Discovering DNA As the Genetic Material
DNA, deoxyribonucleic acid, is accepted as the genetic material holder; in the 1950s that was not the case. Alfred Hershey and Martha Chase devised an experiment to finally end the debate between proteins and DNA being the genetic material. Proteins were thought to be the genetic material because of the complex structure and the twenty different amino acids used. DNA was relatively simpler with the same structure and only four nucleotides. Two previous famous experiments, the Griffith experiment and Avery-McCarty experiment, helped lay the ground work for this experiment. In the Griffith experiment, mice were injected with heat-killed S strain along with the R strain of the Streptococcus pneumonia bacteria which managed to kill the mice, showing DNA was taken up from the environment called transformation. In the Avery-McCarty experiment, enzymes were used to digest different macromolecules like polysaccharides, lipids, and DNA among other things. Only when DNA was digested did the mice survive when injected with the heat-killed S strain and R strain. These two experiments helped solidify that DNA was the genetic material, but there were still skeptics. The Hershey-Chase experiment finally settled the argument between proteins and DNA being the genetic material.
In the Hershey-Chase experiment, there are three essential concepts needed to understand the data and experimental techniques.The first concept is about viruses and their replication, another being about the radioactive labels they used, and the final concept about centrifugation. Escherichia coli (E. coli), a common bacteria, and phage T2 (a virus that infects bacteria) were used in the Hershey-Chase experiment. These were specifically used because of the fast reproduction time while also having a cheap maintain cost. The phage T2 cells injected their genetic material into E. coli and the bacteria reproduced more new viral particles using the instructions from the genetic material. After a while, the bacteria will have formed all the products needed to make new viruses. The virus products assembled and eventually lysed the bacteria. To track the DNA and proteins, Hershey and Chase used radioactive labels; the labels used were phosphorus with an atomic weight of 32 (32P) and sulfur with an atomic weight of 35 (35S). Phosphorous usually weighs 31 atomic mass unit (a.m.u.) and sulfur usually weighs 32 a.m.u. Phosphorous was chosen because it is DNA but never in proteins and sulfur was chosen because it is in proteins but never in DNA. The T2 phage viruses were labeled with 32P in their DNA for one part of the experiment and 35S in their proteins in a different part of the experiment. Each label would clearly indicate the results telling whether DNA or proteins was injected into the E. coli. Another big part of the experiment was a machine called a centrifuge. The machine works by spinning in circles very fast which allows the material inside the test tube to separate according to weight. Heavier objects will be on the bottom and lighter objects will be at the top. The top of the test tube, called the supernatant, would contain the phage carcasses (ghosts) and the bottom, called the pellet, would contain the bacterial cell. Combining all these ideas and concepts together helped easily see the answers from the experiment.
The beginning of the experiment performed by Hershey and Chase were to radioactively label the T2 phage with the 35S. After this was done, Hershey and Chase allowed the T2 phages to infect the E. coli for a sufficient amount of time to let the virus to reproduce; they also did this part again using 32P in a different test tube. The next part of the experiment was to put the bacteria in a household blender to break the cell wall open but to not ruin the DNA or other organelles inside. After this was performed, each tube was centrifuged. When the T2 phages were labeled with the 32P, most of it was found in the pellet with the bacterial cells indicating the DNA was injected into bacteria. When the T2 phages were labeled with 35S, most of it was found in the supernatant with the phage ghosts indicating the proteins never entered the bacteria. These results were the final case and evidence in that DNA was the genetic material, not proteins.
DNA has been known since the 1800s, but in 1952 Alfred Hershey and Martha Chase performed their experiment which ended this debate. Every scientist knew that DNA was the genetic material rather than proteins because of the E. coli and viruses. Once DNA was known to be the genetic material, scientists could study how it works and decipher the code of DNA called nucleotides. Also, they could find out how proteins and RNA (ribonucleic acid) arise from DNA. Once further investigation and experiments are conducted, the diversity of life can be figured out and how related one organism is to another.