The beginnings of molecular biology: General studies
DNA was largely an academic subject and not the source of dinner table conversation in the average household. In 1995 this changed when media coverage of the O.J. Simpson murder trial brought DNA fingerprinting to homes across the world. Two years later, the cloning of Dolly the sheep was headline news. Mendel studied the inheritance of seed change.
We know today that wrinkled seeds possess an abnormal form of starch. The diagram shows Mendel’s genetic hypothesis to explain the 3: 1 ratio of dominant: recessive phenotypes observed in the F2 generation of a monohybrid cross. True-breeding (homozygous) round (R) seeds and true-breeding wrinkled (r) seeds were planted. Plants were cross-pollinated and allowed to grow and mature after nearly 16 years with no further advances in characterizing Griffith’s transforming principle, an important breakthrough occurred. An in vitro assay was developed that provided the means by which the nature of the transforming factor in heat-killed S cells could be directly investigated without having to inject mice and wait for them to die. The assay involved selection of transformed cells from untransformed cells by their resistance to agglutination (clumping) by serum containing antibodies directed against R cells. Oswald Avery, Colin MacLeod, and Maclyn McCarty used this assay to show in 1944 that Griffith’s transforming principle was DNA.
An important event in the history of the characterization of DNA was the emerging availability and utility of radioisotopes in basic science research in the early post-World War II years. Radioisotopes allowed Alfred Hershey and Martha Chase to carry out a classic experiment in 1952 showing that the genetic material of a virus that infects bacteria, bacteriophage T2 (literally “bacterium eater”), is DNA of bacteriophage T2 (phage for short) was known to be contained within a protein coat, so Hershey and Chase designed an experiment to determine whether the protein or DNA carried the genetic information to make a new phage. First, they selectively labeled phage DNA with the radioactive isotope 32-phosphorus (32P) and phage protein with 35-sulfur (35S). DNA contains phosphorus but no sulfur; while protein is composed of some sulfur (in the amino acids methionine and cysteine) but no phosphorus. Next, they incubated bacteria (Escherichia coli) with the labeled phage. During infection, the phage attaches to the bacterium and injects its DNA.
At this point, Hershey and Chase encountered a major problem. They were not able to tear the empty phage coat away from the bacterial cell wall after injection of its DNA. Without this step they could not complete their experiment. In an ingenious moment, they tried the recently invented kitchen blender and found they could separate the empty phage coats from the bacteria. Fred Waring, a popular band leader, financially backed development of the blender which bears his name. This step removed the 35S since the phage protein did not enter the bacterial cell, but left the 32P phage DNA inside the bacteria. After synthesis of phage components from the phage genetic material and their assembly, lysis of the bacteria occurred.
Journal of Biochemistry & Biotechnology