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Name: Obaid
Status: Other
Grade:  Other
Location: N/A
Country: United States
Date: October 2007


Question:
Is the structure of bacterial DNA is the same as humans (I mean is it a double helix?). If it is a double helix, why the hydrogen bonds between nitrogen containing bases doesn't breakdown in bacteria living in 95 degrees C or above? Human DNA unwinds and becomes single strand when heated above 70 dergees C ( i.e. in polymerase chain reaction).



Replies:
All DNA in life is similar in structure, but there are some differences. Some organisms chemically modify their DNA for various reasons, for example. Bacterial and human DNA are both double-helical. Bacterial DNA is typically in the form of plasmids, a circular form of double-stranded DNA. Human DNA is in a linear form.

PCR uses enzymes from a type of bacterium that can live up to 80C. The enzymes (not the whole bacterium) remain functional at the higher temperature cycles of PCR, 95C. The bacterium would not survive for a long period of time at 95C, but PCR doesn't use the whole bacterium, just its enzymes.

There is a third type of organism, archaeans, which are neither eukaryotes (like humans), nor true bacteria. Some archaeans can survive up to 121C -- autoclave temperatures. I am not sure we know all the adaptations they have for high temperature. I would speculate these organisms wrap proteins around their DNA (or their DNA around proteins) to prevent the DNA from denaturing in this extreme temperature, or it may also coil its DNA in a way to limit its denaturation. Archaeans are not the source of enzymes used in PCR.

Hope this helps,
Burr


Good Question! Thermophilic organisms tend to have DNA with more GC base pairs than organisms living at more moderate temperatures. GC base pairs are held together by 3 Hydrogen bonds while AT base pairs contain only 2 Hydrogen bonds. Synthetic DNA consisting of only GC base pairs does not denature until the temperature is around 90 degrees C.

Ron Baker, Ph.D



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