L2 Flashcards
(22 cards)
What does Cdc28 do in C. cerevisiae?
Interacts with phase specific cyclins
It is a protein kinase
The cyclins help the kinase phosphorylate the right substrates at the right phase
What are Cln and Clb?
Are cyclins that are regulatory subunits that bind to the kinase Cdc28, changing the substrates that the kinase regulates
What options do S. cerevisiae cells have in G1?
Starved cells enter G0 arrest
Haploid cells mate
Diploid cells sporulate
Once its committed to its decision it cannot change it
Has to rule out the other decisions to ensure it only does 1 route properly
Haploid cells in S. cerevisiae
Conditional mutations
Cdc28-ts blocks cell early in G1 phase
Cdc7-ts blocks replication (S phase)
If you block Cdc28 in a haploid cell they could still mate and undergo G0
Blocking Cdc7 stops the ability to mate or enter G0 - stops mating happening while its trying to replicate
Passing Cdc28 is critical to getting into the cell cycle
Diploid cells in S. cerevisiae
Conditional mutations
Analogous to the haploid cell
Blocking Cdc28 - cells still undergo meiosis and can go into G0
Blocking Cdc7 - blocks the stationary phase so cells can no longer arrest in G0 but cell can still undergo meiosis
Cdc28 therefore isn’t critical in diploid cells
Meiosis is blocked if the pathway is blocked further along the gene
What is START?
Decision about whether conditions are appropriate for cell division
Not a distinct point
Series of developmental choices based on conditions
Why is it essential to co-ordinate cell growth with the cell division cycle?
Cells can divide faster than the cell mass doubles
• Cells gradually get smaller
• Called mitotic catastrophe
• Cells run out of space
Why can cells with asymmetric cell division not use a ‘double mass’ mechanism?
Mother and daughter cells will always be different sizes
Mother cell will always get bigger
Daughter cell will always get smaller
Doesn’t work in yeast or humans
Cell size control in G1 phase in S. cerevisiae
Cell cycle is different for the 2 cells
Mother cell has a shorter G1 phase
Daughter cell in held in G1 until it grows big enough to be the same size as the mother
Uses a cell mask mechanism to check the size of the cell in G1
What is Cln3?
A G1 cyclin
Regulates cell size in the G1 phase of S. cerevisiae
Binds to Cdc28
How did mutations in Cln3 lead to an insight into how cell mass in coordinated?
Dominant mutation is usually gain of protein function and cell becomes more active - cells enter the cycle at a much smaller size.
If you increase the amount of CLN3 by 4 times it gives the same result as the dominant mutation - shows the dominant mutation is a gain of function
If you make a deletion of the gene, the cells are viable but are a lot larger than the WT cell
Cln3 cyclin plays an important role in determining the cell mass control mechanism
Why does the dominant (D) mutation of CLN3 result in a shorter protein?
Removes a sequence that results in its degradation
Loses ability to degrade protein so means theres more of it around
Makes the cell think the cells are bigger than they are - is conned
What happens if you overexpress cyclins in human cells?
Drives cells out of the cell cycle a lot faster
What happens if nutrients run out during the cell division cycle?
Can lead to cell death due to ATP depletion
What are the cells 2 main defences against nutrient shortages?
Sufficient internal stores - requires a checking mechanism
Respond to depletion by arresting the cell cycle at specific points
What 2 mutations are involved in arresting the cell cycle due to nutrient depletion in S. cerevisiae?
Cdc25 - guanyl nucleotide exchange factor
Cdc35 - adenylate cyclase
How is Cdc35 activated?
1) Cdc25 exchanges GDP for GTP on Ras - is a guanyl nucleotide exchange factor
2) Ras is active when GTP is bound
3) Activated Was activates Cdc35 which then produces cAMP from ATP
Mutations in either Cdc25 or Cdc35 will block the production of cAMP
What does cAMP do in regulating the cell division cycle?
cAMP regulates a protein kinase
The protein kinase consists of 4 subunits:
• 2 catalytic
• 2 regulatory (inhibitory to kinase)
cAMP binds to the regulatory subunit which then comes off the catalytic subunit to activate the kinase by phosphorylation
If you have mutations in Cdc25 or Cdc35 then cAMP isn’t produced so the kinase isn’t produced so can’t activate the cell division cycle
What does nitrogen starvation do?
Inhibits Ras
Causes sporulation
How do you stop haploid cells from sporulating?
Haploid cells produce Rme1 to inhibit lme1 which then inhibits sporulation
Diploid cells inhibit Rme1 which opens up the option of taking the sporulation route
what does cAMP-dependent protein kinase do?
either promotes the cell cycle or inhibits lme1
What happens when you block Ras?
All downstream pathways are blocked
