DNA Surviving Fire

[Sharwood]

This is going to sound like I'm up to something, but I saw an episode of Law & Order: SVU ages ago, where a woman had been burnt with cigarettes. They found the cigarettes, and they had a guy's fingerprints all over them, but they couldn't prove anything because none of her DNA was on the cigarettes; apparently, it had been burnt away.

The reason I remembered this, was today I was reading the newspaper, and there was something about a man being found guilty of arson based on DNA evidence, which reminded me of that episode. Now, granted, it didn't say the DNA evidence was from the place that burnt down, it could have been on a petrol can, outside, anywhere, but it got me wondering about how resilient DNA traces are.

I know they can't be destroyed just using household cleaning products, but say someone breaks into a car, cutting his arm in the process, then sets the car on fire after joyriding. Would that destroy his DNA? And if not, is there any way to do so? Or is it, you touch something, your DNA is there for life?

Please, no wild guesses at what crime I'm planning to commit, there aren't any.

 

[nonconformist]

Fire will generally denature DNA to the point where is useless, but you're thinkign about crime scenes as being mch too localised.
His DNA may have been fund outside on a cigarette he smoked, or on a part of the building or premises that wasn't destroyed or overly damaged.

DNA will be destroyed in odies that are compeltely burned.

DNA is fairly resiliant if kept under the right conditions. It can last theoretically forever in a very cold, dry environment, while it may be completely destroyed by a week in a swamp.

 

[Brighteye]

It seems likely to me that the heat from the fire in the car would destroy the DNA, but I couldn't say for sure. Given how easy it is to amplify even a small amount of DNA, there's an element of luck involved.

 

[nonconformist]

It seems likely to me that the heat from the fire in the car would destroy the DNA, but I couldn't say for sure. Given how easy it is to amplify even a small amount of DNA, there's an element of luck involved.

Yeah, but there are minimum threshold limits for hat evidence to be allowed in court depending on Loci

 

[Sharwood]

Fire will generally denature DNA to the point where is useless, but you're thinkign about crime scenes as being mch too localised.
His DNA may have been fund outside on a cigarette he smoked, or on a part of the building or premises that wasn't destroyed or overly damaged.

DNA will be destroyed in odies that are compeltely burned.

DNA is fairly resiliant if kept under the right conditions. It can last theoretically forever in a very cold, dry environment, while it may be completely destroyed by a week in a swamp.

Oh I'm aware his DNA could have been anywhere. My point was that it simply reminded me of that old SVU and got me thinking about fire and DNA.

And what do you mean by "denature." Make the proteins separate or something?

 

[nonconformist]

And what do you mean by "denature." Make the proteins separate or something?

Yah, once a threshold temperature is passed, all proteins and polyamides experience a change on account of overcoming intermolecular forces, leving a useless jumble of mess.

 

[ori]

DNA denaturing usually refers to melting the doublehelix resulting in single stranded DNA - this happens at around 60-90°C - but that does not destroy the DNA in a way that would prevent the "finger print" from being determined.
DNA usable for PCR can be recovered from bodies incinerated in a gasoline fire or after cremation - so if the temperature does not get too hot at the part of your imagined car that contains your DNA sample you might be out of luck

However heat is a pretty good way of destroying dna - you just need to make sure that you exceed about 400°C for a sustained time period in all parts of the evidence - allowing for thermal breakage of the covalent bonds of the DNA

 

[Sharwood]

So, if the DNA was on a part of the car that was constantly burning, it would be denatured, but on a part that burnt out quickly, it would be fine?

 

[AngryZealot]

That's right. Heat would only separate the double helix of the DNA. The two individual strands would still be intact, and that's all you need for amplification.

 

[Sharwood]

So you need to pretty much completely destroy it, not just denature it?

 

[AngryZealot]

You would have to destroy it quite substantially. The human genome contains over 3 billion base pairs (spread over 23 DNA molecules). The fragments used for forensic investigations aren't really more than 1000 base pairs. The chances of breaks in DNA affecting the region of interest are quite small unless the DNA is seriously trashed. Even then there are ways of piecing together broken fragments. It's why they're able to get DNA sequences of Neanderthals and Mammoths even after 10000+ years.

 

[Sharwood]

So how would one go about destroying it? Since I'm guessing you'd basically have to stand there adding wood for the fire thing to work.

 

[LesCanadiens]

So how would one go about destroying it? Since I'm guessing you'd basically have to stand there adding wood for the fire thing to work.

Thermonuclear weapons.

 

[AngryZealot]

Like ori said, heating it >>100 C for a time will do the trick. DNA exposed to the air will also be degraded over time by radiation. Probably the biggest factor is life itself. Organisms are very careful about what they allow in and out. Since foreign DNA is typically bad, microbes will make DNases, which are enzymes that destroy DNA. Some will probably feed off the DNA as well. A savvy criminal might dump DNase over everything he touched, but time will do the trick as well.

 

[Sharwood]

And where would you get DNase? I doubt it's sold over the counter.

 

[AngryZealot]

You could get a small vial from pretty much any biotech firm for <$100. They might find it curious that the shipment isn't going to a normal institution, but it's not regulated at all so I doubt they would check up on it unless you started ordering a bunch of other stuff. I'm not really sure how effective it would be. I don't know how long it takes human cells to break open outside the body, and unless they do that the DNase won't work.

 

[Brighteye]

You could always lyse the cells with washing up liquid or something.

 

[Sharwood]

You could get a small vial from pretty much any biotech firm for <$100. They might find it curious that the shipment isn't going to a normal institution, but it's not regulated at all so I doubt they would check up on it unless you started ordering a bunch of other stuff. I'm not really sure how effective it would be. I don't know how long it takes human cells to break open outside the body, and unless they do that the DNase won't work.

Is there any way to make them break open? Speed it up?

 

[AngryZealot]

Soap, as Brighteye said.

For all the trouble you'd go through though, wouldn't it be easier to just wear gloves?

 

[Sharwood]

Soap, as Brighteye said.

For all the trouble you'd go through though, wouldn't it be easier to just wear gloves?

I was thinking something more along the lines of cutting your arm on a piece of glass or something.