Telomeres Flashcards
What is telomere? What is telomere structure and function?
- telomeres are positioned at the end of chromosomes
- confer stability on a linear molecule, protect chromosome ends, prevents fusion of chromosome ends, allow telomeres to be extended
what is the relative size of telomeres?
- vary from 2 - 20 kb in humans
- in human somatic tissues, telomeres shorten 50 to 100 bp per cell division
What is the meaning of tandem repeats at chromosome ends?
- repeated sequences that can be as large as 100 - 1000 repeats
- sites are CA rich at the ends
- the 3’ strand that overhangs is G/T rich and generates 14-16 bases of a single strand overhang (result of a specific limited degradation of a G/A rich strand on the 5’ end)
What happens to the CA-strand versus GT- strand?
- the C/A strand is degraded from the 5’ end to produce the G/T rich protruding 3’ end
What are differences between G-hairpins and G-quartets?
- the G hairpin is used as a primer
- the G quartet is 4 repeat units that are thought to interact through the remaining Gs to form a helical array at the end of the chromosome
what tissues do not have telomerase activity? where is it present in?
- most somatic tissues have no telomerase activity but is present in 90% of tumors
- often involved in immortalization of cells
what shows research promise in anticancer agents?
- telomerase inhibitors show promise as anticancer agents but must be careful not to harm tissues that have a high turnover rates of cell division
how can human cell lines be immortalized?
- many human cell lines can be immortalized by the introduction of the telomerase gene (hTERT)
- other tissue types require hTERT plus the SV40 T antigen to be immortalized
- normal tissues types can be transformed to onocgenic lines by introduction of hTERT, SV40 T antigen, and rasV12
what is the G quartet and how does this occur?
- guanine bases have an unusual capacity to associate with one another
- stacks of quartets of G residues
- each quartet contains 4 G.s that hydrogen bond with one another, forming a planar structure, following another 4 Gs from the next planar stacked below the first one
- connected via hydrogen bonds
What is D-loop versus T-loop?
- the D loop is a mechanism the mitochondrion uses to replicate itself
- the t-loop is a D-loop like structure that is the 3’ single-stranded end of telomere TTAGGG invades upstream region of telomere and displaces homologous repeats from duplex DNA
- enables no free ends on the tail of the telomere - the tail is paired with the homologous strand
What protein is crucial for the T-loop formation?
TRF2
- telomere binding protein makes complex with other proteins and stabilizes chromosome ends
What is the activity of RAP1?
- binds to the CA repeats at the ends of telomeres
When is the meiotic telomere cluster/bouquet formed?
Before synapsis/pairing of chromosomes during meiosis in many organisms
what is involved in the meiotic telomere cluster/bouquet?
Fibers that act across the nuclear membrane
What type of fibers are in the meiotic telomere cluster/bouquet?
Microtubules, actin or others
Why is telomerase needed?
needed to extend the ends of chromosomes to prevent degradation of useful genes
- in the gametes telomerase is used in order to prevent this degradation and pass genes onto offspring
Stn 1
protein keeps the degradation of the C/A strand from going too far
what is telomerase and its components?
- represents a solution to the replication of a linear template
- extends the 3’ OH end of the G/T strand - which acts as a primer
- contains a covalently attached RNA (192 bases) which serves as a template
- the template RNA contains 15-22 bases equivalent to 2 repeats of the CA rich repeating consensus
- acts as a specialized reverse transcriptase
Do somatic tissues have telomerase activity? Do tumor cells?
both have telomerase activity
What protein the Shelterin complex recruits Sir3/Sir4 proteins?
Rap1
What proteins bind histone H3 and H4 tails?
Sir3/Sir4
What protein facilitates formation of 30 nm and above fibers?
Sir3
What protein structure resembles lamin, a feature that helps it insert itself into the laminar matrix during formation of telomere bouquets?
Sir3
How can Drosophila acquire white patches within red eyes?
- genes transferred neat heterochromatin are sometimes silenced by spread of heterochromatin
- RAP1 binds C A/T repeats at telomeres
- SIR3/SIR4 bind to RAP1
- SIR 3/4 bind to H3/H4 tails
- SIR3/SIR4 polymerize
- SIR3/SIR4 attach to matrix
- white gene inactive = white patches
- white gene active = red patches
