An Easy Way To Distinguish DNA from Protein: An Experiment for General Chemistry

Abstract

We propose a classroom experiment to complement a classroom activity described by Ellis et al. in this Journal. In the Ellis activity students extracted long polymer fibers from the cells of common foods. In this experiment, the identity of the long polymer fibers as either DNA or protein can be determined by doing three accessible, inexpensive, and easy tests. Two of the tests, temperature and the acid-medium effect, are based on the physicochemical properties of DNA (reversible denaturalization), and the third test is a qualitative determination of proteins (absent in DNA). The results of these three tests provide evidence to distinguish the isolated molecule of DNA from proteins that might look like DNA.KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Biochemistry, Physical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Food Science Nucleic Acids/DNA/RNA, Proteins/Peptides E llis et al.1 described a classroom activity that illustrates DNA extraction from animal and plant tissues. In this activity, students extract DNA from the cells of common foods (beef, liver, and onions) in three steps: (i) breaking the cell membrane with mechanical crushing and by the addition of a buffered detergent solution, (ii) separating and removing the cell debris by centrifugation, and (iii) precipitating the DNA by adding cold ethanol. Students obtain long polymer fibers of DNA forming a fluff-like layer in the last step. What is the evidence that supports that the long polymer fibers are DNA? How can we prove that those white fluffs are DNA without using a spectrophotometer? Students may wonder about the presence of proteins in the extract or even think that the precipitate is protein instead of DNA, so how can we distinguish DNA from proteins?Proteins are the most abundant biological macromolecules, occurring in all cells and in all parts of cells. Proteins are polymers of amino acids, with each amino acid residue joined to its neighbor by a specific type of covalent bond, the peptide bond. Proteins can be easily broken down (hydrolyzed) to their constituent amino acids by a variety of methods, so that the earliest studies of proteins focused on the free amino acids derived from them. Twenty different amino acids are commonly found in proteins and they have a carboxyl (−COOH) group and one amino (−NH 2 ) group attached to the same carbon atom (the α-atom). Amino acids differ from one another in their side chains, or R groups, which vary in structure, size, and electrical charge. Proteins are described in terms of four levels of organization; primary, secondary, tertiary, and quaternary. A description of all covalent bonds (mainly peptide bonds and disulfide bonds) linking amino acid residues in a polypeptide chain defines its primary structure. The most important element of the primary structure is the sequence of amino acid residues. The secondary structure refers to particularly stable arrangements of amino acid residues giving rise to recurring structural patterns...

Protein Quantification in Complex Matrices

“…Exogenous compounds containing nitrogen have been added to food products to artificially raise the measured amount of protein, and this practice has led to serious health effects . The distinction between proteins and other components in food samples is important and has been the subject of several interesting and pedagogical activities. − Given the very diverse properties and biological activities of proteins, it is also relevant to perform more detailed analysis, , including the separation and characterization of the different proteins in food samples. In this context, the quantification of gluten proteins is a particularly interesting activity …”