University of Illinois researchers have developed a technique for imaging cells under an electron microscope that yields a sharper image of the structure of chromatin, the tightly wound bundle of genetic material and proteins that makes up the chromosomes.
Their findings appear in Nature Methods.
Scientists have known for more than a century that proteins, such as histones, aid in packing DNA into the nucleus of a cell. Human cells contain 2 to 3 meters of DNA, which must be kinked and coiled enough to fit into a region 1/10 the width of a human hair.
Despite the use of powerful, high-resolution imaging techniques such as electron microscopy, the mechanism by which this chromatin packing occurs remains a mystery. The densely coiled chromatin fibers are very difficult to visualize, and little is known about how they condense during cell division, or unwind to allow gene expression.
In developing their method, the Illinois team tackled a key difficulty in imaging cells using electron microscopy. Traditional studies fix the cells with potent chemicals (called fixatives) to preserve their structure for viewing under a microscope. But standard fixation methods interfere with another step in the imaging process: the use of tagged antibodies to label key components of the cells.
These antibodies, which target and latch on to specific proteins in the cell, can be tagged with fluorescent labels for detection in light microscopy, or with metal particles (gold, in this case) for electron microscopy.
If you fix the cells first, you have a dramatic drop in the efficiency of these immunochemical reactions, said Igor Kireev, a visiting scientist in the department of cell and developmental biology and lead author of the paper.
And if your target is inside the condensed chromatin, the antibodies have no way to penetrate.
Instead of fixing the cells before staining with antibodies, the researchers first
|Contact: Diana Yates|
University of Illinois at Urbana-Champaign