[MODULE 2] Checkpoints

What are the characteristics of cell cycle switches?

• binary (complete, irreversible)

• remarkably robust

• highly adaptable to different cell types and signals

What are the three major regulatory transitions in the cell cycle?

• Start/restriction point

• G2/M Transition

• Metaphase to Anaphase Transition

_________ the first checkpoint in G1, where the cell commits to DNA replication. Removing signals after this point does not stop the cell cycle.

Start/Restriction point

Checkpoint that checks whether all DNA is replicated and the environment is favorable for division. If not, it can enter G0 or undergo apoptosis.

G2/M Transition checkpoint

Ensures proper chromosomal alignment and checks for chromosomal abnormalities before separation of sister chromatids.

Metaphase-to-Anaphase Transition

What types of cells undergo mitosis without cytokinesis, resulting in multinucleated cells?
______ - responsible for platelet production, skeletal muscles, liver cells, and some plant cells.

Megakaryocytes

Processes included in the M phase

Nuclear division (mitosis) and cytoplasmic division (cytokinesis)

How much time is typically spent in interphase during the cell cycle?


22-23 hours

What are the three phases of interphase?


G1 phase

S phase

G2 phase

What types of errors does the G2 phase check for?


Unreplicated or damaged DNA.

It is an artificial thymidine analog incorporated into newly synthesized DNA in S-phase cells.

5-ethynyl-2'-deoxyuridine (EdU)

How do cyclin levels change during the cell cycle?


• 
  Changes in protein levels result in cyclic assembly and activation of cyclin-Cdk complexes.

• G1 cyclins respond to extracellular signals and govern G1/S-cyclins activity.

What are the central components of the cell-cycle control system?


• Cyclin-dependent protein kinases (Cdks)

• Cyclin subunits

• APC/C and SCF ubiquitin ligases

• Oscillations in cyclin-Cdk complex activities control cell-cycle events.

pace the cell cycle by phosphorylating intracellular proteins, initiating major cell-cycle events

Cyclin-Dependent Protein Kinases (Cdks)

What activates Cdks to move forward with cell cycle by adding an additional phosphate group

Cdk-activating kinase (CAK)

adds inhibitory phosphorylation to Cdks, preventing their activity before mitosis (G2/M); prevents cells with DNA damage from entering mitosis prematurely

Wee1 kinase

removes inhibitory phosphates from Cdks, activating them at the onset of mitosis

Cdc25 phosphatase

inactivates cyclin-Cdk complex , inhibiting progression through the G1/S checkpoint.

CKIs (Cdk Inhibitor Proteins), ex: p27

TRUE OR FALSE:
Cdk is always present in cells but activation varies depending on amount of Cyclin

TRUE

It enhances the progression through the cell cycle by amplifying the signals that move the cell to the next stage; moves away from homeostasis

Positive feedback

Examples of positive feedback regulators

CDK4,6 + Cyclin D1,2,3

CDK2 + Cyclin E

Inhibit progression and moves toward homeostasis

Negative feedback

Examples of negative feedback regulators

INK4 (p15,16,18,19)

KIP/CIP (P21,27,57)

Cyclin G2

_______ - reverses the effects of Cdks by dephosphorylation, acting as an anti-cell division mechanism

Protein Phosphatase 2A (PP2A)

How does M-Cdk activate during mitosis?

M-Cdk activation involves positive feedback, where phosphorylated M-Cdk stimulates Cdc25 and inhibits Wee1, enhancing mitotic progression

_____ tags proteins (e.g., S or M cyclins) for degradation in proteasomes, leading to Cdk inactivation.

Ubiquitin

initiates the transition from metaphase to anaphase by degrading proteins like securin, leading to chromatid separation.

Anaphase-Promoting Complex (APC/C)

• Activated by Cdc20 and Cdh1 to transition from metaphase to anapase

• induces the disassociation of cohesin complex

Anaphase-promoting complex/cyclosome (APC/C)

triggers initial ativation of APC/C at metaphase to anaphase; stimulated by M-Cdk activity

Cdc20

Maintains APC/C activity after anaphase and throughout G1, inhibited by Cdk activity

Cdh1

POSITIVE OR NEGATIVE FEEDBACK
APC/C-Cdc20 turns itself off by inactivating Cdks, leading to APC/C dephosphorylation and inactivation in anaphase

Negative

• Involved in the destruction of G1/S-cyclins and CKIs, promoting the transition from G1 to S phase and facilitating DNA replication.

• Protein complex for ubiquitination

SKP, CULLIN, F-BOX (SCF)

cyclin (Cyclin A) bound to Cdk2, initiates DNA replication once per cycle

S-Cdk Complex

• Shorten every division

• Gives signal to cell that it is old

• Old cells more likely to have mutations (e.g. cancer in old people)

Telomeres

What prevents endless cell division in normal cells?

Hayflick limit