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.