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Usually DNA sample manipulation is realized with an ice box at hand in order to avoid degradation, and also its storage is set at -20ºC. Nevertheless, DNA has been obtained from really ancient samples that were not in a cold environment and even our DNA is at 36ºC. Furthermore, when performing PCR, samples are put into a massive heat shock and at high temperatures. Then how needed are those protocols in order to grant correct DNA quality? how much can a sample be left without need to worry? and how does this degradation happen (DNAse free environment)

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Really awesome question ! –  biogirl Dec 17 '13 at 14:33
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1 Answer

up vote 4 down vote accepted

DNA decomposition isn't a phenomenon that is too common. In nearly all cases, DNA is broken apart to the accidental contamination with nucleases. Long-term storage of DNA in the -20 is also suggested in order to significantly lower the rates of these nuclease enzymes.

Additionally, DNA only truly fragments at very high temperatures (>90-100 degrees Celsius) due to the nature of the covalent chemical bonds that form DNA. Nearly any organic molecule will degrade at very high temperatures, and some will degrade at lower temperatures due to unstable structures or heat-labile bonds.

Of course DNA will deanneal at somewhat-high temperatures due to its double standedness , but allows for reannealing to the initial molecule once the temperature is brought down.

One additional reason for potential DNA degradation is DNA shearing, which will occurs when you tear up your DNA physically due to exposure to rigorous shaking, resuspending or other "violent" techniques.

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and how common are nucleases? –  Cobblestone Dec 17 '13 at 9:42
    
Not that common, but just think about how it only takes one enzyme to destroy your entire sample. A "source" of nucleases can be from your fingertips too, which is why its very important to wear gloves when dealing with DNA directly. –  LanceLafontaine Dec 17 '13 at 12:44
    
DNA is a incredible stable makro molecules. It survives in out cells and we can do things like PCR (which need serial heating and cooling steps) with it without degradation. As long as you work clean, your DNA is not at risk. We usually keep our working stocks of plasmids or primers in the fridge at 4°C to avoid repeated freeze-thaw cycles. –  Chris Dec 17 '13 at 15:43
    
@Chris, exactly. If you treat your DNA well, it would treat you well (experimentally, that is). :) –  LanceLafontaine Dec 17 '13 at 16:14
    
@JunkDNA One important source of DNAses and RNAses when isolating nucleic acids are the cells/tissues themselves from which the DNA/RNA is isolated. Pure nucleic acids in an appropriate buffer are very stable, in my experience RNA samples can be stable for years at 4°C or even above (small RNAs with stable structures). –  Mad Scientist Dec 17 '13 at 19:47
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