At the end of every chromosome sits a cap made of repeating DNA letters: the telomere. For a long time it was measured as an average length across all chromosomes, as if judging a library by the average thickness of its books. But in humans, three ends, 17p, 20q, and 12p, are consistently shorter than the rest, and they are the ones more often linked to genome damage. The average number hid these differences.
The new tool TeloXplorer reads each chromosome end separately, like an archivist who files scrolls not on a common shelf but by name. It works with long reads, DNA fragments that a sequencer delivers whole rather than in pieces. The program finds telomeric repeats, determines the boundary between the telomere and the adjacent region, and then measures three parameters at once: length, repeat variants, and the level of chemical marks (methylation).
Testing on the HG002 cell line showed that telomere lengths range from 0.66 to 9.61 thousand base pairs depending on the chromosome end and the parental version. Family trios confirmed that variant repeats are inherited according to Mendel's laws, and methylation is tied to TAR1 repeats. Analysis of 232 people revealed that African populations have higher variant diversity than non-African ones, a trace of the ancient out-of-Africa migration.
Telomeres can no longer be described by a single average length. The inherited architecture of chromosome ends sets the framework within which changes during aging and disease occur. For researchers of aging and cancer, this means a shift from aggregate measurements to per-end ones. Already today, one can search for which chromosome ends predict cancer risk or the rate of aging, if the tool is applied to cohorts with clinical data.
Let's return to the library: now we know that every book has its own thickness, its own typeface, and its own marginal notes. These records are inherited, but they are read differently in different families. To read them is to understand why some books decay faster than others.
Human telomeres contain TTAGGG repeats, but in Arabidopsis they are slightly longer: TTTAGGG. And in yeast, the repeats are so irregular that they are described as TG₁₋₃, that is, from one to three guanines in a row.
In Douglas Adams's novel The Hitchhiker's Guide to the Galaxy, the characters search for the question to the answer "42". Telomeres also store encoded history, but instead of the number 42, they contain repeat variants that can only be read with long reads and the right tool.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.