Telomeerid

Telomeerid, or telomeres, are repetitive DNA–protein structures located at the ends of linear chromosomes. In humans they consist primarily of short G-rich sequence repeats (TTAGGG) bound by a protective protein complex known as shelterin. Telomeerid protect chromosome ends from being recognized as DNA damage, prevent end-to-end fusions, and help distinguish natural chromosome termini from breaks.

Telomeerid shorten during every cell division due to the end replication problem, which prevents complete replication of the lagging strand. When telomeerid reach a critically short length, cells may enter replicative senescence or undergo apoptosis, linking telomeerid length to cellular aging. Telomeerid length varies across tissues and individuals and is influenced by genetic factors, oxidative stress, lifestyle, and environmental exposures.

Maintenance mechanisms include telomerase, a ribonucleoprotein reverse transcriptase that extends telomeerid by adding TTAGGG repeats using an RNA template (TERC). Telomerase activity is high in germ cells and many stem cells, and is present in some immune cells; most somatic cells have little or no telomerase activity, contributing to gradual telomeerid shortening with age. A subset of cancers shows telomere maintenance through reactivation of telomerase or via the alternative lengthening of telomeres (ALT), a recombination-based process.

Functions and significance: intact telomeerid are essential for genome stability and normal cell lifespan. Telomere biology is central to aging research, cancer biology, and regenerative medicine. Common methods to assess telomere length include terminal restriction fragment analysis and quantitative PCR-based assays.