Replication at the Ends of Chromosomes

Since DNA in chromosomes is a linear molecule, problems arise when replication comes to the ends of the DNA. Synthesis of the lagging strand at each end of the DNA requires a primer so that replication can proceed in a 5 to 3 direction. This becomes impossible at the ends of the DNA and 50-100 bp is lost each time a chromosome replicates. Thus, at each mitosis of a somatic cell, the DNA in chromosomes becomes shorter and shorter. Ultimately, after a limited number of divisions, a cell enters a nondividing state, called replicative senescence, which may play an important role in biological aging. 

Telomeres in human chromosomes consist of tandem repeats of the sequence TTAGGG. In most adult somatic cells, telomerase activity is very low or absent. Even in 

---