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Crystal structure of parallel quadruplexes from human telomeric DNA, molecular representation. Telomeric ends of chromosomes, which comprise noncoding repeat sequences of guanine-rich DNA, are fundamental in protecting the cell from recombination and degradation. Disruption of telomere maintenance leads to eventual cell death, which can be exploited for therapeutic intervention in cancer. Telomeric DNA sequences can form four-stranded (quadruplex) structures, which may be involved in the structure of telomere ends. structures. All four DNA strands are parallel, with the three linking trinucleotide loops positioned on the exterior of the quadruplex core, in a propeller-like arrangement. In the centre three co-ordinated metal ions.