As people age, their cells divide less frequently and eventually stop dividing altogether. This process is partly controlled by the length of telomeres, which are repetitive DNA sequences at the end of each chromosome. Telomeres protect chromosomes from damage and degradation, but each time a cell divides, the telomeres get shorter. When they get too short, the cell can no longer divide and dies. This may lead to the deterioration of tissues and organs and is a major contributor to aging.