Cell Cycle and Apoptosis Flashcards
4 major phases of the cell cycle
G1, S, G2: Interphase
M
G0 phase
Cell is no longer dividing
Quiescent (dormant)
Can re-enter cell cycle
M phase
Cell division
2 parts: mitosis (nuclear division) and cytokinesis (cytoplasmic division)
Prophase/Prometaphase
Chromosomes condense
Nuclear envelope breaks down
Mitotic spindle (microtubules) forms
Kinetochore (protein structure at centromere) connects chromosomes to microtubules
Centrosomes (organizing centers) form at opposite sides of cells
Metaphase
Centromeres line up at metaphase plate
Anaphase
Sister chromatids separate
Kinetochores pull chromosomes along microtubules
Telophase
Two daughter nuclei form
G1 phase
Cell grows and makes more macromolecules and organelles
S phase
DNA is replicated
G2 phase
Cell grows and makes more materials as in G1
3 major checkpoints in the cell cycle
End of G1
End of G2
Partway through M
Cell machinery is always surveying the state of the cell
G1 checkpoint
Make sure that there is enough material to replicate DNA
Look for DNA damage
G2 checkpoint
Make sure that DNA has been duplicated completely and correctly
M checkpoint
Make sure that mitotic spindles are attached properly and that there is enough material to make daughter cells
2 major proteins involved in checkpoints
Cyclins
Cyclin-dependent kinases (Cdks)
Cyclin-Cdk complex
Cyclin binds to Cdk -> Cdk changes shape -> kinase ability is activated (Cdk can phosphorylate key proteins that initiate the next step in the cell cycle)
More cyclin present- greater Cdk activity
Specificity of cyclin-Cdk
Specific cyclin-Cdk complexes exist for each checkpoint
Different targets are phosphorylated to activate next step
Cdc6 and cyclin-Cdk complex
G1: Cdc6 (protein) is bound to origin recognition complex (ORC), which is bound to origin of replication
Cdc6 + ORC + origin of replication = pre-replicative complex
S: S-Cdk triggers S phase -> Cdc6 is phosphorylated and then degraded -> replication fork assembled -> DNA replication
Cdc6 phosphorylation: beginning of transcription
Once Cdc6 is phosphorylated, origin of replication can’t be used again
Regulation of cyclin-Cdk complex
Cdk is phosphorylated and then dephosphorylated to turn on
Cyclin is degraded to turn off
Cdk inhibitor proteins can hold back or turn off completely
Extracellular cues
Phosphorylation of cyclin-Cdk complex
Cdk is phosphorylated and de-phosphorylated according to cellular cues
Cyclin binds to Cdk -> protein kinases phosphorylate Cdk -> activating protein phosphatase removes inhibitory phosphate, leaving activating phosphate -> active cyclin-Cdk complex
Ubiquitylation of cyclin-Cdk complex
Allows for Cdk to be deactivated
Ub-ligase attaches ubiquitin to cyclin, tagging it for proteosome
Proteosome degrades cyclin
Inhibitor proteins
Proteins inhibit formation of cyclin-Cdk or cyclin-Cdk activity
DNA damage activates protein kinases that activate p53 protein -> active p53 binds to regulatory region of p21 gene -> transcription and then translation of p21 gene -> p21 protein binds to cyclin-Cdk complex, inhibiting it
Survival factors
Inhibit apoptosis
Mitogens
Stimulate cell division by activating corresponding cell cycle cascade
Mitogen binds to mitogen receptor in cell membrane -> mitogen-activated protein (MAP) kinase cascade -> activation of Cdk -> Rb protein is phosphorylated -> Rb lets go of transcription factor -> cell proliferation
Growth factors
Stimulate cell growth
Activate signaling cascade -> increased production of macromolecules
Components of cytoskeleton that aid in cell division
Mitotic spindle
Contractile ring
Contractile ring
Actin and myosin assemble on cytoplasmic face of plasma membrane
Slide like muscle contraction, pulling membrane in
Phragmoblasts
Found in plants
Construct cell wall as final step in cell division
Phragmoblast microtubules position Golgi-derived membrane to form new cell wall
3 classes of microtubules
Kinetochore microtubules
Aster microtubules
Interpolar microtubules
All 3 types array from centrosome
Kinetochore microtubules
Attach to centromere and pull chromosomes apart as they (MTs) shorten
Aster microtubules
Pull centrosomes apart
Interpolar microtubules
Push poles apart using sliding force
Timing of assembly of mitotic spindle
Centrosomes replicate during interphase and migrate during M phase
Dead cells
Loss of integrity of plasma membrane
Cell, including nucleus, has undergone complete fragmentation into discrete apoptotic bodies and/or its corpse or fragments has been engulfed by an adjacent cell
Causes of cell death
Damage to cell (DNA, ER, oxidative stress, etc.)
Cell is no longer working efficiently
Development (“webbing” between fingers disappears in embryo)
2 types of cell death
Necrosis
Apoptosis
Necrosis
“Messy” death
Acute injury causes cell to lyse and dump its components
Compromising of cell membrane
Apoptosis
"Neat" death Cell is engulfed by phagocytic cell and cellular components are recycled Compartmentalized No damage to neighboring cells Complex signaling cascade
Vacuoles in apoptosis
Vacuoles become enlarged: cell partitions itself off
What happens to cell during apoptosis
- Blebbing of cell: cytoskeleton uncouples from membrane and ball-like structures (blebs-contain cell components) form
- Cell shrinks
- Nucleus fragments
- Chromatin condenses
- DNA fragments
2 major pro-aptotic pathways
Extrinsic: extracellular cues induce
Intrinsic: cellular damage induces (causes activation through mitochondria)
Intrinsic apoptotic pathway
- Cell stress activates BAX
- BAX causes permeabilization of mitochondria
- Cytochrome C leaks out (cytochrome C: electron transport chain- end of energy production, committed step)
- Apoptosome forms
- Apoptotic processes activated by caspases
Extrinsic apoptotic pathway
- 1st messenger binds to death receptor
- Caspases activate BAX
- BAX causes permeabilization of mitochondria
- Cytochrome C leaks out
- Apoptosome forms
- Apoptosis by caspases
OR
Caspases that activate BAX also activate caspases that cause apoptosis
Bcl2 protein
Anti-aptotic: inhibits BAX
Caspases
Proteases (cleave proteins)
Activated by cleavage
To activate: cleave procaspase
Regulate apoptosis using irreversible method (cleavage is permanent): cell is damaged- don’t want to go back
1 molecule of this can cleave many molecules of this, which in turn cleave more molecules of this
Ways to measure apoptosis
Cleaved caspase 3 (Western blot: Ab on cleaved caspase)
TUNEL stain: label ends of chopped-up DNA and look at it under microscope
Phosphoserine exposure: phospholipids are flipped to outer side of membrane during apoptosis (usually on inner)
Phosphoserine exposure
Annexin 5 binds to phosphoserine
Use Ab to Annexin 5 to label apoptotic cells
Measure Ab binding by flow cytometry (control graph: no shoulder; experimental graph: shoulder)
