Ans. is 'a' i.e. There are many cyclin inhibitors which prevent cell to enter into S phase in adult * After a fixed number of divisions, normal cells become arrested in a terminally non dividing state known as replicative senescence.How does normal cell count their divisions ??It is not entirely clear, how normal cells count their divisions but it has been noted that with each cell division there is some shortening of specialized structures called telomers at the ends of chromosomes.Telomers are short repeated sequences of DNA present at the linear ends of chromosomes that are important for ensuring the complete replication of chromosomal ends and protecting chromosomal termini from fusion and degradation.When somatic cells replicate a small section of the telomere is not duplicated and telomeres become progressively shortened.As the telomers become shorter, the ends of chromosome cannot be protected and are seen as broken D.N.A., this signals cell cycle arrest.The loss of telomere function leads to activation of p 53 dependent cell cycle checkpoints causing proliferative arrest or apoptosis.Germ cells continue to divide because in these cells telomere shortening is prevented by sustained function of the enzyme Telomerase.Telomerase is an enzyme (specialized RNA-protein complex) that maintains the length of the Telomer by nucleotide addition.Telomerase activity is expressed in Germ cells and is present at low levels in stem cells but it is usually absent in most somatic tissues.Therefore as cells age their telomers become shorter and they exit the cell cycle, resulting in an inability to generate new cells to replace damaged ones.One more question which crops upIf loss of telomerase is the basis of the finite life span of cells, how do cancer cells continue to divide indefinitely ?In cancer cells telomerase is reactivated and thus the telomers are not shortened.