MICROSCOPIC SEQUENCING OF DNA          
   
   If it were possible to take a picture of a DNA molecule, then the order
   of its bases might be easily determined. Dave Allison and Bruce Warmack,
   both of the Health and Safety Research Division, are taking this
   unconventional approach to sequencing DNA.                 
   
   "Researchers in our group are using a new class of microscopes to
   develop sequencing technologies," Warmack says, "and, for the first
   time, we're enjoying a good deal of success at imaging complete DNA
   molecules." Allison and Warmack are working with two scanning probe
   instruments, an atomic force microscope (AFM) and a scanning tunneling
   microscope (STM), to determine whether DNA strands can be imaged well
   enough to identify individual DNA base pairs.                    
   
   The AFM drags a fine metal probe (about 10 to 50 nanometers in width--a
   fraction of the diameter of a human air) across a sample bound
   chemically to a supersmooth surface. The AFM measures variations in the
   height of the probe as it is dragged across the surface of the sample
   using a laser focused on the probe's tip. After the AFM makes 400 passes
   across he sample at 1-nanometer intervals, the data it produces are
   converted to a computer-generated image.                   
   
   The STM, in much the same way as the AFM, uses an electromagnetic probe
   to scan a sample at narrow intervals and construct a computer image of
   the resulting data.       
   
   Sample preparation for the AFM requires little more than evaporating DNA
   out of a water solution onto a mica substrate, but a persistent problem
   with the STM has been developing a procedure for the routine preparation
   of samples. Because the substrate used with the STM must be an
   electrically conductive material, such as gold and mica, nonconductive
   materials with "sticky" surfaces have been ruled out. Recent progress in
   this area has been the development of an organic molecule that bonds to
   both the gold substrate and the DNA.                
   
   The group has yet to determine exactly how they will distinguish among
   the various bases that make up DNA once they are imaged, although they
   have some schemes in mind. "Up until now," says Allison, "the challenge
   has been just finding a way to routinely see the DNA."                  
   
   It appears their efforts are beginning to pay off; in October, the
   research group became the first in the world to image an entire genomic
   (gene-containing) molecule using an STM; in this case the molecule was
   a plasmid (a circular DNA molecule) from an E. coli bacterium.             
         
   
   "We are also looking forward to using the STM to observe biological
   reactions as they occur," Warmack says. "No other microscope can do
   that."
   
   
   ------------------------------------------------------------------------
   
   Please send us your comments.
   
   Date Posted:  1/10/94  (ktb)