What is the role of telomeres in protecting chromosome ends and how are they linked to aging?

• A. Telomeres cap chromosome ends to protect genes during replication; they shorten with each cell division and are linked to aging; telomerase maintains them in some cells.
• B. They are replication origins that duplicate DNA.
• C. They are centromeres essential for spindle attachment.
• D. They code for aging proteins directly.

Answer: (A)

Telomeres sit at the ends of chromosomes and act as protective caps of repetitive non-coding DNA. They guard important genes from being lost during DNA replication and prevent end-to-end fusions between chromosomes. But because DNA polymerase cannot fully replicate the very ends of linear chromosomes, each cell division trims a bit off the telomeres. Over many divisions, this shortening reaches a critical length, at which point the cell typically enters senescence or undergoes programmed cell death, linking telomere length to cellular aging and tissue aging. Some cells, like germ cells and many stem cells, express telomerase, an enzyme that adds telomeric repeats back onto the ends, helping maintain telomere length and extend their capacity to divide. Most ordinary somatic cells have little telomerase activity, so their telomeres shorten with age, contributing to aging processes.