gene product is functional, as a result the cells undergo the cell cycle properly
-Would expect to see yeast in all four stages of the cell cycle
Larger vs Smaller yeast?
Larger = further into the cell cycle
Smaller = Less far into the cell cycle
Describe yeast with a temperature sensitive Cdc mutant at restrictive temperatures, what would the yeast look like?
-The mutant Cdc gene product will be unfunctional, leading to abnormal progression through the cell cycle
-Progression through the cell cycle stops at a specific point where the mutant protein is needed for further progression
Why are restrictive temperature yeast abnormally large?
-They continue to grow without undergoing mitosis
What is a non-cdc mutant yeast?
Yeast, that lack genes/processes essential for overall biosynthesis and growth
Example of a non-cdc mutant yeast?
Yeast deficient ATP production
How do non-cdc mutant yeast arrest?
Don’t all arrest at one stage, the cells will halt at various stages of the cell cycle depending on their biochemical reserves (ex. How much ATP they had to begin with)
Normal vs Abnormal population of budding yeast
Normal: yeast all in different stages of the cell cycle varying in size
Abnormal: All cells arrestin in the same phase
Frog Oocyte prior to fertilization?
The frog oocyte grows in the ovary of the female frog for several months without dividing(grows to 1mm). The oocyte grows larger but remains a single cell. It is accumulating nutrients and materials it will need once fertilized
Frog Oocyte upon fertilization?
The oocyte enters a phase of rapid cell division, where it divides repeatedly without significant growth in size. First few division occur once every 30 mins
What results from the rapid cell divisions in the frog oocyte?
Production of a multicellular tadpole within a day or two post fertilization
Does the ize of the embryo change as the rapid cell divisions occur?
NO, the cells just get gradually smaller and smaller
When does the growth of the tadpole begin?
When it starts feeding
Why mus the cell cycle of the frog oocyte be rapid?
Since the oocyte is fertilized outside the body, the cell cycle must be fast to ensure that the egg is not eaten by an animal
How to get a cell-free system of the cell cycle from frogs?
- Break open activated frog eggs(centrifugation), the cytoplasm from the egg is then collected.
- In a test tube, add the cytoplasm, ATP and nuclei from the frog sperm
- The sperm nuclei will then decondense their DNA and go through repeated cycles of DNA replication followed by mitosis
- This indicates the cell cycle is functional even outside the cell
What can the cell-free cell cycle of frogs be used for?
To discover different proteins involved in the cell cycle by purifying them out of the test tube
Two reasons why mammalian cells are hard to study ?
- Cannot knockout genes easily due to diploidy
- If you attempt to grow mammalian cells in a cell-free environment, 99% of the cells will not grow since they are in an unfamiliar environment (not surrounded by other cells), get stessed and die
How are embryonic stem cells easier to work with?
-Embyronic stem cells(ex. fibroblasts) are in the growing phase thus they may grow succesfully in a cell-free environment
What type of human cell always grow in a cell-free environment?
Cancer cells(reverted to their embryonic stage just want to grow and can adapt to grow in pretty much anything)
How to use 3H-thymidine and autoradiography to visualize the stage the cell cycle is in?
- Expose tissue of interest to 3H thymidine for a short period of time and then visualize the cells using autoradiography
How to conclude that a cell was actively dividing in S phase using 3H-thymidine and autoradiography?
If the cell was ever in S phase during exposure period of 3H then the cell will contain silver grains over the nucleus indicating that the cell incorporated the 3H-thymidine into its DNA. We can thus conclude that the cell was actively dividing
How to use immunofluoresence and BrdU-labelling to visualize the stage of the cell cycle that a cell is in?
- Expose cells to BrdU for 4 hours and then fix and label them with a fluoresent antibody that binds BrdU
- If the cells were actively dividing during the exposure period on the immunofluoresence the cells will be fluorescently labelled because they will have incorporated BrdU into their DNA
Appearance of mammalian cells in different stages of the cell cycle?
S-phase, G1 phase or G2 phase(jnterphase): mammalian cells appear flat
M-phase: mammalian cells appear round
Mitotic index?
The amount of cells that are in mitosis and are making daughter cells
What does flow cytometer sort?
Sorts cells based on their fluoresence
How does the flow cytometer work?
- Cells are exposed to a dye that becomes fluorescent when it binds DNA
- The amount of fluoresence is directly proportional to the amount of DNA present in each cell
Three categories of the flow cytometer and their amounts of DNA?
- G1 phase: Cells only have one copy of DNA(unreplicated)
- S-phase: Cells have between 1 and 2 copies of DNA
- G2 phase: cells have 2 copies of DNA (replication has occurred)
Where can most cells be found?
In the G1 phase since it is slower that G2 and S phase
T/F: Depending on how you treat the cells(ex. stress them), you can push more cells into various phases?
True
How can flow cytometry be used to detect dying cells?
When cells apoptose the cell is degraded along with its DNA, thus there will be a peak just below the G1 peak(meaning cells have less than one copy of DNA)
Cell cycle control system?
Rotates clockwiseand triggers essential processes when it reaches specific points
Can you go back in the cell cycle?
No
What does the cell cycle control system trigger from phase to phase in the cell cycle?
- From G1 to S-phase: controller triggers DNA replication machninery
- From G2 phase to M phase;controller triggers mitosis machinery
- From M phase to G1 phase: controller triggers anaphase and then cytokinesis
What are checkpoints?
Negative regulators of the cell cycle that stop the cell cycle in response to signals from the environments and information about the completion of cell-cycle events
-Arrests the cell at specific places
Are checkpoints essential for life?
No, but they do decrease the risk of having mutated cells
The three most prominent checkpoints?
- Between G1 and S phase
- Between G2 and M phase
- Metaphase checkpoint
Checkpoint between G1 and S phase?
This checkpoint ensures that the environment is favourable (is there enough nutrients, material and energy to do DNA replication?)
-If the environment is unfavourable the cell arrests in G1
Checkpoint between G2 and M phase?
-Is all the DNA properly replicated?
-Is the environment favourable?
-If the conditions are not good the cell arrests in G2
Metaphase checkpoint?
-Are all the chromosomes attached to the spindle?
-If not this can result in cells with uneven numbers of chromosomes(Down syndrome)
-Cell would arrest in metaphase
The two key components of the cell-cycle control system?
- Cyclin dependent kinases(Cdk)
- Cyclins
What is a cyclin dependent kinase?
-These phosphorylate target proteins leading to their activation via a conformational change
-ATP dependent
-Inactive until bound by cyclins
When are Cdks inactive/active?
Inactive: during G1 of the cell cycle
Active: during G2, S and M phase
What are cyclins?
-Produced via protein sythesis and bind/activate Cdks and help tirgger different events of the cell cycle
-when not needed they are destroyed via proteolysis which also terminates cdk activity
-Variety of cyclins and their concentrations fluctuate throughout the entire cell-cycle
S-Cdk
- S-cyclin concentration increases during G1
- S-cyclin will bind Cdk to form the S-Cdk complex
- S-Cdk triggers the DNA replication machinery via phosphorylation
- After DNA replication, S-cyclin is degraded and Cdk inactivated
M-Cdk
- M-cyclin concentration increases during G2
- M-cyclin binds to a Cdk which results in M-Cdk
- M-Cdk phosphorylates target proteins resulting in the activation of mitosis machinery
- After mitosis, M-cyclin is degraded and Cdk activity terminated
How do the Cdks know which protein to phosphorylate?
- Substrate for the cyclin-Cdk complex are only expressed when it is present (ex. when S-Cdk is present only proteins responsible for DNA replication are present)
- Cyclins create specificity: Cdk alone does not know which substrate to bind but the cyclin is specific to the substrate and wwill only allow the cyclin-Cdk complex to bind specific substrates
T/F: In yeast there is only one type of Cdk?
True, because the variety of cyclins determine the Cdks specificity/target protein
Inactive state of Cdk?
No cyclin is bound to the Cdk but the Cdk is bound by ATP. The active site is blocked by a region of the protein called the T-loop.
Partially active state of Cdk?
Cyclin binds to the Cdk causing the T-loop to move out of the active site
Full active state of Cdk?
Phosphorylation of Cdk at a threonine residue in the T-loop activates the enzyme by changing the shape of the T-loop which improves its ability to bind its substrate
The two enzymes that regulate the cyclin-Cdk complex?
- Wee1 kinase
- Cdc25 phosphatase
Wee1 kinase?
Active cyclin-cdk complex is turned off when the Wee1 kinase phosphorylates two closely spaced sites just above the active site(inhibitory phosphates)
Cdc25 phosphatase?
The inactive cyclin-Cdk complex is turned back on by Cdc25 phosphatase which removes the inhibitory phosphates placed by Wee1
CKI?
Binds the entire cyclin-Cdk complex and inactivates it by distorting the active site of the Cdk and inserting into the ATP binding site
Example of a CKI?
p27
E3 ligases such as SCF
- E3 ligases such as SCF are unregulated and constitutively active in cells.
-Their role is to ubiquitinate targets and send them for degradation
-One of the targets of SCF can be CKIs when they are phosphorylated
How are CKIs regulated?
- When a CKI is phosphorylated it is targeted by SCF which recruits E1 and E2 to help transfer ubiquitins onto the CKI
- The ubiquitylated CKI protein is then recognized and degraded in a proteasome
APC?
-Promotes anaphase in cells
-Usually inactive in the cell
-E3 ligase that can only be activated by an activating subunit(targets M-cyclin)
What happens when cells lack APC?
They are stuck in metaphase of mitosis
Describe the activation of APC?
- During late mitosis, APC is activated by the activating subunit
- APC then targets M-cyclin for degradation by attaching Ubs
T/F: Both APC and SCF contain binding sites that recognize specific amino acid sequences of the target protein?
True
How is regulation of APC different from SCF?
SCF is always present and its activity is determined by the presence of the phosphorylated CKI
APC is always inactive and its activity is determined by an activating subunit
What happens when you fuse cells in the S phase to cells in the G1 phase?
The S phase cells go to S phase and the G1 cell also enters S phase
What happens when you fuse cells in G1 to cells in G2?
Both cells remain in their respective states and enter proceeding phases on their own time
What happens when you fuse S phase cells and G2 phase cells?
S phase nucleus continues DNA replication and G2 phase nucleus remains in its state
What is causing the G1 phase cells to enter S phase when fused S phase cells?
S phase cells have S-Cdk which is driving the G1 cell into DNA replication because the G1 cells have the S substrate
Why does the G2 phase cell not enter S phase when fused with the S phase cell?
G2 cells have just undergone S phase and thus they don’t have the substrates present for S-Cdk to bind
What is the origin recognition complex(ORC)?
-A protein complex that binds specific sites on DNA
-Start of the replication fork
-It acts as a binding site for the replication site to assemble
ORC phosphorylated vs non phosphorylated?
Phosphorylated: DNA replication complex cannot assemble
Non-phosphorylated: DNA replication complex assembles
How many ORCs are present in mammalian cells?
There are thousands to allow for efficient replication of our DNA in 24 hours and not weeks
Is the ORC always bound?
Yes, unless the cell withdraws from the cell cycle and becomes specialized
How does the pre-replicative complex form?
- In the beginning of the G1 phase there is no Cdk activity but then it starts to build up
- The Cdc6 then associates with the ORC and recruits the Mcm ring complexes(DNA helices). This forms the pre-replicative complex which is inactive
How does the pre-replicative complex become active?
- In the beginning of the S phase there is high Cdk activity and S-cyclin comes in an form S-Cdk complexes
- S-Cdk complexes phosphorylate Cdc6 causing it to come off the ORC and get degraded by SCF proteolysis.
- ORC then gets phosphorylated which causes the replication complext to start replicating the DNA in both directions
When does replication stop?
Once another ORC is reached
What happens once replication stops?
- The Mcm rings are exported to the nucleus
- The S-cyclin is degraded
- The M-cyclin concentration increases to prevent the ORC from becoming unphosphorylated and rereplicating
When can Cdc6 and the Mcm rings return to the ORC for replication?
Once mitosis is over and the M-Cdk has been inactivated
How is M-Cdk activated?
- Cdk1 associates with M-cyclin as the M-cyclin levels gradually rise at the end of the G2 phase forming an inactive M-Cdk complex
- The M-Cdk complex is then phosphorylated on an activating site by CAK and on a pair of inhibitory sites by Wee1 kinase. Inactive M-Cdk
- The complex is activated at the end of G2 by the phosphatase Cdc25 which removes the inhibitory phosphate
How is Cdc25 phosphatase activated at the end of the G2 phase?
It is stimulated by Polo kinase to its active form which is phosphorylated
How do active M-Cdk complexes result in more active M-Cdk ?
Active M-Cdk further stimulates the activation of Cdc25 and inhibits the activity of Wee1 kinase
How do the cyclins concentrations change vs the cdks?
Cyclins: three major types of cyclins oscillate throughout the cell cycle
Cdks: do not change and never exceed the amount of cyclin
What are the three cyclin-Cdk complexes and when do they form?
- G1/S-cyclin levels rise at the end of G1 and form the G1/S-Cdk complex which triggers the start transition
- S-Cdk complexes form at the start of the S phase and trigger DNA replication
- M-Cdk complexes form during G2 but held inactive until they are activated at the end of G2 and trigger mitosis at the G2/M transition
What triggers the rise in G1/S-cyclin?
By a signal for cells to duplicate
APC/C complex?
Initiates the metaphase to anaphase transition
How do we know APC is needed to exit metaphase?
Because an APC inhibitory was once added to eggs undergoing mitosis. The inhibitor cause the cell to arrest in metaphase of mitosis this indicates the APC must be needed to degrade something for the separation of sister chromatids
What happens when you add mutant M-cyclin that cannot be degraded to mitotic frog extracts?
This causes the cells to arrest in mitosis just after sister-chromatid separation. This indicates that the destruction of M-cyclin is require for exit from mitosis but not sister chromatid separation
Structure of both cohesins and condensins?
-Two identical DNA and ATP binding domains at one end
-Hinge region at the other end joined to the binding domains by two long coiled-coil regions
Functions of Cohesins ?
Crosslinkers that crosslink two adjacent sister chromatids together
Function of condensing?
Crosslink DNA by binding to it and condensing it into chromosomes
How are the sister chromatids separated by APC?
- APC is activated by Cdc20 which leads to the ub and destruction of securin, which normally holds separase inactive.
- Separase(protease) cleaves cohesin holding the sister chromatids together
- The pulling forces of the mitotic spindle then pull the sister chromatids apart
- Cohesin cleavage is also facilitated by its phosphorylation via Polo kinase adjacent to cleavage sites just before anaphase begins
What drives the sister chromatid separation by APC?
M-Cdk which produces Cdc20 to activate APC
Mad2 protein
-Acts as another checkpoint
-Binds to unattached kinetochores and tells the cell not to separate the sister chromatids since they ar enot fully attached
-Sends a negative signal to prevent separation of sister chromatids and blocks production of Cdc20 to prevent activation of APC and separase
What happens to Mad2 once a sister chromatid becomes attached by the kinetochore?
The Mad2 protein disappears
Embryonic cell cycle?
-Only have S phase and M phase
How do embryonic cells regulate their cell cycle?
- In early embryonic cell cycles, Cdc20-APC activity rises at the end of metaphase
2.This triggers M-cyclin destruction by APC - The loss of M-cyclin then leads to APC inactivation after mitosis because it is the thing stimulating Cdc20
- This then allows the M-cyclin levels to rise again for the next round of mitosis
Goal of adult cells?
Want to be in the G1 phase longer so that they have the opportunity to do something else(specialize)
How do cells maintain the G1 phase?
- During mitosis M-cyclin levels are high but then as we reach anaphase Cdc20-APC activity increases which degrades M-cyclin
- Degradation of M-cyclin leads to inactive M-Cdk and less Cdc20-APC activity
- Normally, now the M-cyclin would rise again but in G1 cells it is instead degraded by Hct1-APC keep the level low
Why do cells need G1 phase?
In order to do their functions, they cannot be dividng and carrying out functions within the body
Two molecules that repress M-Cdk in the G1 phase and what they do?
- Hct1-APC: degrade M-cyclin
- Sic1: CKI that keeps cyclin-Cdk inactive, in mammals this is p27
How does the cell exit the G1 phase?
-When conditions are right there is a rise in G1-Cdk and G1/S-Cdk which leads the inhibition of Hct1-APC and Sic1 by phosphorylating them
How is S phase initiated?
- G1-Cdk activity causes the phosphorylation Rb which prevents it from binding E2F.
- E2F then activates transcription of S phase genes including a gene for G1/S-cylin and S-cyclin
- The rise in G1/S-cyclin and S-cyclin results in a positive feedback loop further enhancing Rb phosphorlyation
- E2F als forms a positive feedback loop on itself, stimulating transcription of its own gene
Rb protein?
-tumour suppressor protein
-Determines if cells grow or not
Rb in adult vs embryonic cells?
Adult: turned on/off as needed
Embryonic: Rb is usually off allow for constant cell growth
What happens if Rb is mutated?
E2F is constitutively active which drives S phase events and causes the cell to undergo constant proliferation
Are cell division and growth coupled in cells?
Yes, normally
When cell division and growth are coupled what happnes if the cell is nutrionally deprived?
When cells are nutrionally deprived they grow at a slower rate. This causes the rate of progression through the cell cycle to slow down so that the cells have more time to grow. Cell size remains unchanged and is only slightly reduced when nutrition is reduced. Cell will recieve delayed signals to divide once the cell has reached its desired size.
What happens when cell division and growth are not coupled in a nutritionally deprived cell?
Cell division continues at an unchanged rate. This causes the daughter cells to become gradually smaller and smaller because there is no longer a checkpoint to make sure the cell is at the right size prior to mitosis
-Seen in cancer cells
How are cell division and growth coupled in yeast cells?
The cells contain a fixed number of Cln3-binding proteins that bind DNA and Cln3 molecules(inhibiting them). As the cell grow, the total number of Cln3 molecules increases with the total cell protein. When the cell is small, all the Cln3 bind the binding sites and there is excess Cln3-binding sites. As the cell grows it reaches a threshold where the number of Cln3 molecules matches the number of Cln3 binding sites. Once the number of Cln3 molecules exceeds the number of binding sites then the free/active Cln3 molecules can finally bind Cdk and trigger the next cell cycle because now the cell is at the proper size
How can DNA damage occur?
- X-ray
- UV
-Carcinogens
-Or intracellularly by the production of ross reactive oxygen species which are byproducts of metabolism
How does DNA damage arrest cells in G1?
- When DNA damage occurs the kinase, ATM, phosphorylates p53 which prevents Mdm2 from binding and degrading it.
- p53 then accumulates and stimulates transcription of the gene that encodes CKI protein p21
- p21 binds and inactivates the G1/S-Cdk and S-Cdk complexes resulting in the cell arresting in G1
- In some cases, DNA damage can resulting in the phosphoryation of Mdm2 or decrease Mdm2 production causing an increase in p53
p53?
A transcription factor that targets gene p21(CKI) and activates it
-Activated in cases of DNA damage
-Normally, constantly degraded by Mdm2 (E3 ub ligase)
p21?
-CKI activated by p53 during DNA damage
-Binds G1/S-Cdk and S-Cdk and prevents the cell from entering S phase
Why is p53 constantly made in cells?
It needs to be activated very rapidly when DNA damage occurs
Mutations in p53 or ATM kinase?
-people are at a higher risk of developing cancer because they are likely to accumulate DNA damage
-Drives tumorigenesis
Factors required for all cells?
- Survival factors: regulate apoptosis(if not present cell dies)
- Growth factors: regulate cell growth/size
- Mitogens: regulate DNA replication and cell division
Apoptosis in mouse paw?
Cells between the digits apoptose to eliminate the tissue between developping digits
Apoptosis in Tadpole to Frog?
As a tadpole becomes a frog cells in the tadpole tail are induced to apoptose so that the tail is lost
Two types of cell death?
- Necrosis
- Programmed cell death
Necrosis
-Accidental death
-Cell blows up and releases its contents to the extracellular environment, this can cause harm and might cause other cells to die or replicate
Programmed cell death?
-Results due to intracellular/extracellular signals to die
-Controlled
-Cell will eat itself(protein becomes AA and DNA becomes nucleotides), this is a recycling machine. This cell will then be eaten up by a phagocytic cell and the contents will be used to make another cell.
Mitogens?
Small peptides delivered by the bloodstream that bind to a receptor
How mitogens activate Myc?
- The mitogen will bind to a receptor on the cell surface this leads to activation of Ras and MAP kinase cascade
- One effect of this pathway is the increase in the gene regulatory protein Myc
- Myc then increases transcription ofseveral genes
What does activation of Myc do?
- Increases the transcription of the gene encoding cyclin D which leads to G1/Cdk activation which results in Rb phosphorylation and increased E2F and S phase entry
- Increases transcription of the gene encoding SCF which results in degradation of p27 which also activate G1/Cdk
- Increases transcription of the gene encoding E2F
Net result of Myc activation in cells?
Entry into S phase
What happens when there is excessive Myc production?
-Excessive mitogenic pathway stimulation results in cell cycle arrest by p53 or if stimulated for too long apoptosis
How is p53 activated due to excess mitogenic pathway stimulation?
- Abnormally high levels of Myc cause activation of p19ARF, which binds and inhibits Mdm2 which causes p53 levels to increase
- Depending on cell type and extracelular conditions, p543 then causes either cell-cycle arrest or apoptosis
Telomeres?
Cap each end of the chromsome and protect them
Telomerase + what happens if a cell lacks it ?
Add telomeres to the ends of chromosomes, if you lack this enzyme your chromosomes will get shorter with each division and the cell will then recognize the DNA as damaged and p53 will be activated and induce cell cycle arrest
Immortalization
Cells with telomeres on their ends are immortal and can proliferate constantly
Do human fibroblasts have telomeres?
No their chromosomes shorten with each division and they stop dividing after 50-60 divisions
How do growth factors promote cell growth?
- Growth factor binds to a receptor on the cell surface. The receptor then becomes activated and leads to the initiation of a signalling cascade.
- Downstream of PI-3 kinase is activated then another kinase is activated. These result in the activation(phosphorylation) of eIF4E and S6 kinase
- These then increase mRNA translation and stimulate cell growth
Effect of growth factors on neurons vs lymphocytes?
-Both cell types have the same amount of DNA but a neuron is much larger
This is because the neuronal growth factor withdraws the neuron from the cell cycle permanently and allows the cell to grow without dividing
How do survival factors work in neurons?
- The body produces a ton of nerve cells but only has three target cells
- The three target cell secrete survival factors but only enough for three nerve cells
- All of the other nerve cells that body produced will die
Why are nerve cells over expressed?
To make sure that all of the target cells are contacted by nerve cells then the extra nerve cells will be automatically eliminated
What does binding of the survival factor to the cell surface receptor do?
Leads to the activation of various protein kinases, including protein kinase B (PKB)
Two functions of activated PKB?
- PKB phosphorylates and inactivates the Bcl-2 family member Bad. This allows Bad to dissociate from Bcl-2 and suppresses apoptosis
- PKB phosphorylates proteins of the Forkhead family that stimulate transcription of genes that encode proteins that promote apoptosis when not phosphorylated
Bad protein?
When Bad is not phosphorylated it binds Bcl-2 and inhibits it promoting apoptosis
When Bad is phosphorylated it dissociats from Bcl-2 suppresing apoptosis
Contact inhibition?
When cells start to touch during proliferation they will stop proliferation, they form monolayers
Cancer cells and contact inhibition?
Cancer cells typically lack contact inhibition therefore they will continously proliferate on top of one another
What happens when you place fibroblasts in a fresh media?
The fibroblasts will rpoliferate until they fill out the plate in one layer of cells(monolayer)
What are serums and why are they needed in the media?
Serums: contain survival factors, growth factor and mitogens
Needed so that the cells in the media proliferate and survive
What happens if you pump a fresh layer of media over the monolayer of cells?
The cells will continue to divide, however the cells will become gradually smaller because they must maintain the monolayer
-The proliferation is halted when the medium is depleted of mitogens
How do we get serum?
-Take blood out of an animal and spin down the cells so you have a supernatant that contains high concentration of growth factors, mitogens and survival factors
T/F: If you take serum from a growing organism it will have a lot more factors than an adult?
True
What can limit cell proliferation?
-Cells must have something to adhere to in order to grow/proliferate, in cells they adhere to ECM
What happens when you try to grow cells in agar gel?
Cells have nothing to attach to so very few/none of them will grow/proliferate
What cells don’t need to adhere to anything to grow?
Hematopoietic cells from our blood system which are suspended in liquid and grow/proliferate
What happens when an adhesion cell becomes cancerous?
Often the cell will revert to becoming like a blood cell so that it can invade all types of tissue and be able to grow/proliferate when not attached to anything
