(A) In mammalian cells, telomeres are structures at the ends of all linear chromosomes that consist of hexanucleotide repeats [(TTAGGG)n] and several associated protein complexes. The two components of telomerase are illustrated — TERC and TERT. NBS, Nijmegen breakage syndrome; MRE, meiotic recombination 11 homolog; L22, ribosomal L22 protein; TEP1, telomerase-associated protein 1. (B) The relationship of telomere length to cell division is not constant. There is a relatively constant loss of telomere length during normal cell division in the absence of compensatory mechanisms in most cells. However, telomere length can also increase with cell division in some lymphocyte subsets. Human T cells can partially sustain telomere length during cell division. Cellular stress can accelerate telomere loss rates.