The ends of the linear chromosomes of eukaryotes cannot be replicated by DNA polymerase. This problem is overcome through the use of telomerase, an enzyme implicated in aging and cell lifespan. Telomerase contains a noncoding RNA that acts as a moving template to add a repetitive DNA sequence at the chromosome tip, preventing the loss of essential DNA. In mammals, the ends of the chromosomes fold back to form a circular structure called a T‑loop.

Select the answers that correctly describe telomeres and the actions of telomerase.

a. Telomerase is a specialized reverse transcriptase and its presence within eukaryotic cells indicates that an early eukaryote was infected by a retrovirus.
b. Inappropriate activation of telomerase can result in cellular immortality, one of the cellular changes implicated in the development of cancer.
c. Telomerase activity is turned off in most human cells, causing the telomeres to gradually shorten as the individual ages.
d. The T-loop protects the chromosome ends from degradation by nucleases and double strand break repair enzymes.
e. Telomerase activity results in a telomere that is completely double-stranded with no single stranded region that would be susceptible to nucleases.