The  enzyme  telomerase  can synthesize
   short   DNA   sequences  (telomeres)  at  the  3'  end  of  eukaryotic
   chromosomes, preventing chromosome shortening in germ cells.

   The  enzyme  adds  repeats  of  TTGGGG  sequences  that  fold  back on
   themselves by forming unorthodox G-G hydrogen bonds.

   The gap is filled by a DNA polymerase and ligase.

   The hairpin loop is then cleaved off, preserving the original duplex.

   This  allows  gametes  and malignant cells, as well as some "immortal"
   cultured cells, to continue duplicating the linear DNA.