3.2. MECHANISM FOR TELOMERE SHORTENING
The mechanisms of DNA replication in linear chromosomes are different for each of the two strands (called leading and lagging strands). The lagging strand is made as series of discrete fragments, each requiring a new RNA primer to initiate synthesis. The DNA between the last RNA priming event and the end of the chromosome cannot be replicated because there is no DNA beyond the end to which the next RNA primer can anneal, thus this gap cannot be filled in (this is referred to as the "end replication problem"). Since one strand cannot copy its end, telomere shortening occurs during progressive cell divisions. The shortened telomeres are inherited by daughter cells and the process repeats itself in subsequent divisions.