THE unique temperature at which a given fragment of DNA unwinds could be used to speed up identification of suspects at police stations and allow hair samples and other evidence to be profiled at the scene of the crime.
Forensic scientists create a DNA profile of an individual from a sample of saliva or hair by extracting the DNA and making multiple copies of the short tandem repeats, or STRs, at 13 signature regions on the human genome. STRs are typically two to five base pairs long and are repeated many times head to tail, as in GACT-GACT-GACT. What differs between individuals, and is key to a DNA fingerprint, is the number of these repeats at the key points, and hence the length of the strands. When you look at all 13 points together, each person’s eprofile will be different.
The length of a DNA strand is measured by drawing the sample through a polymer gel using a high voltage – the longer the strand, the slower it moves through the gel. But this process usually takes about a day and has to be done by a forensic scientist using a large, delicate machine, so scene-of-the-crime analysis is not possible.
That can delay investigations as suspects are often released before the DNA results come through, says Stuart Hyde, who heads DNA operations for the Association of Chief Police Officers in the UK. “The sooner you can get a DNA profile done, the sooner you can take action.”
So Tom Brown, a chemist at the University of Southampton, UK, is working with London-based DNA analysis company LGC to develop a technique that instead uses the temperature at which the STRs unwind to deduce their length. The process can be carrid out in one tube, is potentially faster and does not require forensic expertise, he says.
Brown created a synthetic stretch of DNA and tagged it with a fluorescent molecule that emits a bright green colour when it binds to another strand of DNA, but only a weak colour when unbound. He then mixed the labelled synthetic strand with the STR strands.
DNA analysis can take so long that suspects are released before the results
By heating and cooling the mixture, he forced the STRs to first unwind and then to rebind to the fluorescently labelled strands instead of their original partners.
When they were heated again slowly, he recorded the temperature at which green light faded, signifying that the double strand had separated.
As the chemical forces holding the two strands together depend on the length of the strands, longer strands unwind at higher temperatures. An STR with seven repeats unwinds at 53 °C, one with eight at 57 °C and one with nine at 60 °C. However, as STR strands get longer, the temperatures at which they unwind are very close together and hence difficult to resolve. Brown hopes to space these temperatures further apart. By adding nitrogen-containing chemical groups to bases at the 16th position and higher on the synthetic strand, he can increase the contribution made by each base to the attractive forces holding the two strands together.
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