This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Telomerase is ? 
Select and explain. 
A. DNA dependant RNA polymerase, B. RNA dependant DNA polymerase, C. RNA dependant RNA polymerase, D. DNA dependant DNA polymerase
B: RNA dependant DNA polymerase
Answer: B: RNA dependant DNA polymerase 
Explanation: Telomers and TelomeraseTelomers are sho repeated sequences of DNA present at the end of eukaryotic chromosomes.In normal cells, telomeres get shoened in each replicated cycle. After the telomers get shoened by a critical length, cell stops dividing and enters into Go phase (senscence).However, germ cells, cancer cells and stem cells have an enzyme telomerase (telomerase activity) which prevents telomer shoening.Thus telomers are not shoened in these cells, and therefore these cells have ability to self replicate extensively.Telomerase is a reverse transcriptase (RNA dependent DNA polymerase) and is responsible for telomer synthesis and maintaining the length of telomers (replication of end of chromosome).Thus, telomerase provide longetivity to the cells which contain this enzyme.Telomerase is absent from most of the somatic cells and hence they suffer progressive loss of telomers and they exit the cell cycle.