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Published: Saturday, 8/3/2002

Offenders finding it harder to hide as DNA detectives hone their skills

BY JENNI LAIDMAN
BLADE SCIENCE WRITER

Don't smoke. Don't eat. Don't spit. And definitely, whatever you do, don't bleed.

That's just the beginning of a very long list of things the perfect criminal should avoid.

Break these rules, and a lawbreaker might as well sign his or her work.

Yesterday, a 36-year-old Temperance man was indicted, in part, with DNA found on chewing gum at the scene of a burglary. And if that sounds far fetched, how about DNA identifications made from a bit of slime on a toothpick, or a bite mark on a pizza?

Roger Kahn, deputy supervisor of laboratories for the Ohio Bureau of Criminal Investigation, said BCI has used these things and more to help police departments make arrests.

It's not our genes that turn state's evidence in police laboratories, Dr. Kahn said. Instead it's the part of your DNA often referred to as “junk.''

In the nucleus of every cell of your body - except your red blood cells, which have no nucleus - there are 46 bundles of information called chromosomes. The chromosomes are made of chemicals called nucleotides.

In total, you have about 6 feet of DNA per cell. You can think of the nucleotides as beads on this 6-foot string. There are some 6 billion of these beads. They come in four different types: Thymine, Adenine, Guanine, and Cytosine. Geneticists generally refer to them by their initials: T, A, G, and C.

It's the specific arrangement of these A's, T's, G's and C's that make up your genes. So you might have a gene that reads ATAGGGGATACCCAAATG. (Actually, they're usually a lot longer.) This code is translated to make a protein. The protein then combines with other proteins to get something done in your cell.

But in humans, there are plenty of A's, T's, G's, and C's that code for nothing. They serve no biological purpose that anyone has managed to identify yet. But they're mighty useful in crime labs.

With a mere 100 cells - a scratch of material too small to see - analysts get to work. First, they wash the DNA out of the cells, then “amplify” the number of DNA copies until there are enough strands of DNA to read reliably.

Researchers look at 13 specific portions of DNA. Each of these portions contains 100 to 400 nucleotides - or beads. Specifically, the experts are interested in parts of our code where the same four letters repeat over and over again. Maybe the section reads AATC, AATC, AATC, and so on.

Scientists want to know how many times this pattern repeats. Although you and the guy next door may have the same number of repeats on one section of DNA, the chances of any two people having the same number of repeats on all 13 sections of DNA studied are something like 1 in a quadrillion. The analysts also look for a Y chromosome. If they find one, they know this gene donor is a male.



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