Re: extracting DNA and RNA

Dear Lynn,

Thank you for your question. First I'll refer you to other MAD Scientist essays on DNA extraction. Then I'll evaluate techniques for destroying DNA (freezing, heating, exposing to UV light, and decomposing DNA). Finally I'll explain destruction of RNA, and briefly describe how it is extracted.


EXTRACTING DNA

In this essay a forensic molecular biologist describes techniques she uses to extract DNA from crime scene samples.

In thi s essay by Sharon Shriver, additional information is given, with references provided.

Both of these essays are somewhat technical. Techniques designed for children for DNA extraction from plants can be found through links in thi s MAD Scientist essay by Andreas Matern If you'd like a simplified description of DNA extraction techniques used in the first two essays, or any other techniques, please submit another question to MadSci Network.


DNA breaks pretty easily. The problem with trying to destroy all the DNA in an area is that there is usually a whole lot of DNA to break. If even one strand survives, it is possible to amplify it using PCR techniques also described in the above links.

FREEZING

Freezing can break DNA. The water inside the cell crystalizes during the freezing process. These sharp crystals expand in volume, tearing apart the cells and their contents. Unless it has some other protection, the DNA could be torn apart. Condensed DNA, especially in our "germ cells" (eggs and sperm), may have extra protection because it is wound up tightly around structural proteins. Below 0 degrees Celsius, no additional damage is done; it is the process of freezing that causes the damage. Usually some DNA survives freezing.


HEATING

Heating any large compound will increase the rate at which molecular bonds break. At moderately high temperatures the bonds between complimentary DNA strands break. In fact, the famous Polymerase Chain Reaction (PCR) uses this phenomenon in one of its steps to make many copies of a single DNA strand. At very high temperatures, greater destruction of the DNA can occur. (Basically, it burns.)

Very high temperatures are used to sterilize objects, such as surgical equipment, in machines called autoclaves. While no additional damage is done below 0 degrees Celsius, there is no upper limit for DNA damage by heat. More heat means more destruction, until the level at which all DNA has been destroyed. Autoclaves typically run at 121 degrees Celsius for 15 minutes, which completely sterilizes solid objects.


UV LIGHT

In the laboratory, UV light is also used to sterilize objects. This powerful wavelength range disturbs the bonding in DNA, causing breaks along the DNA chain. Of course, this also occurs outside with sunlight, which contains UV light. (Exposure to UV light from the sun can lead to the formation of cancerous cells when your own DNA is broken.)


DECOMPOSITION

Perhaps the most common example of DNA destruction comes from your message: microorganisms. Fungi are particularly destructive to DNA. There are many examples of blood from crime scenes being stored unrefrigerated. Fungi in the samples grow through the evidence and digest everything present. Later, when scientists attempt to extract DNA from the sample, they can find nothing but fungal DNA.


RNA

As for RNA, that's slightly different. While cells typically have only one copy of DNA, it is filled with many, many copies of RNA. Despite its abundance, however, RNA is very difficult to work with. One reason is that there are many proteins (called RNAases) designed to break apart RNA. Even our skin is covered with these RNA-destroying proteins. On top of RNAases, RNA can be destroyed in all the same manners as described for DNA.

To learn the RNA's sequence, the sample must be carefully handled in a very sterile laboratory. The RNA is treated with an enzyme called reverse transcriptase, which was originally isolated from retroviruses. Reverse transcriptase takes RNA and copies it as DNA. The DNA can be copied many times with PCR, and then sequenced for information.



DNA and RNA are both easily destroyed. However, to be truly sterile, extreme levels of heat are usually used. Such extreme measures are required because even a single strand of DNA can be detected by current molecular techniques.

I hope I've answered your questions on DNA and RNA destruction and extraction. If not, please write us another question.


Thank you for your question,

John Carlson