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Name: Jenny
Status: Student
Age: 17
Location: N/A
Country: N/A
Date: April 2003


Question:
Why does DNA decompose when heated to too high a temperature?



Replies:
The most of the hydrogen bonds that join the two DNE stands break from the heat. With more heat the actual bonds between the nucleotides can break -fragmenting a strand. Very few organic molecules can with stand high temperatures for very long. Heat provides the activation energy to break covalent bonds and denature/destroy molecular structure. Proteins are particularly sensitive. There are exceptions, as in the case of thermophillic bacteria. There are cases or heat shock-proteins or "chaperone" proteins that can protect regular proteins from breakdown - to a point.

Lou


DNA doesn't actually decompose when heated. It just melts. DNA comes in two mirror image strands that you could visualize as a zipper. The chemical bonds that make up each strand of the zipper are permanent joins, but the teeth that connect the two strands are much weaker and sensitive to heat. So when you expose DNA to heat (for instance, by boiling it), the two strands of the zipper separate. By very slowly cooling that denatured DNA, you could actually get the strands to reanneal or zip up again.

Christine Ticknor, Ph.D.
Ireland Cancer Center
Case Western Reserve University


DNA in its native form is composed of two molecules that have the characteristic of being complementary such that one strand associates with the other in a particular fashion, forming a "double helix." The two molecules are stabilized in this structure due to noncovalent bonds, mostly hydrogen bonds and vanderWaals forces. These bonds are not strong; accordingly, when the temperature rises sufficiently, the double helix is said to "melt" into its two component molecules, and will reassociate upon slow cooling under appropriate salt conditions. So, the issue is not decomposition so much as disassociation upon exposure to too high a temperature; and the reason for the disassociation is that the energy input of the heat overcomes the ability of the noncovalent bonds to keep the molecules together.

Don Silvert


Heat "cooks" organic material, DNA or others, especially in the presence of oxyge. So DNA can "burn" in the usual sense of the word forming CO2, N2 and other compounds. Even in the absence of O2, heat can cause DNA, or other molecules, to change its structure either by losing some degradation product, or just changing structure so that it cannot replicate. The "technical" term for proteins is "denaturing", which is a catch-all phrase for "losing its chemical structure.

Vince Calder


Chemical bonds have a certain stability based on the type of bonding. Some bonds are quite strong The following are covalent bond strengths.

Bond Bond Strength(kJ/mole); Bond Bond Strength (kJ/mole)
Cl-Cl 239 H-Cl 427
H-H 432 C-H 413
N N 941 N-H 391

Bond strength can be overcome by adding heat...The GC and AT bonds in DNA are hydrogen bonds (non-covalent) and can also be overcome with heat.

these hydrogen bonds are about 71 kilojoules per mole...relatively weak (but strong in large numbers) and can be broken and reformed by heating and cooling. This is the secret behind the polymerase chain reaction.

Faletra, Peter



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