After a fixed number of divisions, normal cells becomes arrested in a terminally nondividing state. How does the normal cells count their divisions i.e. How, do the cells find out that now they do'nt need to divide further.Cells achieve this through telomer shorteningTelomers are short repeated sequences of DNA present at the linear ends of chromosome that are important for ensuring complete replication of chromosome ends. It consists of many arrays of TTAGGG repeats at tellninal of 3' ending strand. It confers stability to chromosomes. With each successive division there is some shortening of the telomers. Once the telomers are shortened beyond a certain point the ends of the chromosome are seen as broken i1 NA, which signals cell cycle arrest. The broken NA leads to activation of p53 dependent cell cycle check points causing proliferative arrest or apoptosis. Thus telomere shortening functions as a clock that counts cell division. How do the germ cells keep on dividing throughout their life??? In germ cells telomere shortening is prevented by the sustained function of the enzyme telomerase. Telomerase is a reverse transcriptase and is responsible for telomer synthesis and maintaining the length of telomer. It is RNA dependent DNA polymerase. It has one segment that is complimentary to TTAGGG repeat and is used as template for the replication of telomeric sequence. This enzyme is absent from most somatic cells and hence they suffer progressive loss of telomers and they exit the cell cycle. Telomerase is present in Germ cells thus explaining the ability of these cells to self replicate extensively. If loss of telomere is the basis of finite life span then how do cancer cells continue to divide actively???? In cancer cells telomerase activity is reactivated. Telomerase activity has been detected in more than 90% of human tumours. Thus telomerase activity and maintenance of telomere length are essential for the replicative potential in cancer cells.