A short forensics question

[Zebo]

Hi,

 

I hope this is the right forum for this. If not, redirect me please.

 

Imagine someone is stabbed with a knife, and this knife is thrown into a lake. About a week later, the knife washes up on the shore of the lake.

 

A. -- Will it still be possible to extract DNA from this knife (blood, fingerprints)?

 

B. -- And after two months?

 

C. -- If the knife was covered in blood, will that blood still be visible somehow to the bare eye?

 

Thanks in advance.

Edited December 23, 2010 by Zebo

[ewmon]

Most likely, the blood (being hydrophilic) would eventually dissolve/wash off the knife and/or the microbes in the lake would eventually ingest it, whereas the oils that compose the fingerprint (being hydrophobic) might survive longer. The time frame for these events is problematical (kinds and numbers of microbes, water temp, etc), but the knife "washing up on shore" means it was also subjected to currents, wave action and friction and saltation along the lake bottom which that would add to the "washing away" effect.

[Mr Skeptic]

I think it depends on how soon the knife was thrown in the lake. Dried blood is nasty stuff to get off.

[Zebo]

Thanks so far, this is helpful.

 

How about: the victim is stabbed, and the perpetrator keeps the knife for a day. Then he tosses it in a lake, and, two months, later, it washes up on the shore of the lake (or someone finds it at the bottom). Will it still be possible to extract DNA from the knife? (Also, the time span is May/June/July: mid-summer, hot.)

 

(In case anyone's wondering: no, I'm not a serial killer, it's for a student film, a final exam.)

 

 

[Mr Skeptic]

Well I don't really know, but I'd guess that the blood would still be detectable, and similarly bloody fingerprints too, but the DNA would probably be very contaminated due to bacteria even if it were not destroyed. If there's some deep crevices in the knife, that might be enough to protect DNA. Now I don't really know, but that would be plausible to me and I wouldn't question it in a movie (they always have far worse things). I think the more unusual thing in this scenario would be the killer keeping the bloody knife for a day.

[Zebo]

Yeah, but that can be made plausible though: perhaps he can't get rid of it, as he has (very intrusive) company; or perhaps he thinks he threw it away in a lake, yet it landed on the back of tortoise (joking, but you get the idea).

 

You bring up an interesting point though: bacteria. Imagine the killer throws the knife in a sewer (and the sewer is, via via, connected to the lake). Would bacteria be able to 'eat' the DNA, that is destroy it?

[Ahsan Iqbal]

I think it depends on the knife too. If it had a shiny glossy surface, then probably, it might lose all the proof but if it was rough, there are chances of at least something left within the microscopic crevices.