Telomeres define the chromsosome ends. The two criteria which identify a telomere are:
1) It must lie at the chromosome end.
2) It must confer stability on a linear molecule.
Several telomeric sequences are known from various eukaryotic organisms. Each telomere consists of a long series of short, tandemly repeated sequences. The telomere construction follows a universal principle. All of the telomeric repeat units are written in the general form Cn(A/T)m, where n>1 and m is 1-4.
Plants have C3TA3 and humans have C3TA2 basic telomere repeat sequences/units. The number of copies of the basic repeat unit in telomeres varies across species, from chromosome to chromosome within a species, and even on the same chromosome at different stages of the life cycle.
In some species, the telomeres terminate with a single stranded region of the DNA strand with the 3′ end (so-called overhang). Terminal bases of this single-stranded end exhibit unique patterns of methylation that probably contribute to the formation of a unique hairpin or folded structure at the tip of the telomeric DNA. Additional repetitive DNA sequences-referred to as telomere-associated sequences-are present adjacent to the telomere.