Moonbear said:
Well, red blood cells don't contain a nucleus, so no DNA there, and there really shouldn't be much blood in urine unless the "perp" has something seriously wrong with them. I'm not sure if there would be a sufficient concentration of cells of any kind in the urine to find them in a sample collected off a surface. There'd probably be far more outside contaminants picked up with it, making it rather difficult if not impossible to sort out the cells of the "perp" from the cells of whatever else is on that car surface (bacteria, bird poop, bug splatters, the owner's sloughed skin cells, pollen, etc.), and things like road salts and oils would probably make it really difficult to clean up the sample. This would be quite different from collecting urine in a sterile container. Maybe if there was an obvious puddle left behind, something could be done with that?
Normal urine can still contain white blood cells, more so if it's a contaminated specimen (vaginal or urethral meatus). Wbcs have a nucleus.
Besides, normal urine always contains sloughed off transitional epithelial cells, which are all nucleated. Centrifugation ("spinning down") will help to concentrate the matter of interest.
Your point about contamination is a fair one, but it certainly would be possible to purify nucleic acid and sort through the mess in the sample. Don't forget that current forensic experts routinely harvest blood from crime scenes for DNA analysis. Blood that's shed and left ex vivo is an
excellent culture medium for environmental bacteria. Yet crime labs don't find it too difficult to analyse blood smears that have been out in the open for a few hours, or even days. I don't think uninfected urine is going to present a huge problem in this respect.
And you're right that the only practical current use of nucleic acid testing would be in the various DNA fingerprinting methodologies, where one would already have to have a group of suspects handy to test a particular sample against.
But let's walk a little way along the realm of (current) science fiction here. The Human Genome Project has succeeded in sequencing the entire genome, and a very gross functional mapping has already been done. Who is to say that in a few decades down the line, we wouldn't be able to directly extrapolate defining physical characteristics from the complete genome sequence ? It would involve pretty complex mathematical models, but I daresay we'll be able to do simple things like eye color and gross facial structure prediction in a decade or so. Eventually, everything in the phenotype can theoretically be predicted except something having a profound and unpredictable environmental influence (like digital fingerprints).
So fast forward 50 years down the road. One could certainly envision a complex (and expensive) analyser that could isolate the predominant human DNA and sequence it, salvaging gaps in sequence from degradation from other strands by a complicated automated algorithm. From the complete sequence, a perfect physical portrait of the subject is generated, and further algorithms can be used to artificially "age" the portrait. In this way, a manhunt can be initiated from a drop of urine.
The whole process will be neater with better specimens like blood, but I don't think this sort of scenario (with a urine sample) is too far fetched in the next 50 years or so.
), or if this is one of those "I saw it on CSI and wonder if it really works that way" kind of questions.
