Human quadruplex telomeric DNA, molecular model. A telomere is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Telomeric ends of chromosomes, which comprise noncoding repeat sequences of guanine-rich DNA, are fundamental in protecting the cell from recombination and degradation. Atoms are represented as spheres and are colour-coded: carbon (grey), nitrogen (blue), oxygen (red) and phosphorus (yellow). One of three potassium ions is shown in purple.