Re: Electrical Signals in DNA

Hi Karan,

I have come accross a lot of material and research about the conductvity of DNA.

It is unfortunate that you didn't include any references to the materials you have been reading. It is difficult to give you a good answer to your quesiton without knowing more about the information that you already have.

Although it is still not clear what kind of conductor the DNA molecule is, I have observed that in all the experiments the electrical signal was applied externally and a variety of different setups.

From this, it sounds like you have been reading about electrophoresis, a process in which a DNA molecule moves toward the positive terminal in a solution with an electric current running through it.

I would like to ask that is there any electrical signal coursing throught the DNA molecules in reality [as in, naturally]? On what basis do the researchers decide that the signal applied should be a.c. or d.c, its frequency and other such characteristics?

The answer to your questions lie in the structure of the DNA molecule. DNA (deoxyribonucleic acid) is a polymer, which means that it is a string made up of repeated units (monomers). In the case of DNA, each monomer is a nucleotide phosphate molecule. The nucleotide phosphate is a complicated molecule in and of itself, in that it is composed of a deoxyribose sugar, one of four different nitrogenous bases, and a phosphate group. It is important to your answer to remember the the phosphate group is negatively charged.

When these nucleotide phosphates are linked together to make a DNA molecule, the phosphate of monomer A is connected to the deoxyribose of monomer B, and the phosphate of monomer B is connected to the deoxyribose of monomer C, etc. If you try to draw this out, you will see that there is a "backbone" comprised by alternating phosphates and sugars; the individual bases point away from this backbone into the surrounding solution.

Now, if you remember that the phosphate groups are negatively charged, you will see that the DNA molecule has a net negative charge that is directly proportional to the number of monomer units in it.

So, to get back to your question, NO!, the DNA molecule does not have an electric current/signal coursing through it, but it does constitute a very large, and very highly charged ionic molecule, one that would be strongly affected by an electric current.

It is this phenomenon that is exploited in the process of gel electrophoresis. Because the DNA has a net negative charge, it will stretch out and move toward the positive terminal when it finds itself in an electric current running through a solution (remember, the DNA is an ion, so it is only charged when in solution. In order to dry DNA down, one must form a salt.). The gel is a highly-crosslinked matrix that is permeable to water, so that the solution can move through it, but the crosslinks will retard the movement of DNA.

The longer a piece of DNA is, the harder it will be to move through the gel, and the slower it will move toward the positve terminal. This allows DNA to be separated by size, because smaller DNA molecules will move farther. The DNA moves through the gel in a process known as Biased Reptation, which means that it moves like a snake, with the end of the molecule leading the way. Unfortunately, linear DNA molecules have two ends, so that really long DNA molecules can get caught in tightly linked gel matrices, with both ends trying to reptate their way toward the positive terminal.

With this in mind, I think you can see that the electric current used has to be DC. If you were using an AC current, the DNA would move in one direction, and then immediately in the other. It wouldn't make any progress. The voltage used depends on the concentration of other ions in the solution, the concentration and type of gel used, and the ambient temperature of the electrophoresis setup. A higher voltage will mean faster migration of the DNA, but as the current runs, the solution will heat up. If you run your voltage too high for too long, the temperature may get high enough for the gel to melt, the solution to evaporate, and even for the plastic gel-box be damaged.

In some cases, for large pieces of DNA the size of chromosomes, current is run alternately in orthagonal directions. This is called "pulsed-phase", "pulsed-field", or "orthagonal" gel electrophoresis. The current will first run from the upper right to the lower left, and then stop, and will then run from the upper left to the lower right. When the current is removed, the DNA molecule will relax into a coiled shape, and when the current is re-applied, it will stretch out again and begin reptating through the matrix. With the current starting and stopping, and coming from alternating angles, the leading ends of the DNA molecule can reptate themselves out of places where they would otherwise get stuck.

You can find much more about the different types of gel electrophoresis by searching our archives (see for example 913866987.Mb), searching the internet, and by reading a good college-level Molecular Biology textbook, such as Molecular Biology of the Cell by Alberts et al, or Molecular Cell Biology by Lodish et al. For example, read Gel Electrophoresis Resolves DNA Fragments of Different Size and Pulsed-Field Gel Electrophoresis Separates Large DNA Molecules, from Lodish.

Keep asking questions!