Telomeres are specialized structures at the ends of chromosomes that contain repeated DNA sequences and proteins. They protect chromosome ends from being recognized as DNA damage. During cell division, the lagging strand of DNA cannot be fully replicated, resulting in telomere shortening over time. This eventually leads to chromosome instability and cell death. Telomerase is an enzyme that maintains telomere length by adding DNA repeats to chromosome ends and counteracts replication-induced shortening. Its expression allows stem cells and cancer cells to divide indefinitely.

1. Telomeres are Specialized Structures at the Ends of Chromosomes
Telomeres contain multiple
copies of short repeated
sequences and contain a 3’-
G-rich overhang
Telomeres are bound by
proteins which protect the
telomeric ends initiate
heterochromatin formation
and facilitate progression of
the replication fork
from Gilson and Geli, Nature Rev.Mol.Cell Biol. 8, 825 (2007)

2. Functions of Telomeres
Telomeres protect chromosome
ends from being processed as a
ds break
End-protection relies on telomere-
specific DNA conformation,
chromatin organization and DNA
binding proteins
from Gilson and Geli, Nature Rev.Mol.Cell Biol. 8, 825 (2007)

3. The End Replication Problem
Leading strand is synthesized to
the end of the chromosome
Lagging strand utilizes RNA
primers which are removed
The lagging strand is shortened
at each cell division
from Lodish et al., Molecular Cell Biology, 6th ed. Fig 6-49

4. 5’
3’
Template DNA new DNA RNA primer
5’
3’

5. Chromosomal DNA replication
Template DNA
new DNA
5’
3’
5’
3’
*

6. Replicating chromosome ends
At this end of the chromosome,
which strand cannot be
completely replicated?
A. Top strand
B. Bottom strand
C. Both strands
D. Neither strand
E. Not enough info
template DNA new DNA RNA primer
5’
3’
5’
3’
5’
3’
5’
3’

7. Replicating chromosome ends
At this end of the chromosome,
which strand cannot be
completely replicated?
A. Top strand
B. Bottom strand
C. Both strands
D. Neither strand
E. Not enough info
template DNA new DNA RNA primer
5’
3’
5’
3’
5’
3’
5’
3’

8. Replicating chromosome ends
template DNA new DNA RNA primer
The lagging strand cannot be
completely replicated.
Template for lagging DNA strand
Template for leading DNA strand
3’
5’

9. Genes lost!
Essential genes here
- cells die!
Information loss during replication

10. Essential genes here
“page” = Telomere repeat
- cells die!
Delaying
information loss during replication

11. Telomerase!
Telomere repeat
Solving the information loss problem
Essential genes here

12. The Action of Telomerase Solves the Replication Problem
from Alberts et al., Molecular Biology of the Cell, 4th ed. Fig 5-43
New Okazaki fragments are
synthesized using the extended template

13. Telomerase Extends the ss 3’-Terminus
Telomerase-associated RNA base pairs
to 3’-end of lagging strand template
Telomerase catalyzes reverse
transcription to a specific site
3’-end of DNA dissociates and base pairs
to a more 3’-region of telomerase RNA
Successive reverse transcription,
dissociation, and reannealing extends
the 3’-end of lagging strand template
New Okazaki fragments are
synthesized using the extended template
from Lodish et al., Molecular Cell Biology, 6th ed. Fig 6-49

14. Loss of Functional Telomeres Results in Genetic Instability
from O’Sullivan and Karlseder, Nature Rev.Mol.Cell Biol. 11, 171 (2010)
Dysfunctional telomeres
activate DSB repair by NHEJ
Fused chromosomes result in chromatid
break and genome instability

15. from Lodish et al., Molecular Cell Biology, 6th ed. Fig 25-31
Stem cells and germ cells contain
telomerase which maintains telomere size
Somatic cells have low levels of
telomerase and have shorter telomeres
Loss of telomeres triggers
chromosome instability or apoptosis
Cancer cells contain telomerase
and have longer telomeres
Loss of Telomeres Limits the Number of Rounds of Cell Division

16. from de Lange, Genes Dev. 19, 2100 (2005)
Shelterin Specifically Associates with Telomeres
Shelterin subunits specifically
recognize telomeric repeats
Shelterin allows cells to distinguish
telomeres from sites of DNA damage

17. Telomere Termini Contain a 3’-Overhang
from de Lange, Genes Dev. 19, 2100 (2005)
A nuclease processes the 5’-end
POT1 controls the specificity of the 5’-end

18. Loss of Functional Telomeres Results in Genetic Instability
from O’Sullivan and Karlseder, Nature Rev.Mol.Cell Biol. 11, 171 (2010)
Dysfunctional telomeres
activate DSB repair by NHEJ
Fused chromosomes result in chromatid
break and genome instability

19. from Lodish et al., Molecular Cell Biology, 6th ed. Fig 25-31
Stem cells and germ cells
contain
telomerase which maintains
telomere size
Somatic cells have low levels
of
telomerase and have shorter
telomeres
Loss of telomeres triggers
chromosome instability or
apoptosis
Cancer cells contain
telomerase
and have longer
telomeres
Loss of Telomeres Limits the Number of Rounds of Cell Division

20. Telomerase is widely expressed in cancers
80-90% of tumors are telomerase-positive
Telomerase-based Cancer Therapy
Strategies include
Direct telomerase inhibition
Telomerase immunotherapy