Telomere

Telomeres are regions of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. They are a widespread genetic feature most commonly found in eukaryotes, protecting the terminal regions of chromosomal DNA from progressive degradation and ensuring the integrity of linear chromosomes by preventing DNA repair systems from mistaking the ends of the DNA strand for a double-strand break.

The existence of a special structure at chromosome ends was independently proposed in the late 1930s by Hermann Joseph Muller, studying fruit flies, and Barbara McClintock, working with maize. Muller, building on earlier work on chromosome rearrangement, inferred that a terminal gene functioned to seal breaks in radiation-damaged chromosomes and coined the term "telomere" from the Greek for "end part." In the early 1970s, Soviet theorist Alexey Olovnikov recognized that chromosomes could not completely replicate their ends, a problem known as the "end replication problem." He suggested that DNA sequences are lost with each cell replication until a critical loss halts division. His theory of marginotomy proposed that tandem repeats at telomere ends create a buffer determining how many divisions a cell clone can undergo, and predicted that a specialized DNA polymerase could extend telomeres in immortal tissues like germ lines, cancer cells, and stem cells. He also noted that organisms with circular genomes, such as bacteria, lack this problem. In the mid-1970s, Elizabeth Blackburn discovered the unusual nature of telomeres, finding simple repeated DNA sequences composing chromosome ends. The end replication problem arises because DNA polymerase can only synthesize DNA in one direction and requires a primer; on the lagging strand, the last primer cannot be replaced by DNA, leaving a gap. Telomeres, being non-coding repetitive sequences, act as buffers, progressively degrading instead of vital coding sequences. This problem is exclusive to linear chromosomes; most prokaryotes with circular chromosomes lack telomeres, though a few bacterial species with linear chromosomes possess structurally different telomeres, such as protein-bound ends or hairpin loops. At the very end of a telomere, a single-stranded overhang can invade the double-stranded region, a structure coordinated by the shelterin protein complex.