Watch a rigged game of tug-of-war inside the egg cells of mammals

There’s a tug-of-war going on inside reproductive cells. When females make an egg, they can only donate half of each of their chromosome pairs (the other half comes from dad). But how does the cell choose which one? Using fluorescently tagged proteins to watch cell division in mouse egg cells, researchers have discovered that repetitive sequences of DNA near the center of chromosomes—known as centromeres—can either be “strong” or “weak,” they report in Science. Strong centromeres have more places for the so-called spindle fibers that pull chromosome copies apart in cell division to attach, but they don’t operate by brute force. Instead, strong centromeres get what they want by cheating: If their chromosome is oriented in such a way that it’s likely to get discarded rather than reach the egg, the strong centromere lets go of its spindle fibers, causing its counterpart centromere to release its hold, too, and the game starts over. This way the strong centromere can re-orient itself toward the side that will become the egg. So what makes some centromeres stronger than others? Researchers say stronger centromeres are “bigger”—they repeat the same DNA sequence many more times than weak centromeres might—and they are more sensitive to which direction the spindle fibers are pulling them. That makes them more likely to let go when things aren't going their way, and thus rig the tug-of-war.