Dear Satish,
Please read the following text:
In Hyperthermophiles and not humans:
It has been suggested that the DNA in hyperthermophiles is protected against denaturation by specific mechanisms, such as positive supercoiling by reverse gyrase or stabilization by histone-like proteins. Indeed, reverse gyrase has been detected in all hyperthermophilic archaea and bacteria tested up to now, and the archaeal histone-like proteins HTa from Thermoplasma acidophilum or HMf from Methanothermus fervidus increase the Tm of linear DNA molecules in vitro. However, the role of reverse gyrase or histone-like proteins in DNA stabilization at very high temperature remains to be demonstrated. In particular, the greater thermostability of positively supercoiled DNA versus negatively supercoiled DNA has never been experimentally verified. Furthermore, the effect of various histone-like proteins on linear DNA in vitro might not be significant, since linear DNA lacks the topological constraints between the two DNA strands which are typical of DNA in vivo. Finally, it is well known that DNA experiences several degradative processes at high temperatures, such as hydrolysis of the phosphodiester bonds, depurination and cytosine deamination. These pathways of thermodegradation should have an important effect on topologically closed DNA stability.
References & Links:
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http://nar.oxfordjournals.org/content/22/9/1681.full.pdf
https://en.wikipedia.org/wiki/Unique_properties_of_hyperthermophilic_archaea
http://thelivingcosmos.com/Extremophiles/Thermophiles_12May06.html
Hoping this will be helpful,
Rafik
No matter how heat-tolerant the DNA may be, it is not fire-proof, since it is an organic substance. If DNA is recoverable from burnt bodies, it is because some portion of the body was not completely burnt. The DNA may have been fragmented by exposure to high temperature, but the pieces may still be large enough to be amplified by PCR.
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