Bad Boys, Whatcha Gonna Do When CSI Comes For You?

About a year ago, someone broke into my car, smashed a window and stole my radio. When I went to the police, they sent me to their forensics division to try and extract fingerprints. It was nothing like the high-tech CSI labs you see on television; two bored officers with old computers, huge paper piles on their desks, and a small suitcase-sized kits with some low-tech tools. Although the person broke into my car was smart enough to use gloves, there are times where DNA from the crime scene can frame a suspect.

One method to match a suspect’s DNA with a sample extracted from a crime scene is agarose gel electrophoresis. Here is the main concept: You get a DNA sample from the crime scene and from your suspects. You mix all the DNA samples (the crime scene sample and the suspects’ DNA) with some restriction enzymes that cut the DNA strands on a specific base sequence. Since every person’s genetic code is unique, this sequence will appear on different locations in the DNA, the strands will be cut on different locations resulting in different sized segments for each person. Next, you take all the samples and place them in a device filled with a special gel and connect it to an electric current. DNA is negatively charged so the segments will start migrating through the gel. Long segments have a greater charge but a greater mass, so the only thing affecting the speed of the segments is their size; short segments travel more quickly than long ones. Since every person’s sample will contain segments with different sizes, every person’s DNA segments will travel in different speeds creating a unique pattern in the gel. About an hour later, you stop the electric current and photograph the gel using ultra violet lighting. Now you can match the suspects’ unique patterns to the pattern generated from the crime scene sample and get your man. Note that this method did not require you to discover the exact DNA sequence of each of your suspects, and just used the fact that each person has a unique genetic code.

And what about identical twins? Well, although they have identical DNA (so this method can not distinguish between them), they do not have identical fingerprints.

p.s. If one Brazilian butterfly can cause a tornado in Texas, Iran should invest in insects not nukes.


Uri Kalish said...

Hi Zvik,

Actually, your wife and I talked about the Sanger method for determining DNA sequence and not the subject of this post which is gel electrophoresis for comparing DNA strands.

Maybe your wife should write a biology blog?

p.s. We’ll try to minimize the jumpy jumpy on the floory floory.

toomanytribbles said...

i love your blog! more, please...

Uri Kalish said...

Thanks :-)
I'll do my best...