Telomeres

Question 1

What is telomere? What is telomere structure and function?

Answer 1

• 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

Question 2

what is the relative size of telomeres?

Answer 2

• vary from 2 - 20 kb in humans
• in human somatic tissues, telomeres shorten 50 to 100 bp per cell division

Question 3

What is the meaning of tandem repeats at chromosome ends?

Answer 3

• 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)

Question 4

What happens to the CA-strand versus GT- strand?

Answer 4

• the C/A strand is degraded from the 5’ end to produce the G/T rich protruding 3’ end

Question 5

What are differences between G-hairpins and G-quartets?

Answer 5

• 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

Question 6

what tissues do not have telomerase activity? where is it present in?

Answer 6

• most somatic tissues have no telomerase activity but is present in 90% of tumors
• often involved in immortalization of cells

Question 7

what shows research promise in anticancer agents?

Answer 7

• telomerase inhibitors show promise as anticancer agents but must be careful not to harm tissues that have a high turnover rates of cell division

Question 8

how can human cell lines be immortalized?

Answer 8

• 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

Question 9

what is the G quartet and how does this occur?

Answer 9

• 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

Question 10

What is D-loop versus T-loop?

Answer 10

• 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

Question 11

What protein is crucial for the T-loop formation?

Answer 11

TRF2
- telomere binding protein makes complex with other proteins and stabilizes chromosome ends

Question 12

What is the activity of RAP1?

Answer 12

• binds to the CA repeats at the ends of telomeres

Question 13

When is the meiotic telomere cluster/bouquet formed?

Answer 13

Before synapsis/pairing of chromosomes during meiosis in many organisms

Question 14

what is involved in the meiotic telomere cluster/bouquet?

Answer 14

Fibers that act across the nuclear membrane

Question 15

What type of fibers are in the meiotic telomere cluster/bouquet?

Answer 15

Microtubules, actin or others

Question 16

Why is telomerase needed?

Answer 16

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

Question 17

Stn 1

Answer 17

protein keeps the degradation of the C/A strand from going too far

Question 18

what is telomerase and its components?

Answer 18

• 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

Question 19

Do somatic tissues have telomerase activity? Do tumor cells?

Answer 19

both have telomerase activity

Question 20

What protein the Shelterin complex recruits Sir3/Sir4 proteins?

Answer 20

Rap1

Question 21

What proteins bind histone H3 and H4 tails?

Answer 21

Sir3/Sir4

Question 22

What protein facilitates formation of 30 nm and above fibers?

Answer 22

Sir3

Question 23

What protein structure resembles lamin, a feature that helps it insert itself into the laminar matrix during formation of telomere bouquets?

Answer 23

Sir3

Question 24

How can Drosophila acquire white patches within red eyes?

Answer 24

• 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

Question 25

What is lamin?

Answer 25

matrix protein

Question 26

Can telomerase keep cells young?

Answer 26

Telomerase prevents this decline in some kinds of cells, including stem cells, by lengthening telomeres

Question 27

Is Arabidopsis survival affected by telomerase mutations and how?

Answer 27

Arabidopsis becomes progressively smaller and abnormal; does not survive past 10 generations