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Thread: What destroys dna?

  1. #1
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    What destroys dna?

    I was watching a movie and after being shot a criminal sprayed the blood with ammonia to keep the detectives from using his blood for a dna sample. Was this hollywood fiction? I didn't think ammonia would destroy blood dna samples...

  2. #2
    Registered Senior Member maxzuk's Avatar
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    I've heard that a topical solution of hydrogen peroxide will destroy DNA,

    I've also heard that bleach will do the job.

  3. #3
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    Quote Originally Posted by maxzuk
    I've heard that a topical solution of hydrogen peroxide will destroy DNA,

    I've also heard that bleach will do the job.

    Huh. I figured it was hollywood hype. I wonder why all criminals don't carry around bleach, hydrogen peroxide, or ammonia. Anyone know if this is fact or fiction?

  4. #4
    Leave your World Behind Tristan's Avatar
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    It might not necessairly destroy the DNA as much as it would contaminate the sample and make it unusable in the chemical reactions needed for anlysis (PCR?)

  5. #5
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    DNA is a relativly fragile chemical. Hydrogen peroxide would probably destroy it, if it was concentrated enough. A sodium hydroxide solution would certainly work.

  6. #6
    What the hell, man? Idle Mind's Avatar
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    Was the movie "the Boondock Saints"?

  7. #7
    Mature human red blood cells do not have nucleii, and therefore no DNA. From a bloodsplatter, the amount of available DNA would be locked up in phages and tissues that got torn loose.
    While complete and utter destruction of DNA would be rather difficult to perform (especially in a shootout), you could use a PCR inhibitor, (though I'm uncertain of using straight ammonia, NH3). BY inhibiting PCR, you could make it extremely difficult to replicate DNA.
    PCR stand for polyermerase chain reaction, which basiacally amplifies DNA, or replicates it a gazillion times.
    I run lots of PCRs, and one of the chemicals we use as an inhibitor is BSA, or benzyl sulfonamide. BSA contains an ammonia group in it, and too much BSA will prevent the PCR.

    So it is entirely plausible that NH3 would goof up a CSI's attempted PCR, especially if the NH3 concentration was high enough.
    There are methods that you can use though to extract DNA, or remove PCR inhibitors, so by using ammonia you would only slow down the cops' investigation.

    Despite what Nasor says, DNA is a surprisingly robust chemical. If you were to damage the DNA, which would be easy, you could prevent entire genomic sequencing. However, the type of genetic analysis that CSI does is on very small, well known regions of DNA, so partially fragmented DNA may not be a problem.

    Sodium hydroxide would screw over CSI's efforts.

  8. #8
    voltage gated ion channel Hercules Rockefeller's Avatar
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    Quote Originally Posted by Roman
    Mature human red blood cells do not have nucleii, and therefore no DNA. From a bloodsplatter, the amount of available DNA would be locked up in phages and tissues that got torn loose.
    Ever heard of white blood cells?

    You most definately can get DNA from blood samples.

    But I don’t have the knowledge to definatively answer the original question. Bleach (ie. dilute sodium hypochlorite) will degrade DNA and greatly reduce the likelihood that it can be used for biological analysis, such as PCR. It will also degrade proteins and most likely prevent immunological detection of blood group antigens. But when it comes to merely detecting the presence of blood, there are some very sensitive fluorescent dyes that can detect even minute traces of blood, even after it has been cleaned and scrubbed away from visible sight. Bleach will react with DNA but it won’t degrade it down to individual nucleotides. It really takes a lot of chemicals and a lot of ‘elbow grease’ to beat forensic DNA dyes.<P>
    Last edited by Hercules Rockefeller; 03-06-05 at 12:01 AM. Reason: factual correction

  9. #9
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    Yeah, it was Boondock Saints. Most of the responses are a little above me.

    I don't know what sodium hypochlorite is, or NH3, or Sodium hydroxide. Interesting though.

  10. #10
    What the hell, man? Idle Mind's Avatar
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    NH3 is ammonia. It is a nitrogen atom with three hydrogens attached to it. Sodium hypochlorite is bleach (in a dilute form, as Hercules Rockefeller stated). There is an oxygen bonded to a chlorine, but since only one of oxygen's two binding sites is filled, the oxygen carries an extra pair of electrons and has a negative charge. The sodium has a positive charge and interacts ionically with the hypochlorite.

    NaOH is sodium (Na) hydroxide (OH). It is a very strong base (opposite end of pH scale to an acid, with similar effects in that they both can be very destructive).

    Hope that clears some things up. Ask more questions if there is still more you don't understand. After all, we're all here to learn.

    Boondock Saints was a great movie, by the way.

  11. #11
    Secular Humanist Huwy's Avatar
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    boondock saints was sweet movie

  12. #12
    Save the whales motherfucker sargentlard's Avatar
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    How long does traces of DNA last in a crime scene?

    After how long can blood still be detected, even after it has been wiped away from human sight?

  13. #13
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    Thanks! That does clear some things up Idle Mind. These forums are pretty d*mned interesting.

  14. #14
    DAMNIT!!
    Too late.

    Oh well- interesting question:
    After how long can blood still be detected, even after it has been wiped away from human sight?
    Years and years.

    Trying not to get exited here (murder, favorite subject), but forensic cases have been known to remain cold for up to 30 years and then solved by blood evidence still at that scene

    The question of what would destroy DNA has already been more or less answered, but now where the simple detection of blood is concerned.
    There is a chemical sprayed on any suspect area called luminol
    http://people.howstuffworks.com/luminol1.htm

    its a compound made primarily of carbon and hydrogen with nitrogen in the formula (which you can tie in to this legend of spraying ammonia)- and they mix this with hydrogen peroxide which would further destroy DNA; either way, DNA is a very fragile molecule.

    However, DNA or not, this compound binds the heme group in hemoglobbin( more like the iron atoms in that protein) found in blood cells so it wouldn’t matter one way or the other if DNA were present or not.

    So you spray it on, let it set, and turn the lights out. Everywhere that blood was present- even if it was wiped out by strong chemiclas such as bleach or ammonia- would show up as fluorescent streaks glowing in the dark.
    Which is eery.

    I've seen crime scenes where you can literally see how and where the killer wiped off blood, just by looking at each glowing area you can imagine the sponge in his hand as he wiped.
    And you could also, 30 whole years later, trace where the body was dragged by the glowing strip on the floor.

  15. #15
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    Quote Originally Posted by gendanken
    However, DNA or not, this compound binds the heme group in hemoglobbin( more like the iron atoms in that protein) found in blood cells so it wouldn’t matter one way or the other if DNA were present or not.
    That's very interesting. What if you treated the blood with something to strip the iron center out of the heme group, like EDTA with sulferic acid or some other strong chelator?

  16. #16
    Nasor:
    That's very interesting
    Isn't it??!!
    Muhahahhahaahah.

    Then again not- science is making it very hard for clever murder.
    Count your stars I can use this as an excuse for not killing your family.
    ~kidding~

    What if you treated the blood with something to strip the iron center out of the heme group, like EDTA with sulferic acid or some other strong chelator?
    From my understanding of this, the iron’s main use its its ability to catalyse the fluorescence.

    This means that without it, you could still be able to detect blood there but only with more complicated tests- iron just makes this easier by making the detection visible to the naked eye.

    Even so, as wonderful as this sounds it has its own problems- detergents and even bleach can make luminol glow so that there are cases where this chemical is sprayed (say a laundry room, and I've seen it happen) where blood was never present, putting the innocent under suspicion.

    So the use of EDTA or sulfuric acids can do one of two things:

    1. Destroy the iron atoms in the protein, which would render the sample undetectable in the dark
    2. Or actually enhance the glow, the way some detergents do

    Either way, you'd still be able to detect if blood had been there if it had.

    Sprays like this can also be used to detect saliva and semen- so if you think you'll get away with it 'cause ya strangled the bitch, hence no blood, think again.

  17. #17
    about there blood dectecting dyes. Wouldnt sodium hydroxide DESTROY not only the blood but basically (unless it was glass or stainless steel) most of the substance the blood is on? We use it at work to clean grills and it eats ANYTHING, your left with the shiny steel underneath no matter how much crap was on the grill

  18. #18
    Wouldnt sodium hydroxide DESTROY not only the blood but basically (unless it was glass or stainless steel) most of the substance the blood is on? We use it at work to clean grills and it eats ANYTHING, your left with the shiny steel underneath no matter how much crap was on the grill
    Pause.

    Then what would be the point of checking?
    That's like trying to palm a fingerprint off an amputee.

    This is for surfaces that appear clean to the nake eye- carpets, floors, tables- that most people wipe off with simple bleach or soap.

  19. #19
    that was kind of my point. Yea it would get all the blood but the cops would know SOMETHING had been going on due to the half gone floor and furniture

    but out of interest say you kill someone over a sink (stainless steel) and all the blood goes into the sink then you could destroy it with grill cleaner because it would clean it all up

  20. #20
    but out of interest say you kill someone over a sink (stainless steel) and all the blood goes into the sink then you could destroy it with grill cleaner because it would clean it all up
    Easier said than done, Assman.

    Blood spatter.
    You can't bash a skull in without blood exploding out everyfuckingwhere- yes, I know, 'Duh'.

    You'd have to kill a person in a bag to not leave any trace evidence, period.

    (They can not only detect the teeniest hints of blood but by reading the shapes of each drop they can tell how you killed, what you used, and where you were standing when you did it.)

    And if you're going to kill them in a bag then what's the point of klling them "over the sink", stainless steel or not?

    Do what us women do- POISON.
    But even then, they'll read the hair and pinpoint exactly what you posioned them with and for how long. They can actually make a calender out of a corpses hair (usually with arsenic).
    You can't kill anyone anymore.

    FUCK SCIENCE.

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