cours 7 - Cell Cycle Intracellular

Question 1

Why Control the Cell Cycle?

Answer 1

• The cell cycle is a complex system of coordinated processes that must occur in a specific sequence.
• If performed incorrectly or out of sequence, results may be catastrophic.
• Regulatory proteins and biochemical switches control progression through the cell cycle.
• This system monitors intracellular and extracellular environments.

Question 2

Describe the Cdks and how they Control the Cell Cycle

Answer 2

• Cdk = “cyclin-dependent kinase”.
• Activity of cyclin (and thus Cdks) rises and falls with cell cycle.
• Molecular switches that regulate important events:
  
  • DNA replication
  • mitosis
  • chromosome segregation
  • cell proliferation

Question 3

describe Cycline major classes and their effect on the cell cycle

Answer 3

• The cyclic activity of cyclin regulates cdk, when [cyclin] ↑, CDK activates.
• Activity of cyclin rises and falls with cell cycle.
• Cyclins in all eukaryotes (4 major classes):
  
  • G1/S cyclins: bind Cdk near end of G1 and lead cell into DNA replication (comes down before S phase).
  • S-cyclins: bind Cdk during S phase and are required for DNA replication, control early mitotic events but [S-cyclin] comes down before metaphase because we don’t want them to replicate during mitosis.
  • M-cyclins: promote the events of mitosis. decrease after metaphase
  • G1-cyclins: (in most cells) promote passage through restriction point in late G1.

Question 4

describe the cell cycle

Answer 4

Question 5

What is a protein kinase (like CDK)

Answer 5

kinase is a protein that takes phosphate and ATP and adds it to another protein, leading to a change in conformation that lead to a change in functtion.

So a protein kinase activity is subject to change depending on phosphorilation.

A protein phosphatase does the opposite.

Question 6

Describe the activation process of CDK

Answer 6

In CDK there’s a small serie of AA that forms a loop. They are subject to modification when cyclin is present.

1. Cycline binds and reveles the loop. (CDK is now partially active)
2. CAK (CDK activating kinase) hydrolises ATP and uses the subsequent phosphate to phosphorilate the loop. (CDK becomes fully active)

Question 7

describe the regulation patern of wee1 and CDC25

Answer 7

Wee1 kinase phosphorylate CDK and inhibites it

CDC25 is a protein phosphatase which acts in the opposite direction of wee1 by removing this phosphate

Question 8

Describe the way P27 regulates CDK

Answer 8

P27 is a CDK inhibitor (CKI) because of it’s shape, once phosphorilated, it’ll bind to the CDK and cyclin and interacts with binding sites therefore preventing them to interact with downstream partners.

Question 9

how is the concentration of cyclin regulated?

Answer 9

Ubiquitine mediates a process of destruction of controling how much cycling is present in the cytoplasm.

Before M phase, there’s a lot of cyclin but to get out of M phase, cyclin concentration needs to be reduced. Their will be a signal sent and they will be destroyed by a process of ubiquitination and proteosome. When this is not taking place, there’s a rise in cyclin concentration.

Any protein tag with a polyubiquitine chain will get destroyed by a proteosome. A proteosome is a structure in cell that grind up proteins down to their amino acids components.

Question 10

How is the process of regulation of cyclin regulated? yeah it’s a lot of regulation :P

Answer 10

ubiquitine ligases are the one marking cyclin for destruction

SCF and APC (anaphase promoting complex) are Ubiquitin Ligases

• SCF
  
  • The active SCF complex includes an F-box
  • can lead to destruction of G1/S cyclins and destruction of CKI (like P27)
  • So G1/S is reduces, we enter the S phase but because the CKI is destroyed, it dosen’t inhibit S-CDK. so S-CDK can continue into S phase.
  • leading to a positive effect on the cell cycle (allows it to continue).
• APC
  
  • APC. activating subunit Cdc20, ubiquitination enzyme E1 + E2 and ubiquitin are requiered for the APC active complex
  • APC can lead to destruction of securin, leading to chromatid separation and destruction of M-cyclin.
  • In turn, the cell will now leave M-phase so that leads to a positive effect on tthe cell

Question 11

When are SCF and APC active?

Answer 11

SCF: ↑ from S phase to telophase

APC (Cdh1): ↓ from S phase to telophase

APC (Cdc20): ↑ from anaphase to telophase

Question 12

Describe the mecanism propelling the cell in metaphase

Answer 12

1. The cell is at the end of G2.
2. CDK1 joins with M-cyclin
3. T-loop of CDK1 is exposed and becomes phosphorilated by CAK
4. normaly, wee1 also phosphorilate the MCDK complex and the complexe stays inactive, therefore preventing the entry into mitosis
5. S-CDK phosphorylates cdc25 therefore activating it.
6. When cdc25 (phosphatase) steel the phosphate that wee1 puts, MCDK becomes active again but the rise of MCDK is slow
7. active MCDK inhibites wee1 and creates a positive feedback on cdc25
8. the concentration will eventually be high enough to push the cycle into metaphase

Question 13

What are the the 3 checkpoints in the cell cycle that regulates it?

Answer 13

1. DNA replication checkpoint
2. Spindle attachment checkpoint
3. DNA damage checkpoints (several)

Question 14

describe the regulation process that pushes the cell cycle into anaphase

Answer 14

1. MCDK activates cdc20
2. APC joins CDC20 and is activated
3. Securine is joined to seprase therefore inactivating it
4. Activated APC ubiquitinated securin
5. Securine is degraded by proteosome and separase is active
6. Separase cleaves the cohesin, a protein that holds sister chromatides together, therfore seprating them hahaha
7. Cell enters anaphase

Question 15

Describe the DNA replication checkpoint

Answer 15

1.DNA replication checkpoint

occurs at the end of G2

detects unreplicated DNA and that blocks cdc25 and Mcdk

Question 16

Describe the Spindle attachment checkpoint

Answer 16

2.Spindle attachment checkpoint

are the kinetochores attached to mitotic spindle

if not, MAD2 will bind and prevent the activation of CDC20-APC, therefore blocking the seperation

Question 17

describe the DNA damage checkpoints

Answer 17

3.DNA damage checkpoints (several)

can we go into synthesis

If DNA is damages:

1. Protein kinase is activated and phosphorylate p53 on the MDM2/P53 complex (normally, this complexe is inactive and will be degraded by proteosome)
2. This release Mdm2 abd leads to a stable activated p53
3. P53 migrates to the nucleus and binds to the regulatory region of p21 gene
4. P21 gene will be transcribed and translated leading to a CDK inhibitor protein
5. It will then associate with G1/S CDK and with S-CDK, therefore inactivating them
6. This will have a negative effect on cell cycle so that the cell does not enter S-phase