FAMILY doctors will soon be able to tell within minutes which bacteria are
making you ill. Instant diagnoses like these could become routine, thanks to the
arrival of a computer add-on that can identify traces of DNA in blood
samples.
A British company has squeezed the apparatus needed to perform the polymerase
chain reaction (PCR), which 鈥渁mplifies鈥� DNA, into a unit small enough to slot
into an Apple Mac computer. Conventional PCR equipment occupies a sizeable chunk
of bench space, takes as long as two hours to give a result and can only be
operated by a specialist. The unit built by Molecular Sensing of Melksham in
Wiltshire fits into a disc drive bay. The company says it can be operated by
anyone and gives an answer in 8 to 15 minutes.
Doctors would simply put blood samples into 32 tiny wells in a disposable
鈥淒NA smart card鈥� no bigger than a credit card and pop it into a slot in the Mac.
Each of the wells contains different 鈥減rimers鈥濃�攕lugs of DNA designed to
hunt for genetic sequences unique to a particular target organism, for example
HIV. If the target DNA is present, the PCR unit makes millions of copies of it,
using free nucleotides鈥攖he building blocks of DNA鈥攊n the well.
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In most systems this 鈥渁mplification鈥� is detected by expensive dyes and
optical sensors, but in Molecular Sensing鈥檚 unit the computer spots the target
DNA by measuring an electric current flowing through the cell. When DNA strands
are formed, they liberate magnesium ions
(black dots in Figure) previously bound
to the free-floating nucleotides, increasing conductance.
鈥淚f they鈥檝e pulled this off, it will be terrific,鈥� says Alec Jeffries of the
University of Leicester, who pioneered genetic fingerprinting.