Telomeres

Telomeres

Eukaryotic chromosomes also have a specialized structure at each end. Chromosome ends are called telomeres and in linear chromosomes it is important that these ends be protected from degradation by cellular enzymes. Cells have numerous enzymes that will degrade nucleic acids, often as a defense against viral infection, so it important to protect their own chromosomes.

At the end of the chromosome, one of the two DNA strands consists of a single-stranded overhang of a few hundred up to thousands of nucleotides in length. This is the strand that has a free 3' OH at the end. This overhang consists of multiple repeats of a short that differs from one species to another. In vertebrates, this repeat sequence is TTAGGG (written T₂AG₃).

This single-stranded overhang forms a particular structure called a t-loop by looping back and displacing one of the strands in the double-stranded molecule (which is possible due to the repeat sequence). The structure is stabilized by a number of proteins including POT (Protection of Telomere), TRF1 and TRF2.

A nice diagram of this can be found here at Nature.