A Close Look at Heredity

One of the pictures looks like a Scottish meadow sprinkled with sleeping sheep. In another, more enlarged, the curious objects that looked at first like sheep actually seemed to resemble four tufts of cotton joined at a central point.

The photographs, which were published in a recent issue of Nature, are actually the most revealing and undistorted views man has ever had of the surface structure of heredity-bearing human chromosomes.

The chromosome closeups were made by German scientists at the University of Muenster, using the recently developed scanning electron microscope. Unlike the conventional electron microscope, which forms an image by passing an electron beam through extremely thin slices of a specimen, the scanning device plays a fine electron beam back and forth across the surface of the object being examined. Electrons knocked out of the surface of the specimen by the scanning beam are collected and converted into signals that are projected on a television screen in the form of a picture.

By tilting their chromosome specimens, which were taken from a human white blood cell, the German scientists were able to get a side view and measure their thickness--about four-millionths of an inch at their thinnest, center portion and ten-millionths at the thickest part of their "limbs." In Britain, where scientists at St. George's Hospital Medical School are also using scanning electron microscopes to examine chromosomes, the resulting photographs have suggested that chromosomes have an underlying fibrous structure. From these and other scanning electron closeups, scientists hope eventually to gain new insight into the complex processes by which chromosomes and their constituent genes control heredity.

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