Fluorescent tag can be affixed to proteins or genetic structures of interest.

A glowing tag made of DNA can be used to label a single target molecule in a cell.

Today’s advanced microscopes allow physicists to image individual molecules, and even arrange them in simple patterns. But such equipment is not very useful for biologists, because biological structures are complex and easily damaged by microscope probes.

Mingjie Dai and Peng Yin at Harvard Medical School in Boston, Massachusetts, and their colleagues developed a technique that uses fluorescent DNA strands to label biological molecules in a cell. First the label’s DNA binds to a matching strand on a target molecule. Then a fluorescent-dye particle attached to the label glows under illumination. This ‘blink’ switches on a laser, which triggers a chemical element embedded in the DNA label to form a tight bond with the target molecule.

The researchers tagged nanometre-scale synthetic DNA structures with a 65% success rate. They also used the technique to tag protein filaments called microtubules inside cultured cells.

The tags could be used to ferry activating molecules to cell-membrane proteins called ion channels, which are crucial for transmitting nerve-cell messages. This would reveal the workings of an individual channel in a cell, the authors say.