Which enzymatic mutation is responsible for immoality of cancer cells?

Telomerase is the enzyme responsible for maintenance of telomeres which are specialized structures that cap the ends of chromosomes. Telomeres shoen each time the cell divides and and this in turn limits their life span. Mutation in this enzyme result in immoality of cancer cells. Function of telomeres: They provide a protective cap on chromosome ends that prevents their degradation or deleterious fusion. A special mechanism for replicating the DNA at chromosome ends DNA polymerase I: paicipates in DNA replication, and it also serve as key enzyme of DNA repair. DNA-dependent RNA polymerase is the enzyme responsible for the polymerization of ribonucleotides into a sequence complementary to the template strand of the gene Ref: Encyclopedia of Biological Chemistry, e-only page 380, Weil P. (2011). Chapter 36. RNA Synthesis, Processing, & Modification. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.