Figure 13.17 Telomeres and Telomerase (A) In most cells, the chromosome shortens with each replication because the nonreplicated DNA at the 3′ end of the template DNA is removed. However, in stem cells telomerase uses an RNA template to extend the telomere and prevent chromosome shortening. (B) Bright fluorescent staining marks the telomeric regions on these blue- stained human chromosomes.