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Direct sequencing means that the letters of the genetic code are read
directly, as if with a magnifying glass. A DNA or RNA strand has a
diameter of only two nanometers, so the magnification must be
correspondingly powerful. Deckert�s team uses an atomic force
microscope to achieve this degree of magnification. Steered by the
microscope, a tiny, silvered glass tip moves over the RNA strand. A
laser beam focused on the tip excites the section of the strand being
examined and starts it vibrating. The spectrum of the scattered light
(Raman spectrum) gives very precise information about the molecular
structure of the segment. Each genetic �letter�, that is, each of the
nucleic acids, vibrates differently and thus has a characteristic
spectral �fingerprint�.
The direct resolution of individual bases has not been attainable, but
is also not necessary. The tip only has to be moved over the RNA
strand at intervals corresponding to about the base-to-base distance.
Even if the measured data then consist of overlapped spectra from
several neighboring bases, the information can be used to derive the
sequence of the RNA.
If this method, known as tip-enhanced Raman spectroscopy (TERS), can
be extended to DNA, it could revolutionize the decoding of genetic
information. Previous methods for sequencing DNA are highly complex,
work indirectly, and require a large sample of genetic material. In
contrast, the TERS technique developed by Deckert directly �reads� the
code without chemical agents or detours. It also requires only a
single strand of DNA. �DNA sequencing could become very simple,� says
Deckert, �like reading a barcode at the supermarket.�
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