Exam 3: Cell Division

mitogens

stimulate cell division by triggering G1-Cdk G1/S-Cdk

growth factors

stimulate cell growth (an increase in cell mass)

survival factors

suppress cell death (apoptosis)

The cells of a multicellular organism divide only when they receive stimulatory extracellular signals from other cells, mitogens. In the absence of mitogenic signals, progression into a new cell cycle is _________

blocked

Cells can exit the cell cycle to a nondividing state:

- the expression of cyclins and Cdks is turned off in this state

G0 (quiescent state)

Growth factors mostly activate ________

A. GPCR

B. RTK

RTK

Checkpoints control cell cycle

G1-Cdk checkpoint?

G1/S-Cdk checkpoint?

S-Cdk checkpoint?

M-Cdk checkpoint?

APC/C checkpoint?

G1-Cdk: favorable extracellular environment

G1/S-Cdk & S-Cdk: DNA damage

M-Cdk: unreplicated DNA/DNA damage/DNA re-replication

APC/C: chromosome unattached to spindle

Mechanisms controlling cell cycle entry (at G1 checkpoint)

Late G1 checkpoint

1. Is environment favorable?

Rb inhibits cell cycle progression in non-proliferating cells

Transcriptional activator E2F turns on genes involved in S phase including S-Cdk

The Retinoblastoma protein (Rb) binds to and inhibits E2F, thus preventing exit from G1.

G1-Cdk phosphorylates Rb, causing it to dissociate from E2F, leading to progression into S.

Loss of Rb causes cancer in the eye.

Which of the following is a cancer suppresser?

A. Myc

B. G1/S Cdk

C. Rb

C. Rb

How DNA damage arrests the cell cycle in G1

When DNA is damage, protein kinases that phosphorylate (1)___ are activated

(2)____ normally binds to p53 and promotes its ubiquitylation and destruction in proteasomes

Phosphorylation of p53 blocks its binding to Mdm2; as a result, p53 accumulates to high levels and stimulates transcription of the gene that encodes the (3)____ ________ ____.

p21 binds and inactivates G1/S-Cdk and S-Cdk complexes, arresting the cell in (4)___.

1. p53

2. Mdm2

3. CKI protein p21

4. G1

Which of the following is a gene regulatory protein?

•A. P53

•B. P21

•C. G1/S Cdk

A. P53

Intracellular signal, which control the cell cycle, can arrest the cell cycle at the ________.

checkpoints

Checkpoints generally operate through ______ intracellular signals

- signals emanate from incomplete jobs

- only when the job is complete does the negative signal cease

negative

Three main checkpoints in the cell cycle

- Late G1 checkpoint: ?

- Late G2 checkpoint: ?

- Metaphase checkpoint: ?

- G1: p53, mitogens, growth factors

- G2: cdc25

- M: through activation of cdc20/APC complex

which checkpoint mostly protects us from cancer?

•A. Late G1

•B. Late G2

•C. Metaphase

•D. All of the above

D. All of the above

Late G1 checkpoint

Checks for DNA damage before DNA replication. Regulated by proteins such as p53, p21, and Rb.

Late G2 checkpoint

Checks whether DNA replication is complete and whether DNA damage needs repair before mitosis

Metaphase checkpoint

Checks that chromosomes are properly attached to spindle fibers before they separate

apoptosis

programmed cell death

suicide of the cell

billions of cells die in bone marrow every hour

half of the new neural cells die during development

The differences between necrosis and apoptosis

Necrosis- inflammation, cell swells, cell ruptures

Apoptosis- blebbing, DNA fragmentation, nucleus packed up

Apoptosis

Cell shrinks

The cell membrane shows irregular buds known as (1)_____

Cytoskeleton collapses

Nuclear envelope disassembles

DNA breaks into (2)________

The cell breaks apart into several vesicles called (3)______ _____, which are then phagocytosed

(4)_______ promotes phagocytosis by displaying signal on cell surface

- found on inner leaflet surface of plasma membrane and redistributed to extracullar surface

1. blebs

2. fragments

3. apoptotic bodies

4. phosphatidylserine

Mechanism of the cell destruction

procaspases

inactive form of caspases

- stored in the cell

- can be activated by active caspases

caspases

the proteases responsible for carrying out the apoptotic pathway

- act in cascade of protease activation, which serves to amplify and re-enforce their fateful decision

CAD (caspase activated DNAse)

cuts DNA between nucleosomes

How are the first caspases activated?

-procaspases have very low proteolytic activity, insufficient to trigger caspase cascade

-they start to cleave each other when they are aggregated (induced proximity)

Two possible ways to bring procaspases together?

extrinsic and intrinsic

Extrinsic pathway of apoptosis

Binding of a Fas ligand on a T-killer lymphocyte to a Fas death receptor on a surface of the target cell

Upon binding of the ligand to the receptor, caspases start to assemble on an intracellular part of the receptor, which brings them into proximity

procaspases cleave each other and activate caspase cascade

extracellular signal promotes aggregation of procaspases at the cell surface

Intrinsic pathway of apoptosis

Cells can activate their apoptosis program from inside the cell in response to injury, DNA damage, and lack of oxygen, nutrients, or extracellular survival signals

triggered by some of the factors above, involves the release of the mitochondrial protein cytochrome C in the cytoplasm

- cytochrome C normally plays a role in electron transport in mitochondria

release of the cytochrome C triggers the assembly of the apoptosome, which brings procaspases together

Which of the following is activated by induced proximity?

•A. RTK

•B. Procaspases

•C. TGFbeta

•D. All of the above

D. All of the above

___ proteins regulate the intrinsic pathway of apoptosis

Bcl2

Bcl2 proteins

Anti-apoptotic ?

Pro-apoptotic ?

anti-apoptotic: inhibit aggregation of effector proteins thus inhibiting apoptosis

pro-apoptotic: Bax, BAD, Bak, Bok

extracellular survival factors inhibit apoptosis in various ways

a) increased production of anti-apoptotic Bcl2 family protein

b) inactivation of pro-apoptotic BH3-only protein

c) inactivation of anti-IAPs