New DNA test helps police get their man among many suspects

New Worlds: Yissum has introduced a method for forensic profiling even when a crime scene contains DNA from more than two individuals.

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January 16, 2011 03:56
4 minute read.
Strings of DNA

Strings of DNA 311. (photo credit: Wikipedia images)

 
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At crime scenes, DNA from a suspect, the victim and other people are often mixed, thus confusing forensic scientists. Now the Hebrew University’s technology transfer company Yissum has introduced a method for forensic profiling even when the DNA is not “pure.”

Forensic DNA science has advanced significantly in the past decade, and today constitutes the mainstay of forensic science. All serious crime scenes are routinely inspected for DNA evidence, and many cases are solved by matching the DNA found to that of a suspect. But often a crime scene will contain complex DNA mixtures – usually from more than two individuals – making the detection of a particular DNA profile challenging. In such cases, current police practices overlook valuable information that may aid in solving crimes.

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The new technique, developed by HU geneticist Prof. Ariel Darvasi in collaboration with Lev Voskoboinik from the Forensic Biology Laboratory of the Israel Police (as part of his MSc thesis) consists of checking the DNA mixture and the suspected individual’s DNA for 1,000 to 3,000 single “letter” changes (polymorphisms) that are relatively rare in a given population. Their work was recently published in Forensic Science International: Genetics.

“DNA profiling has become an indispensable tool in crime investigations, and is used both to convict and acquit. It is therefore of great importance that crime investigators be able to determine whether a suspect is indeed connected to the crime scene,” says Yissum CEO Yaacov Michlin. “The novel DNA profiling method will enable police to investigate even complex crime scenes containing DNA from multiple individuals and discern with high reliability the presence or absence of DNA from a specific suspect.”

Human DNA consists of long chains comprising various combinations of four small molecules, referred to as A, G, C and T. Forensic DNA analysis is based on the fact that the exact DNA “script” of each human is unique. The human genome, our DNA, comprises three billion letters, 99.9% of which are identical in any two random individuals. Yet the remaining 0.1%, corresponding to about 30 million possible changes arranged in various combinations, enables each of us to carry our own DNA “barcode.” Thus, in a particular position along the DNA chain, one person will have the letter A, whereas another will carry the letter G.

Current DNA profiling methods check a few such “polymorphic” sites, and if they all match a suspect’s DNA exactly, one can establish with extremely high certainty that the DNA found at the scene belongs to that suspect.

But if the DNA found is a mixture from more than two individuals – which is often the case – current DNA analysis methods are inefficient. The invention can establish if a suspect’s DNA is present in a DNA mixture from up to 10 individuals.



The procedure involves analyzing the DNA mixture for 1,000 to 3,000 polymorphic sites where DNA letter changes are rare (i.e. found only in about 5% to 10% of the population). Thus, a suspect will carry about 100 to 200 of these rare letter changes. If the DNA mixture collected at the crime scene does not include the suspect’s DNA, there’s almost no chance that all of his/her 100 to 200 specific and rare letters will be present. On the other hand, if all these rare polymorphisms are present, then the only logical explanation is that the mixture included the suspect’s DNA.

NANOSIZE SENSORS

Electro-mechanical sensors tell the airbag in your car to inflate, or rotate your iPhone screen to match your position on the couch. Now a research group at Tel Aviv University’s engineering faculty is making the technology even more useful. Prof. Yael Hanein, Dr. Slava Krylov and their doctoral student Assaf Ya’akobovitz have set out to make sensors for microelectromechanical systems (MEMS) much more sensitive and reliable than they are today. And they’re shrinking their work to nanosize.

More sensitive sensors means more thrilling videogames, better-functioning prosthetic limbs, cars that can detect collisions and dangerous turns before they occur, and – in the defense industry – missiles that can reach a target far more precisely.

Able to sense the movement of individual atoms, the researchers’ new MEMS sensing device uses small carbon tubes about one-billionth of a meter long. Creating these tiny tubes using a process involving methane gas and a furnace, Hanein has developed a method that enables the atoms to “arrange themselves” on the surface of a silicon chip and accurately sense tiny movements and changes in gravity.

In the device the team developed, a nanometer-scale tube is added onto much larger micrometer-scale MEMS devices.

Small deformities in the crystal structure of the tubes register a change in the movement of the nano object, and deliver the amplitude of the movement through an electrical impulse. “It’s such a tiny thing,” she says. “But at our resolution, we are able to feel the motion of objects as small as a few atoms in size.

Originally developed mainly for the car industry, miniature sensors are all around us, she adds. “We’ve been able to fabricate a device where the nano structures are put onto a big surface – and they can be arranged in a process that doesn’t require human intervention, so they’re easier to manufacture.

We can then drive these nano-sensing tubes to wherever we need them to go, which could be very convenient and costeffective across a broad spectrum of industries.”

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