Cell Death and Renewal Flashcards
What role do cohesins and condensins have in chromatin condensation?
Cohesins bind to DNA in S phase and maintain the linkage between sister chromatids following DNA replication. As the cell enters M phase, the condensins then replace the cohesins along most of the length of the chromosome, so that the chromatids are linked only at the centromere. The condensins then induce chromatin condensation by forming DNA loops, leading to the formation of metaphase chromosomes.
Which kinases regulate chromatin condensation?
Cdk1/cyclin B helps trigger chromatin condensation, nuclear envelope breakdown, and spindle formation. Aurora B kinases attach chromosomes to spindle microtubules, regulate the separation of sister chromatids, and are involved in cytokinesis. Polo-like kinases are coordinate multiple aspects of mitosis, including spindle formation, chromosome segregation, and cytokinesis.
What structural changes occur between G2, prophase, and metaphase?
G2: DNA is fully replicated, not condensed. Centrosomes are duplicated, the nuclear envelope is intact, and the mitotic spindle is not formed.
Prophase: the chromatin condenses and centrosomes begin to move to opposite poles. The mitotic spindle fibers (microtubules) start forming from the centrosomes. The nucleolus disappears but the nuclear envelope is still intact.
Metaphase: the nuclear envelope is broken, the mitotic spindle is fully formed, and the chromosomes are aligned perfectly at the metaphase plate. The kinetochore microtubules attach to kinetochores on the sister chromatids, and the spindle assembly checkpoint happens.
What type of remodeling occurs at the nuclear envelope during mitosis and what are the enzymes and structural proteins involved?
Cdk1/cyclin B phosphorylates the nuclear lamins as well as proteins of the nuclear pore complex and inner nuclear membrane. Phosphorylation of the lamins causes the filaments that form the nuclear lamina to dissociate into free lamin dimers.
Compare the differences between embryonic and typical eukaryotic cell cycles. What could account for the different cell cycle times?
Embryonic cells only have M and S phases (no G1 or G2), and the focus is on rapid cell proliferation rather than growth and differentiation.
How is progression of animal cells regulated through the cell cycle?
with the four DNA checkpoints
What occurs at each phase during the cell cycle?
95% of the cell cycle is spent in interphase where the cell grows at a steady rate: G1 (growth), S (DNA replication), and G2 (protein synthesis in preparation for mitosis). The other 5% of the time is mitosis.
How is regulation of the cell cycle for budding yeast controlled?
Budding yeasts can go through the cell cycle in 90 minutes rather than 24 hours. The cell cycle is regulated at G1 by availability of nutrients, mating factors, and cell size. G1 and S phases are normal, but there is no G2 phase because the mitotic spindle begins to form in S instead.
Contrast plant and animal cytokinesis
In animal cells, a contractile ring of actin and myosin pinch the flexible cell membrane until it splits. In plant cells, a cell plate forms, vesicles from the golgi fuse at center, and new cell wall forms.
What is the difference between endothelial and epithelial cells?
Endothelial cells are the cells that line the lumen of blood vessels (e.g. capillaries, veins, arteries).
Epithelial cells are the cells that line external and internal body surfaces (e.g. skin, GI tract, respiratory tract).
What is the role of Vascular Endothelial Growth Factors (VEGF) in cell proliferation?
Blood vessel (endothelial) cells proliferate as a response to VEGF, because it promotes angiogenesis in oxygen deprived tissues
What are the steps leading up to replacing damaged cells that have shed from the surface epithelium? What types of cells are involved in this process?
- Remove damaged cells: damaged epithelial cells shed from the surface
- Activation of stem cells: Basal layer (the deepest layer of epithelium) contains adult stem cells (undifferentiated, mitotically active, triggered to divide when cell loss is detected)
- Proliferation: stem cells divide to produce:more stem cells (self-renewal) and transit amplifying cells (rapidly dividing intermediate cells)
- Differentiation: transit amplifying cells differentiate into specific epithelial cell types needed at the surface
What is hematopoietic stem cell transplantation and how is it applied to the treatment of cancer?
Stem cells that give rise to all blood cells and are infused into a patient to restore bone marrow function. They are used for leukemias and lymphomas.
What are induced pluripotent stem cells?
Induced pluripotent stem cells are somatic cells that are reprogrammed back into a self renewing (stem cell) state, meaning they can differentiate again.
How are induced pluripotent stem cells made?
It starts by having specific transcription factors (OCT4/SOX2) which repress differentiation genes and activate pluripotency genes.
Why was it necessary to make induced pluripotent stem cells?
Induced pluripotent stem cells are important because embryonic stem cells come from early-stage embryos, raising significant ethical concerns on manipulating them, but taking cells from adult (somatic) cells seems less concerning.
What is transdifferentiation?
Transdifferentiation is the process where one mature, specialized (differentiated) cell type directly transforms into another, without first becoming a stem cell or going through a pluripotent stage. An example are fibroblasts (connective tissue cells) being reprogrammed directly into a neuron or cardiac muscle cell.
What is the medical significance of transdifferentiation?
Transdifferentiation is important for regenerative medicine because it avoids ethically concerning embryonic or even stem cells.
What are the medical applications of stem cell therapy?
The main example is regenerative medicine and the potential to repair or replace damaged tissue in place (e.g. turning scar-forming fibroblasts into heart muscle cells after a heart attack, or converting supporting glial cells in the brain into neurons to treat Parkinson’s or spinal cord injuries)
What role do phagocytes play in the removal of apoptotic cells? What interactions take place between apoptotic cells and phagocytes?
Phagocytes are cells that read the eat me signals on apoptotic cells plasma membrane to know to engulf the apoptotic cells
What are the alternative pathways of programmed cell death? What are the mechanisms that lead to cell death for these alternative pathways?
-extrinsic pathway: secondary activation pathway as a response to external trauma
-pyroptosis: primary response to infectious microorganisms leading to release of inflammatory cytokines and death
-necroptosis: secondary cell death response to situations where apoptosis is inhibited (caspase inhibition)
-autophagy: cell recycling and death- lysosomes, etc.
What role do caspases have during the process of apoptosis?
A family of proteases called caspases bring about the events of apoptosis by digesting cell proteins and cleaving the proteins off (proteolytic executioners of programmed cell death). They create DNA fragments, nucleus fragments, cytoskeleton fragments, and golgi fragments.
What are the roles of effector proteins in the mitochondrial pathway of apoptosis? Focus on the role of Bax and Bak. What steps lead to cytochrome C release?
The Bcl-2 family of proteins, including Bax and Bak, act as effector proteins to regulate mitochondrial outer membrane permeabilization (MOMP). MOMP is irreversible and leads to the release of apoptotic factors such as cytochrome c. Once cytochrome c is released, it binds to Apaf-1, which activates caspase-9. This initiates the caspase cascade, leading to the execution phase of apoptosis.
Anti-apoptotic Bcl-2 family members (e.g., Bcl-2, Bcl-xL) bind to Bax and Bak, inhibiting their activation and preventing apoptosis.
Pro-apoptotic Bcl-2 family members promote Bax/Bak activation and function as activators in the apoptotic process.
BH3-only proteins (e.g., Bid) sense cellular stress and activate Bax and Bak, pushing the cell towards apoptosis.
What role does p53 have in DNA damage-induced apoptosis? What are the main molecules involved in this apoptotic pathway?
DNA damage leads to activation of ATM and CHK2 kinases, which phosphorylate and stabilize p53, increasing levels of p53. p53 then activates transcription of Bcl-2 family members, resulting in the elimination of cells with potentially harmful mutations. This is an intrinsic pathway that responds to spontaneous internal damage of vascular endothelium.
