7 - Telomeres and Telomerase

Question 1

Senescence

Answer 1

No proliferation but also no dying

Question 2

Immortalisation

Answer 2

• Hallmark of cancer, endless replication capacity
• Characterized by having short telomeres (ends of
  chromosomes).
• After an immortalizing event, that results in increased telomere length, the cells will again start dividing.
• Cancerous cells divide continuously in culture and do not enter a senescence phase as they have high telomerase activity.

Question 3

Normal cells that need to replicate many times

Answer 3

• Stem cells need to be able to replicate throughout your life, but at a very low rate, to replace specific cell types.
• Germ cells need to be able to replicate over generations so we `can reproduce, but at a low rate

Question 4

Mititic phase

Answer 4

Can visibly see the cell and chromosomes dividing

Question 5

G0

Answer 5

• Stage of the cell cycle when cells are not dividing
• Can be in senescence or entering terminal differentiation
• Mitogens stimulate cells in G0 to enter cell cycle
• The lack of appropriate mitogens/nutrients or specific anti-growth signals will result in a cell exiting the cell cycle

Question 6

G1 phase

Answer 6

Gap phase between end of cytokinesis and beginning of DNA synthesis.

Question 7

G2 phase

Answer 7

Gap phase between end of DNA synthesis and beginning of mitosis.

Question 8

How long is human genome

Answer 8

3 x 10^9 bp long

Question 9

Direction of DNA synthesis

Answer 9

5’ to 3’ direction

Question 10

Telomeres

Answer 10

• Region at end of chromosome
• Gets progressively shorter with each round of cell division
• The reduction in telomere length is associated with senescence.
• An enzyme called telomerase (hTERT) attempts to maintain telomere length.
• Cancer cells have high telomerase activity enabling cells to keep dividing

Question 11

Four functions of telomeres

Answer 11

• Protect the chromosome ends from degradation and fusion.
• Mask chromosome ends from DNA damage response mechanisms that might trigger apoptosis.
• Help position the chromosomes in the nucleus.
• Provide a means of maintaining chromosome length through many generations of replication.

Question 12

Telomerase

Answer 12

• A ribonucleoprotein with reverse transcriptase activity that catalyzes the extension or lengthening of telomeres
• Carries its own RNA template

Question 13

Unequal Telomeric SCE (Sister Chromatid Exchange)

Answer 13

Homologous recombination dependent telomere maintenance, but one daughter cell suffers even shorter telomeres

Question 14

Strand invasion

Answer 14

• Similar to the D-loop structure at the ends of
  telomeres
• Can lead to initiation of DNA synthesis

Question 15

Shelterin complex

Answer 15

• Protein complex that binds to the telomere and control telomere elongation as well as protect/prevent unwanted responses from ssDNA.
• Required to recruit hTERT complex to telomere

Question 16

Proteins that make up shelterin complex

Answer 16

• TRF1
• TRF2
• RAP1
• TIN2
• POT1
• TPP1

Question 17

TRF1 complex

Answer 17

• Interacts with TPP1 (via TRF1/POT1) to recruit hTERT to the telomere and regulate telomere extension
• When tankyrase is active it polyA-ribosylates TRF1 to release it from telomeres and allows more hTERT in to elongate the telomeres.

Question 18

TRF2 complex

Answer 18

• Prevents NHEJ and homologous recombination from the ssDNA at the telomere
• Interacts with TRF1 complexes to prevent DNA damage checkpoint activation

Question 19

Pathology of defective telomerase function

Answer 19

• Dyskeratosis congenita (DKC)
• Werner syndrome (WS)
• Bloom syndrome (BS)

Question 20

Dyskeratosis congenita (DKC)

Answer 20

• Produces short telomere
• Rare progressive congenital disorder with a highly variable phenotype, sometimes resembling premature aging, initially affecting skin, progressing to bone marrow failure and early mortality.

Question 21

Werner syndrome (WS)

Answer 21

• Rare, autosomal recessive syndrome, characterized by premature aging.
• The WRN gene produces a helicase (unwinds DNA) primarily during repair of double strand breaks and during stalled replication.

Question 22

Bloom syndrome

Answer 22

• Rare autosomal recessive disorder characterized by short stature, genomic instability, and development cancer early in life.
• The BS gene (called BLM) is a DNA helicase.
• BS patients exhibit a striking genomic instability including excessive homologous recombination and sister chromatid exchanges (SCE).

Question 23

Telomerase knockout mice

Answer 23

5th - 6th generation has decreased wound-healing capacity, premature ageing, telomeres very short (even at birth), and have an increased risk of developing cancer.

Question 24

Telomerase inhibitors

Answer 24

• Anti-hTERT cancer Vaccines
• Competitive RNA binding antisense oligonucleotides
• BIBR1532 inhibits RNA template translocation
• Nucleoside analogues
• G-quadruplex-stabilizing ligands