Date: April 2003
Why does DNA decompose when heated to too high a temperature?
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.
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.
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 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.
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Update: June 2012