Electric Forces in Deoxyribonucleic Acid (DNA)

Iliane Miranda Fonseca

The concept of electric force studied in Physics II can be applied to other science fields such as Biology, specifically Molecular Biology. Our body is compound of billions of cells and each one has many molecules that interact with each other to perform a specific function. These interactions occur in water, because this is the main component inside the cell. Water is a polar molecule which has a dipole moment and can interact with other water molecules forming hydrogen bonds, or also can be able to interact with other molecules such as DNA, RNA, and proteins. The electrons in water spend more time around the oxygen atom than around the two hydrogen atoms making the oxygen more negatively charge. 

In the case of the proteins, biochemical compounds consisting of one or more polypeptides, there are six forces or bonds that stabilized its tridimensional structure. These are the following: ionic bond, covalent bond, Van de Walls forces, hydrogen bond, electrostatic forces, and intermolecular forces. One example is the ionic bond which is form when a positive atom (cation) and a negative atom (anion) can hold together under electrostatic attraction because of the opposite charges. As we learned in physics II, “unlike charges attract; like charges repel”. Another example is the intermolecular forces due to molecules with dipolar moment that are attracted with others dipolar molecules electrostatically. 

The DNA contains the hereditary material or genetic information that is passed from one generation to other. This molecule is made up of nucleotides, and each nucleotide is composed of one nitrogenous base, a deoxyribose, and a phosphate group. Adenine (A), guanine (G), cytosine (C), and thymine (T) are the four nitrogenous bases. The DNA consists of 2 strands that are wrapped forming a double helix. The two strands are attracted because of the electrostatic forces between the nitrogenous bases which are positively or negatively charged forming a hydrogen bond. This electrostatic attraction is very important to transmit the genetic information with precision to the other generation. As we can see in the following picture, adenine is always attracted to thymine and guanines to cytosine because this is the most stable form. 

The concept of the Coulomb force acting at a distance can be applied to the DNA using gel electrophoresis. This is a technique in where the DNA fragments or other molecules can be separate based on the mobility of ions in an electric field. Since the DNA is negatively charged when dissolved in water, the electrophoresis will separate the DNA by size. This is because small molecules migrate more easily through the gel matrix while bigger molecules experience a larger resistance. The DNA is placed in a side of the gel and by applying an external electric field it will move to the positive side. The picture at the left is how this technique works and the one at the right is a result of the DNA gel electrophoresis. 

As we can see, Physics is everywhere including our own body. The laws of Physics can apply to almost all other science and engineering fields and for this reason is very important to understand and learn more about Physics. 

References:

1. Deoxyribo Nucleic Acid

http://mrsec.wisc.edu/Edetc/background/DNA/DNAintro.htm (Acceded November 23, 2011)

2. Electric Fields – Coulomb Force at a Distance. http://www.physics.udel.edu/~bcwalker/phys208/lab2.pdf (Acceded November 23, 2011)

3. Giancoli, Douglas C. Physics for Scientists and Engineers with Modern Physics. 4th ed. Vol. II. Upper Saddle River, N.J: Prentice Hall, 2009. Print.