Telomeres and subtelomeres : new insights into the chromatin structures and functions of chromosome ends

Eukaryotic cells possess linear chromosomes. The telomere, a specialized heterochromatin structure, exists at the ends of linear chromosomes. Telomeres contain DNA with an evolutionarily variable repeat sequence: [TG1-3]n in budding yeast, [TTACAG2-5]n in fission yeast, [TTTAGGG]n in most plants and [TTAGGG]n in mammals. Telomere DNA is composed of two parts: double-stranded (ds) DNA in the inner chromosome regions and 3'-protruding G-rich single-stranded (ss) DNA at the very ends of chromosomes. The total length of telomere DNA also varies: - 300 bp in yeasts, - 15 kbp in human germ cells, and - 5 - 13 kbp in human somatic cells. Telomere DNA serves as a platform for various telomere-binding proteins. The G-rich ssDNA recruits a telomeric ssDNA-binding protein complex, including Pot1, which protects chromosome ends and recruits telomerase for telomere elongation.