Intracellularly - Telomeres Flashcards
Telomere
This protects chromosomal ends from erosion over cell divsions and against interchromosomal fusion
What happens when telomeres become too short?
Cell cycle arrest and apoptosis triggering
What are telomeres improtant for?
Genomic integrity.
What happens to telomeres every cell division?
They shorten, inducing instability of chromosomes
How do cancer cells achieve immortality?
Bypass shortening through telomerase production through TERT expression.
What causes increased hTERT expression in cancer cells?
Two mutations in the promoter region about 130BP upstream of TERT translation site.
What are techniques used to induce cell death of cancer cells?
Antisense oligonucleotides
Immunotherapy
G-Quadruplex Stabilisers
Small Molecule Inhibitors
Shelterin Complex
This regulates telomerase activity by binding telomeres and inducing t-loop formation.
T Loop
A cap of 300 BP ssDNA stabilising telomere, preventing ends from being recognised as break points by DNA repair machinery.
What sequence are telomeres rich in?
Tandem repeats of TTAGGG
How long is the teloemre?
About 10-15KB long.
What terminates the telomeres tandem repeats?
A 150-200 NT long overhang.
What is the function of the telomere overhang?
Forms T-Loop by folding back onto the 3’ strand, invading the TTAGGG region.
What are the components of the Shelterin Complex?
TRF1 and 2 heterodimer associated to POT/TPP1 heterodimer linked by TIN2.
TRF1
Controls replication of telomeric DNA
TRF2
Required for T-Loop formation and DDR supression and repression mediated by ATM preventing end-end chromosomsal fusion
POT1
Associates TPP1 and ss 3’ overhang and represses recruitment of RPA
TIN2
Links the POT1/TPP1 heterodimer to the TRF1/2 heterodimer, maintaing structural integrity.
TERRA(Telomeric Repeat ContainingRNA)
A long, non-coding RNA essential for integrity, functioning in heterochromatin reguloation, teloemrase regulation and DDR
How much is TL reduced per cell division?
About 50-150BP
Replicative Senescences
This is where nonmalignant cells stop divding after about 50 cell divsions.
M2 Crisis
When telomeres are so short they cannot form protective structures to protect chromosome ends.
How do cancer cells bypass M1?
Expression of reactivating telomerase expression OR teloemrase independet ALT mechanisms.
ALT mechanisms
A mechanism of which cancer cells use to bypass telomeric degradation.
How many cells in a pool of M1 undergo M2?
1 in 5 million.
What are the components of telomerase holoenzyme?
TERT (telomerase reverse transcription)
hTERC (Human telomerase RNA component)
NHP, NOP10, Dyskerin and GAR1.
What is the general assembly process of Telomerase?
Translocation of hTERT to the nucleus with assembly with hTR then telomerase recruitment.
What do NHP, NOP1, dyskerin and GAR1 do?
Associate with small, nucleoalr RNA important in pseudouridylation
Pseudouridylation
The process of conversion of uridine to pseudouridine within RNA.
TCAB1
Binds hTR sequecne to direct holoenzyme to Cajal Bodies of the nucleolus.
What is the process of Telomerases function?
Diassociates TPP1 and POT1 interactions of shelterin by DAT domains then synthesis from 3’ end.
How does telomerase mediate TPP1 and POT1 diasscoation?
A DAT domain binds amino acid rich Tel patch of TPP1 and binds POT1 and TIN2.
End Replication Problem
This proposes that linear DNA ends are not replicated completely during lagging strand DNA synthesis
What happens to G-rich/C-rich strands in replication?
G-rich by leading strand synthesis and C-rich by lagging strand synthesis.
Why is the lagging strand not completely replicated.
There is no room for okazaki fragment RNA primers to bind, resulting in an overhang and shortened telomere
How does telomerase bypass End Replication Problem?
Increases leading strand to allow okazaki fragment RNA primer to be placed.
What repair systems are DSB telomeres recognised by?
NHEJ
Homologous recombination repair
What is the first step in NHEJ and HRR?
MRN complex recognises DSB, activating ATM
What does ATM do after being activated by MRN?
Phosphorylates DNA repair proteins activating them
What is the major DSB pathway?
NHEJ?
When might HRR be activated?
If NHEJ does not repair quick enough, ATM activates other two pathways by generation of SS 3’ strand overhang.
Why are cancer cells immortal?
Upregulation of catalytic subunit TERT of telomerase.
