Imaging a pair of human chromosomes in 3D

X-ray crystallography routinely yields the structures of proteins with 0.1-nm resolution. But how does one take a detailed look at something far bigger—a chromosome, say, or a cell nucleus? Such objects don’t crystallize, because no two individual examples have the same shape. And at a micron or so in size, they’re too thick for electron microscopy. X rays, of course, pass through whole animals, not just single cells. For the past decade, the practitioners of a technique called x-ray diffraction microscopy have been steadily improving their ability to image single, biological samples. XDM’s latest milestone is shown here. Made at the SPring-8 synchrotron in Sayo, Japan, this image of a pair of human chromosomes is the first from XDM in three dimensions. In general, determining a structure from a diffraction pattern requires obtaining the phases of the scattered photons. Those phases are lost when the diffraction pattern of a crystalline sample is detected; ingenious methods are needed to recover them. In XDM, the phases remain within the continuous diffraction pattern cast by the isolated sample. If the pattern is measured on a fine enough spatial scale, a computer algorithm can yield the structure by iterating between real space and diffraction space. The multiple exposures required to create this three-dimensional image damaged the sample and limited resolution to 120 nm. In principle, further improvements could push the resolution down to 10 nm. (Y. Nishino et al., Phys. Rev. Lett. 102, 018101, 2009 .) — Charles Day