One hallmark of cancer cells is their ability to divideindefinitely, in contrast with most normal somaticcells that undergo senescence after 30 to 50 generations of divisions. We saw in this chapter that one reason for this difference is that many cancer cellsexpress the telomerase enzyme that can mediate telomere lengthening.Interestingly, about 15% of tumors do not expresstelomerase. Instead, they lengthen their telomeres byan alternative pathway. Tumor cells of this class appear to have telomeres that are highly heterogenous inlength; some telomeres have many more TTAGGGrepeats than others.a. Diagram an event involving homologous recombination that would allow some telomeres in thesecells to become longer. What feature(s) of telomeresmake(s) such homologous recombination possible?b. Does this recombination need to occur between homologous telomeres (that is, telomeres of the samearm of the same chromosome)? If such recombination could occur between nonhomologous telomeres, how might you detect it?c. Almost all cells that undergo this alternative telomere lengthening pathway have t-circles: small molecules of circular DNA made up almost exclusivelyof telomeric sequences. Diagram how these circlesmight participate in telomere lengthening
One hallmark of cancer cells is their ability to divide
indefinitely, in contrast with most normal somatic
cells that undergo senescence after 30 to 50 generations of divisions. We saw in this chapter that one reason for this difference is that many cancer cells
express the telomerase enzyme that can mediate telomere lengthening.
Interestingly, about 15% of tumors do not express
telomerase. Instead, they lengthen their telomeres by
an alternative pathway. Tumor cells of this class appear to have telomeres that are highly heterogenous in
length; some telomeres have many more TTAGGG
repeats than others.
a. Diagram an event involving homologous recombination that would allow some telomeres in these
cells to become longer. What feature(s) of telomeres
make(s) such homologous recombination possible?
b. Does this recombination need to occur between homologous telomeres (that is, telomeres of the same
arm of the same chromosome)? If such recombination could occur between nonhomologous telomeres, how might you detect it?
c. Almost all cells that undergo this alternative telomere lengthening pathway have t-circles: small molecules of circular DNA made up almost exclusively
of telomeric sequences. Diagram how these circles
might participate in telomere lengthening
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