telomere

A telomere is like the plastic tip on a shoelace: a protective cap at each end of a chromosome that keeps it from fraying and from sticking to its neighbors. Without these caps the cell might mistake the natural ends of chromosomes for broken DNA in need of repair.

Telomeres are made of a short DNA sequence repeated many times over (in humans, the unit TTAGGG), together with proteins that fold the end into a protected structure. They solve a practical problem of copying: the machinery that duplicates DNA cannot finish the very tip of each strand, so a little is lost each time a cell divides. The telomere is expendable buffer sequence that absorbs this loss, sparing the genes further in.

Because telomeres shorten with each division, they act as a kind of counter that limits how many times most body cells can divide; when they grow too short the cell stops dividing or dies. An enzyme called telomerase can rebuild telomeres and is active in stem cells, germ cells and many cancers, but is largely switched off in ordinary tissues. Telomere biology connects to aging and to cancer, though the relationships are complex and should not be reduced to a single simple cause.