This month, the UK police force collected its millionth DNA sample. But DNA profiling cannot give an indication of the age of a biological sample, and though there are techniques that can, they are not reliable enough to be used as evidence in court, says Clifton Bishop, director of the forensic and investigative science programme at West Virginia University in Morgantown. All require a relatively large sample, only work on blood, and are affected by environmental conditions. Bishop and his colleagues have developed a test based on two forms of RNA.
They discovered that messenger RNA breaks down faster than ribosomal, or rRNA, so it is possible to work out when a sample was deposited by measuring how much of each type is present (Forensic Science International, DOI: 10.1016/j.forsciint.2004.04.071). The test works with just one microlitre of blood and might even be sensitive enough to tell investigators when a fingerprint was left, though the team have not yet tried this.
Bishop is not sure why rRNA breaks down more slowly, but he believes it may be because rRNA is protected by proteins that bind it into ribosomes- part of the cell's machinery for producing proteins. The team has shown the method can be used on samples up to 150 days old, but extensive research is needed before it can be used in court.
For example, the team do not know how the technique is affected by changes in environmental conditions such as temperature and humidity. They will also need to find out exactly how it works. "For this to be bombproof in court, I think the mechanism needs to be explained," says Nigel Watson, an expert in forensic DNA analysis at the University of Strathclyde in the UK.
This article appears in New Scientist issue: 27 November 2004
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