9. Telomere & Senescence

Question 1

Reasons to controlling telomeres

Answer 1

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.

Question 2

Telomere characteristic

Answer 2

Repeat regions, 3’-overhang (several hundreds bases)

Question 3

T-loop

Answer 3

3’-overhang is tucked back into the telomeric tracts.

Question 4

Displacement loop

Answer 4

3’-overhang in T-loop invades telomeric dsDNA, displacing strand, creating a loop.

Question 5

Why are chromosome end not recognized as damaged DNA

Answer 5

DNA-binding proteins

Question 6

Telomere binding proteins

Answer 6

Include TRF1 and TRF2

Question 7

Shelterin complex

Answer 7

Includes TRF and TRF2. Prevents ATR, ATM, Telomore sister-chromatid exchange, telomere replication.

Question 8

Chromatin state of telomere

Answer 8

Highly heterochromatic.

Question 9

DNA damage response proteins’ function in telomeres

Answer 9

Important for proper processing of telomere ends.

Question 10

End replication problem

Answer 10

Primer removal of the last Okazaki-fragment always leave the end of the chromosome shorter.

Question 11

Telomerase

Answer 11

RdDp. Uses own RNA primer with repeats to elongate 3’-strand allowing for elongation of the stunted 5’-strand. Results in 3’-overhang.

Question 12

Telomerase regulation

Answer 12

Processivity of telomerase elongation is regulated by amount of telomere binding proteins such that more TBPs stops telomerase.

Question 13

T-loop and telomerase accessibility

Answer 13

T-loop formation makes 3’-end inaccessible.

Question 14

Telomere attrition

Answer 14

Makes for checkpoint activation to induce replicative senescence.

Question 15

Checkpoint compromisation

Answer 15

Can result in chromosomal instability causing mitotic catastrophe and apoptosis or tumor promotion.

Question 16

Transformation needs

Answer 16

Proliferation, telomere elongation, inactivation of tumor suppressors.

Question 17

TRAP assay

Answer 17

Shows DNA ladder if telomerase is active

Question 18

ALT

Answer 18

Alternative elongation of telomeres using strand exchange of sister chromasomes.

Question 19

Senescence definition

Answer 19

Irreversible exit from cell cycle.

Question 20

Replicative senescence

Answer 20

Depends of no. of cell divisions.

Question 21

Characteristics of senescence

Answer 21

Arrest of growth, G1 DNA content, metabolically active, morphological change, change in chromatin structure, p16Ink4a, SASPs.

Question 22

β-galactosidase activity

Answer 22

In senescent cells

Question 23

Enlarged cytoplasm

Answer 23

Senescence marker.

Question 24

SAHF

Answer 24

Senescence associated heterochromatic foci

Question 25

Triggers of senescence

Answer 25

Oxidative stress, dysfunctional telomeres, strong mitogenic stimuli, chromatin pertubations, non-telomeric DNA damage.

Question 26

Ras

Answer 26

Overexpression induces senescence (strong mitogenic signal)

Question 27

SASPs

Answer 27

Inflammatory cytokines. Kan be associated with tissue repair, againg and tumor promotion.