Download Lecture ** - Telomeres

GEN302, 2nd half of course
Winter, 2006
Dr. Campbell
Topic 2: Telomeres
RR: Beginning to understand the end of the chromosome. Cech TR. Cell. 2004 Jan
23;116(2):273-9.
SR: T loops and the origins of telomeres. (2004) T. de Lange Nature Rev. Mol. Cell
Biology 5: 323-329
The "problem" of linear chromosome ends:
a) 5'->3' DNA replication of "lagging strand"
b) How does cell discriminate between DNA double-strand breaks (which are also
“ends”) and the natural ends of chromosomes?
The solution: telomeres!
a) Telomere structure: Repetitive DNA sequences at ends of chromosomes
• telomeric heterochromatin (gene silencing assay)
• human telomere repeat: GGGTTA (many copies: ~ 10,000 bp)
• binding sites for telomere-specific proteins
b) Telomerase replication mechanism (vertebrates, most other eukaryotic organisms)
• result in short 3' overhanging DNA (needs to be protected from nucleases)
• processing ends into T-loops - is this what distinguishes telomeres from “ends"
caused by DNA damage?
c) Retrotransposon mechanism of telomere protection (Drosophila)
• 2 types of telomere-specific retrotransposons: HeT-A and TART
• transposition mechanism: reverse transcription/ligation of new DNA copy to end
d) Common features of these two different mechanisms
- RNA intermediate > suggests an original retroviral origin
- TERTs: Telomerase Reverse Transcriptases
Discovery of the telomerase-based mechanism
a) Elizabeth Blackburn & colleagues used a protozoa calledTetrahymena
• developmentally regulated chromosome amplification (thousands of “ends”)
b) Biochemical purification of a heat and proteinase K-sensitive "telomere terminal
transferase activity" (description turned out to be inaccurate) from Tetrahymena cell
extracts
c) Testing their model: site specific mutagenesis of RNA template / corresponding
change in telomere sequences
Human telomeres and “replicative senescence”
a) The "Hayflick limit”: most cells can only undergo 40-50 cell divisions
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GEN302, 2nd half of course
Winter, 2006
Dr. Campbell
b) Telomerase activity is down-regulated during early human development - is this a
timing mechanism that limits how long cells can survive before telomere degradation
affects critical genes?
c) Relationship between telomerase activity and cancer in humans- somatic cells
evolving to overcome the Hayflick limit?
• elevated hTERT expression in ~80% of human cancers!!
d) Possible medical applications of re-activating telomerase to restore regenerative and
functional abilities of cells affected by aging, injury or chronic disease
e) Stem cells are not subject to Hayflick limit - how this occurs is not understood.
Molecular mechanisms that link DNA damage responses and telomeres
a) ATM and the MRN DNA DSB repair complex: mutations associated with progressive
neurodegeneration, cancer pre-disposition (ataxia telangiectasia, Nijmegen-breakage
syndrome)
b) Role of ATM/MRN proteins in telomere maintenance
• functional redundancy with DNA-PK repair system, in vertebrates
• essential telomere protection mechanism in Drosophila (no DNA-PK present)
T. de Lange theory of telomere evolution (see SR article for more details)
a) Evolution of circular to linear genomes.
b) Adaptation of primitive DNA repair system to cope with the “new” biological problem
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