Telomeres are sections of DNA at the end of each chromosome that serve as a cap to the genetic material.
Their purpose is critical to the life of the cell in that they serve as protective buffers that keep the ends of the chromosomes from becoming attached to each other or rearranging. If cells divided without telomeres, they would lose the necessary information at the end of each chromosome. In this way, telomeres prevent chromosomal fraying.
Every time a cell replicates, its telomere will become shorter, eventually causing cell death once the telomere attrition has reached a critical length. It is estimated that human telomeres lose about 100 base pairs from their telomeric DNA during each mitosis (cell division). At this rate, after approximately 125 mitotic divisions, the telomeres would be completely gone, which is why normal cells will eventually die after healthy division. Shorter telomeres imply a shorter life span for a cell, essentially giving it a finite lifespan, depending on the number of cell divisions left within each telomere.
Cells can maintain the length of their telomeres with an enzyme called telomerase, which adds genetic material at the end of the DNA strand, thus lengthening the number of times it can replicate, which ultimately prolongs the life of the cell. It is not active in most cells, but is active in stem cells, germ cells, hair follicles and most cancer cells.
Shorter telomeres have been associated with metabolic abnormalities, obesity and several degenerative diseases including cancer, dementia and cardiovascular disease. In vitro studies have shown that telomeres are highly susceptible to oxidative stress, which will shorten telomere length and enhance cellular aging.