The end of a chromosome is like an empty parking spot: it has to be occupied, or the cell will decide the chromosome is broken and start repairing it. In yeast, that spot is taken by the protein Cdc13. Researchers used to think it showed up as a pair right away, like two friends arriving in the same car. But new experiments show that one arrives first, and then the second pulls up next to it.
The scientists attached two glowing labels to the DNA. When the protein binds, the labels move apart and the light changes. That's how they saw it: first one protein sits on the DNA, then a second one appears. If the protein can't connect with a partner, it stays alone and gets washed off easily. But when there are two of them, they hold on tight, even with plenty of salt around.
It's a bit like security work: first one guard checks out the place, then calls a partner so the two of them can stand watch together. Now the scientists want to see exactly how the two proteins hold on to the DNA, and how other proteins help them. But one thing is already clear: parking at the end of a chromosome happens in two steps.
The dissociation constant of the Cdc13 complex with telomeric DNA is about 1 nanomolar, which means that at a protein concentration of just one molecule per billion water molecules, half the DNA is already bound. For comparison, the typical dissociation constant of many enzymes with their substrate lies in the micromolar range, that is, a thousand times weaker.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.