L24: Telomeres, Telomerase, and Aging

Question 1

what are telomeres

Answer 1

heterochromatic regions devoid of genes and made of repetitive DNA sequences - thousands of repeats of the 6 nucleotide sequence , TTAGGG

telomere length shrinks with age due to incomplete replication at each cell division

Question 2

—— enables DNA replication
———- is needed to add a new nucleotide
——- synthesizes new strands in the —- direction

Answer 2

• Base-Pairing Enables DNA Replication
• A free 3’ hydroxy (OH) is needed to add a new nucleotide
• DNA polymerase synthesizes complementary new strands in the 5’->3’ direction

Question 3

major steps for DNA synthesis

Answer 3

1. DNA synthesis begins at replication origins
2. Replication bubble with right & left replication forks
3. Enzyme primase synthesizes RNA primers in 5’-3’ direction providing a free 3’ hydroxy group
4. Replications starts immediately and is continuous on the leading strand
5. Replication is delayed and fragmented on the lagging strand

Question 4

the end replication problem

Answer 4

• The leading strand is synthesized in its entirety.
• The lagging strand cannot be replicated to the end
• W/O a special mechanism ~100bp DNA would be lost
  at each cell division -> 70-100 divisions telomeres
  would be gone

Question 5

telomerase structure

Answer 5

telomerase is a ribonucleoprotein that replicates the ends of eukaryotic chromosomes

TERC: remainder of telomerase RNA, used as a template
TERT (protein): reverse transcriptase = synthesizes DNA from an RNA template

Question 6

telomeres and telomerase prevent linear eukaryotic chromosomes from shortening with each cell division

Answer 6

Question 7

telomere attrition results in —— also known as ——

Answer 7

permanent cell cycle arrest; cell senescnece

• Cells w/o telomerase reach critically short
  telomeres and stop their proliferation
• This is called the “Hayflick limit” and results in cell
  senescence, a form of cellular ‘aging’
• Senescent cells do not die but they permanently
  withdraw from the cell cycle
• Senescence is a form of tumor-suppression
• Restoring telomerase activity in these cells restores
  their proliferative potential
• Cells with high telomerase activity are immortal

Question 8

telmoere length and telomerase activity varies in different cell types

Answer 8

embryonic and plurpotent stem cells maintain their telomere length

tissue stem cells slowly decline in telomere length

progenitor and differentiated cells significantly decline in telomere length

Question 9

ask tutor

senscence of tissue stem cells may contribute to aging - results in early vs late generation TERC (mRNA) knockout mice

Answer 9

early generations: no phenotype due to mice having very long telomeres

late generations: phenotypes show defects in tissue homeostasis
-mice viable for several generations (up to 5)

-male and female infertility increases with generation - germline SCs apoptosis

• -diskeratosys congenita-like phenotypes (a disease with some characteristics of premature aging) due to limited tissue SC proliferation

Question 10

senescence of tissue stem cells may contribute to aging - results in TERT (protein) overexpression mice

Answer 10

live longer and have elevated Stem Cell activity (but have higher incidence of cancer)

Question 11

examples of candidate aging factors

Answer 11

• Epigenetic changes and spurious transcription
• Accumulations of mutations due to replication errors and other
  challenges
• Changes in the ECM or cellular composition of the stem cell niche
• Altered signals that perturb cell-cell communication
• Endocrine signals (young vs old blood)

Question 12

results from heterochronic parabiosis

Answer 12

joining blood circulation of old and young mice

Exposing young mice to an aged systemic environment
induced impaired stem cell activity and tissue decline; Exposing aged mice to a more youthful systemic environment promoted stem cell activity and tissue regeneration/activity