Definition of a Telomere
A telomere is a region of repetitive nucleotide sequences (e.g., TTAGGG in humans) located at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Think of telomeres as the plastic tips (aglets) on shoelaces, preventing them from fraying.
Function in DNA Replication and Cell Division
During each round of DNA replication, the enzymes that duplicate DNA cannot copy the very end of the lagging strand. This means a small portion of the telomere is lost with every cell division. Telomeres act as a buffer, preventing the loss of vital genetic information by sacrificing their own non-coding repetitive sequences instead. Once telomeres become too short, cells can no longer divide and enter a state called senescence.
Practical Example: The Hayflick Limit
A classic example of telomere function is the Hayflick Limit. Leonard Hayflick discovered that normal human cells can only divide a limited number of times (typically 40-60 divisions) before reaching senescence and dying. This limit is primarily attributed to telomere shortening. Cancer cells, however, often reactivate an enzyme called telomerase, which rebuilds telomeres, allowing them to divide indefinitely.
Importance in Aging and Disease
Telomere length is considered a biomarker of cellular aging. Shorter telomeres are associated with older biological age and an increased risk of age-related diseases like heart disease, diabetes, and certain cancers. Research into telomeres and telomerase inhibitors/activators is a significant area in anti-aging research and cancer therapy development.