Protective caps called telomeres sit at the ends of chromosomes. They keep chromosomes from sticking together and falling apart. Previously, scientists measured all telomeres together, as if they weighed a bag of apples and said, "the average apple weighs 150 grams." But one apple can be tiny and another huge. The same goes for telomeres: in humans, three chromosome ends are always shorter than the rest, and those are the ones more often linked to disease.
The new program TeloXplorer reads each chromosome end separately, as if opening a personal diary. It works with long DNA fragments, the kind a sequencer reads in one piece rather than in bits. The program finds the telomere, determines its boundary, and measures three things at once: length, repeat variants (nonstandard sequences within the telomere), and chemical marks (methylation, which affects gene activity).
A test on 232 people from different populations showed that African populations have higher repeat diversity than non-African ones. This is a trace of the ancient migration of people out of Africa. Now scientists can search for which chromosome ends predict cancer risk or the rate of aging. Back to the apples: instead of the average weight, we now know the weight of each apple in the bag, its variety, and even the marks on its skin.
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.