Telomerase
The function of telomeres is to avoid the loss of important DNA from chromosome ends. Every time the chromosome is copied 100–200 meaningless nucleotides are lost, which causes no damage to the organism's DNA. So, the telomeres are shortened after each replication cycle, and then are restored to their proper length by telomerase. Telomerases carry their own RNA molecule, which is used as a template to elongate the telomeres.