9. Telomere & Senescence Flashcards
Reasons to controlling telomeres
Preventing degradation of telomere DNA, fusion with another telomere or DNA ends, prevent chromosome ends from triggering DNA-repair, preventing recombination at or near telomeres.
Telomere characteristic
Repeat regions, 3’-overhang (several hundreds bases)
T-loop
3’-overhang is tucked back into the telomeric tracts.
Displacement loop
3’-overhang in T-loop invades telomeric dsDNA, displacing strand, creating a loop.
Why are chromosome end not recognized as damaged DNA
DNA-binding proteins
Telomere binding proteins
Include TRF1 and TRF2
Shelterin complex
Includes TRF and TRF2. Prevents ATR, ATM, Telomore sister-chromatid exchange, telomere replication.
Chromatin state of telomere
Highly heterochromatic.
DNA damage response proteins’ function in telomeres
Important for proper processing of telomere ends.
End replication problem
Primer removal of the last Okazaki-fragment always leave the end of the chromosome shorter.
Telomerase
RdDp. Uses own RNA primer with repeats to elongate 3’-strand allowing for elongation of the stunted 5’-strand. Results in 3’-overhang.
Telomerase regulation
Processivity of telomerase elongation is regulated by amount of telomere binding proteins such that more TBPs stops telomerase.
T-loop and telomerase accessibility
T-loop formation makes 3’-end inaccessible.
Telomere attrition
Makes for checkpoint activation to induce replicative senescence.
Checkpoint compromisation
Can result in chromosomal instability causing mitotic catastrophe and apoptosis or tumor promotion.
Transformation needs
Proliferation, telomere elongation, inactivation of tumor suppressors.
TRAP assay
Shows DNA ladder if telomerase is active
ALT
Alternative elongation of telomeres using strand exchange of sister chromasomes.
Senescence definition
Irreversible exit from cell cycle.
Replicative senescence
Depends of no. of cell divisions.
Characteristics of senescence
Arrest of growth, G1 DNA content, metabolically active, morphological change, change in chromatin structure, p16Ink4a, SASPs.
β-galactosidase activity
In senescent cells
Enlarged cytoplasm
Senescence marker.
SAHF
Senescence associated heterochromatic foci
Triggers of senescence
Oxidative stress, dysfunctional telomeres, strong mitogenic stimuli, chromatin pertubations, non-telomeric DNA damage.
Ras
Overexpression induces senescence (strong mitogenic signal)
SASPs
Inflammatory cytokines. Kan be associated with tissue repair, againg and tumor promotion.
