Telomeres

Question 1

Telomeres & fusion

Answer 1

Telomeres are refractory to fusion events, never involved in chromosome rearrangements. TRF2 suppresses DNA repair/replication at the telomeres.

Question 2

Telomere shortening

Answer 2

The nucleotides of the 3’ overhang are lost during leading strand replication (uses 5’ as template)

Question 3

Tetrahymena thermophila

Answer 3

model for telomerase search (many small chromosomes, makes effects more obvious)

Question 4

Telomerase reaction cycle

Answer 4

Processivity, hTERT does not dissociate from DNA while template RNA translocates

Question 5

Role of HR

Answer 5

minor compensatory role when TERT is missing in yeast

Question 6

est1, est2

Answer 6

genes required for telomere maintenance

est1 (tpp in humans) recruites telomerase to telomeres, est2 is reverse transcriptase

Question 7

Purification of telomerase

Answer 7

affinity purification using biotin bead attached to DNA probe complementary to telomerase template RNA

Question 8

TRF1/2

Answer 8

prevent ATM binding

Question 9

POT1

Answer 9

prevents ATR binding

Question 10

telomere shortening

Answer 10

Is a barrier to tumorigenesis. if cell continues replication, enters crisis (dicentric chromosomes cannot be separated at anaphase) or becomes immortal.

Short telomeres activate DNA damage response as proteins are lost. Loss of POT1 -> ATR, loss of TRF1/2 -> ATM

Question 11

TERRA

Answer 11

telomere transcribed into long RNA whose function is not yet known.

Question 12

hTERT

Answer 12

telomerase reverse transcriptase. Introduction in cells is enough to elongate telomeres, as all of the other subunits are present in the cells.

expression is sufficient to immortalize cells.

tightly regulated in humans, but not in mice. (They only have to live 2 years, so cancer’s not really a problem.)

Question 13

Cell crisis

Answer 13

Active cell cycling, chromosome end- to-end fusions by MMEJ (alt-NHEJ), chromosome fragmentation and missegregation, frequent cell death.

Question 14

ALT pathway

Answer 14

telomere length in alt pathway is extremely heterogenous. homologous recombination

Question 15

Model for oncogene induced Cellular Senescence

Answer 15

• Oncogenes induce origin firing from within highly transcribed genes
• Conflicts between transcription and replication leads to DNA
  double stranded breaks
• Ensuing checkpoint signaling may cause oncogene-induced senescence.