Telomerase is present in most human cancers, and proliferative stem cells including germline cells. Telomerase plays an essential role in tumorigenesis by maintaining/elongating telomeric DNA, and thus preventing the telomere shortening that results in replicative senescence. Understanding telomerase action in vivo has important implication for both cancer and aging, but there are not robust methods for monitoring telomerase action. By combining a series of cell biological and biochemical approaches, and taking advantage of the enzyme DSN that specifically cuts double-stranded DNA and releases the telomeric overhangs, we have developed a method to monitor telomerase action during one cell cycle. Here, we describe this method using Hela carcinoma cells as an example.