Cell Life Cycle 2 Flashcards
What is the hayflick phenomenon?
Cells in a culture can divide for a certain number of times. Cells from an older donor will divide for fewer times than a young donor. They then stop dividing/senesce.
Why does replicative senescence occur?
Chromosomes have telomeres at there ends which are not properly dealt with in DNA replication, so become shortened with each replication. Eventually become excessively short, so replication stops.
How does telomerase work?
Works in germ/stem cells. Prevents telomeres from shortening by adding (TRAGGG)n to the telomere.
How have tumour cells overcome the Hayflick limit?
They make telomerase, so do not undergo senescence.
What is phase G0?
A stage cells enter when they exit the cell cycle transiently (exit cycle but can return again)
How is necrosis different to apoptosis?
Necrosis is uncontrolled - all of cell contents and enzyme mattered is released during the process. Apoptosis is controlled, as membrane does not break down so cell contents not released into tissues.
What are the features of apoptosis?
Programmed cell death. Energy dependent. Metabolically active. Membrane integrity is maintained. No inflammatory reaction.
What can occur when apoptosis goes wrong?
Syndactly
How does DNA laddering occur?
Occurs as in apoptosis DNA gets chopped into less than 200 base pair fragments as DNAase chops between nucleosomes. Multiples of 200 base pairs form a ladder.
What are caspases?
Special proteases which cleave at cysteine and aspartic acids. All caspases exist in precursor forms - procaspases and can be activated by proteolytic cleavage.
How is procaspase activated?
Two inactive procaspase molecules are cleaved and assembled to form one active caspase molecule.
How does the caspase cascade occur?
When one initiator caspase is activated, it can go on to activate many other caspases (executioner caspases). This cycle goes on and on, causing nuclear lamin cleavage and cleavage of cytosolic protein.
What is the extrinsic pathway of apoptosis?
1) ligand binds to receptor at cell surface.
2) recruits adaptor proteins, which activate caspase 8 (an initiator caspase)
3) caspase 8 activates caspase 3 (executioner caspase)
4) proteolysis of cell proteins and DNA cleavage occurs
What is the intrinsic pathway of apoptosis?
1) cell stress regulates permeability of mitochondria, causing release of cytochrome c
2) cytochrome c forms a complex with caspase 9 (initiator)
3) caspase 9 activates caspase 3 (executioner)
4) caspase 3 causes cellular protein proteolysis and DNA cleavage
Why is regulating mitochondria permeability important?
As apoptotic stimulus will cause increased permeability and promote apoptosis via intrinsic pathway
