Week 12: Telomere Dynamics

Question 1

How is aging related to disease

Answer 1

1. Each species has a different aging limit; is aging an environmental factor?
2. More people are aging longer; however we grow more susceptible to diseases as we age

Question 2

Hayflick’s findings

Answer 2

1. People used to think cells grew indefinitely, however Hayflick found out that cells have a defined replicative capacity
2. This is shown through younger cells having a greater PDL (population doubling limit)

Question 3

Characteristics of senescent cells

Answer 3

1. Resistant to mitogenic stimulation and arrested in G1
2. Cells respond to stressors by going into senesence: change in phenotype and increasing senesence markers
   2a. Such as: large cell volume, low cell density, high stress actin fibres and focal adhesions

Question 4

Telomere structure

Answer 4

1. G rich at 3’ end: TTAGGG: since stranded region which loops back to form DNA duplex which is stabilizes by telomere binding proteins to protect from degradation
2. Shelterin complex: 6 subunit complex that protects chromosome ends

Question 5

Telomere end replication problem

Answer 5

1. Telomeres shorten as we age due to this
2. Unprotected ends of DNA are digested by exonucleases in replication
3. Complete replication of DNA ends is difficult because polymerases only act in 5’->3’ direction leaving the lagging strand w an incomplete 5’ end

Question 6

Telomerase

Answer 6

1. A reverse transcriptase that uses a RNA template (hTR) to add telomeric repeats to the end of chromosomes.
2. Not present in somatic cells; only in stem and cancer cells

Question 7

Telomerase structure

Answer 7

1. consists of 2 subunits (hTERT and hTR) and uses these to extend the 3’ telomere end

Question 8

Mortality barrier 1: senesence

Answer 8

1. At critical telomere length, DNA damage response invoked (p53) causing cells to arrest allowing senescence

Question 9

Mortality barrier 2: crisis

Answer 9

1. Mechanism
   1a. When senesence fails to initiate and telomeres continue to erode (cell keeps dividing)
   1b. Telomere sequence continues to shorten and can’t support shelterin complexes causing sticky unprotected ends that can fuse w other telomere sequences
   1c. Fusions lead to genomic instability and apoptosis
2. Artificial induction of crisis: When a dominant negative form of shelterin complex protein TRF2 is induced: leads to disruption of shelterin; chromosome ends are unprotected; fuse; crisis

Question 10

Abnormal situations where cells avoid senesence and cause telomere shortening

Answer 10

1. Dicentric chromosomes
   1a. Telomeres erode=uncapped chromosomes=fuse with other unprotected caps=dicentric chromosome and an anaphase bridge at telophase
2. Breakage-fusion-bridge cycles
   2a. Telomeres erode=fused chromosomes=break apart in chromatid separation
   2b. Broken ends are susceptible to fusing w other short telomeres (also unprotected)
   Torn apart again in mitosis=mutations

Question 11

How to get out of telomere crisis

Answer 11

1. telomerase reactivation provides a route out of telomere crisis by healing critically shortened telomeres and improving genomic stability, thereby increasing cell viability.
2. The resulting tumour will have active telomerase and a heavily rearranged genome.