What can destroy DNA evidence?

Lots of things! UV light, extreme heat (e.g. fire), any oxidizing chemical (such as bleach). Also, keeping the evidence stored in an air-tight container (bacteria develop and break down DNA). read more

Lastly, the absence of DNA on the scene is not exculpatory in any real way; it's a lack of incriminating evidence, and the jury might be leery of the explanation, but outside of television there are many reasons by DNA evidence might not be available -- in fact, the bias juries have in cases where there is no DNA even has a name: the"CSI Effect". read more

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. read more