Medical Mondays: Where IS this DNA stuff?

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So today, I have a great Medical Mondays question from writer Clara Waibel, who asks:

If someone is attacked, and gets some part of their body scratched, how long until the DNA of the offender vanishes? Would it take just a shower? Or does it remain in the victim's body for a longer period (assuming the victim is still alive)?

So where IS your DNA? We know it's in the cells of your body, but what about fluids? On your skin? Urine? Spit?

DNA, as you all probably know, are the the coding molecules found inside of the nucleus in your body's cells. So. No cell, no DNA. No nucleus, no DNA. As to how DNA can be detected, here are some facts:

Saliva: Your saliva is mostly water, proteins, and electrolytes with some shed cells from the inside of your mouth, so yes, it has DNA. Commonly, DNA samples are taken from scrapings from the inside part of your cheek, in your mouth. 

Blood: Yep, lots of DNA. But here's an interesting fact--the red blood cells that comprise most of the cells in your blood? They don't have a nucleus, and hence, have no DNA. But your white blood cells (aka infection fighting cells) do, and that's where DNA from blood samples come from. Also, platelets don't have nuclei either.

Urine: Your urine probably has some shed cells from the lining of your urinary tract, but usually it's a very small quantity.

Feces: You had to ask, didn't you? Well, you probably have some shed cells from the lining of your intestinal tract, but you also have tons of bacteria with their own DNA too (don't freak out! It's your natural flora. Yes, you are host to a raging party of happy bacteria in there. Fun.)

Skin: The surface of you skin is comprised of compressed skin cells, and yes, they have DNA still in them. In fact, now they can test surfaces you've just touched for DNA (source: Forensic Mag)

Hair: Your hair shaft is made of keratin, a kind of protein. No DNA. However, the follicle from which your hair grows out often comes loose when a hair falls or is pulled from your head. DNA samples can be taken from those follicles.

So going back to Clara's question. If an attacker scratched someone, that victim might have some DNA from his skin on them, but after a shower, it would likely be gone. Also, if there was broken skin (as in, the attackers DNA somehow got inside the victim, such as by blood-to-blood contact), the victim's defenses would destroy that DNA and the cells that crossed into her blood stream.

Thanks for a great question, and have a good Monday everyone!

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