molecular beacon
An ordinary probe glows whether or not it has found its target, so you have to wash away the unbound probe before you can tell. A molecular beacon is a cleverer probe that is dark until it binds its target and only then lights up — a self-reporting switch that needs no washing step, which is what makes real-time detection possible.
A beacon is a single strand of DNA designed with two tricks. First, its two ends are complementary to each other, so when the target is absent the strand folds into a little hairpin (a stem-loop), bringing its ends together. Second, one end carries a fluorescent dye and the other a 'quencher' molecule. While the hairpin is closed, the quencher sits right next to the dye and absorbs its light, so the beacon is dark. The loop part of the hairpin is complementary to the target sequence. When the target is present, the loop base-pairs with it, forcing the hairpin open; this pulls the dye away from the quencher, and the dye is now free to fluoresce. Target present, light on; target absent, light off.
Molecular beacons are widely used in quantitative PCR and rapid diagnostics, because the fluorescence appears only in proportion to how much specific target gets made, in real time, in a sealed tube. Their value is specificity-plus-self-reporting: a beacon is sensitive enough to distinguish even a single-base mismatch, since an imperfect match cannot open the hairpin well. The trade-off is that they must be carefully designed (the stem and loop have to be balanced), and like any probe, a beacon only ever reports the one sequence it was built to recognize.
In a qPCR, a beacon designed against the target stays dark while folded. As each cycle makes more target, more beacons open and glow, so the tube brightens in step with how much specific product has formed — letting the machine track the target in real time without ever opening the tube.
Hairpin closed = dark (quenched); opened by target = glowing.
A beacon is dark not because it is broken but because, when folded, a quencher sits beside the dye and soaks up its light — binding the target pulls the two apart, so the glow is a direct readout of target binding, no wash step needed.