By Steven Gimbel, Ph.D., Gettysburg College
The term ‘gene’ was formed when the Danish researcher Wilhelm Johannsen shortened the term ‘pangene’, which had been coined by the Dutch botanist Hugo de Vries. The gene was discovered to be the functional unit in the chromosome, the seat of genetics. Thereafter, the biological community dedicatedly studied genes and their application.
Willhelm Johannsen used the term ‘gene’ to create a distinction between two other new terms: genotype, which refers to the difference in chromosomal makeup, and phenotype, which refers to the difference in the observable properties of an organism.
This gave birth to a pertinent question: How does the difference in genotype create a difference in phenotype?
Understanding the Gene
The American Biologist Thomas Hunt Morgan researched extensively on this question and found out that the genes which were situated close together on a chromosome were often passed along together from one generation to the next. His student, Alfred Sturtevant, is credited with creating the first map of a chromosome and proving the importance of the distance between genes in the understanding of heritability and birth defects.
The focus on mapping genes had, by now, made the physical architecture a primary factor. To understand the working of the gene, it was important to study the determining factor in it, and this required seeing down to the level of the gene.
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The Gene: Bringing Biology Closer to Physics
The gene was a unit smaller than biologists had been dealing with. So, the x-ray technology being used by chemists and physicists to understand the structure of molecules was called to action.
This intermingling of biology with physics and chemistry meant that scientists such as Max Delbrück, a German physicist, started to become fascinated with the fundamental questions of life. Delbrück believed in reductionism, that biology was just a complex application of physics.
Delbrück had learned quantum theory from Niels Bohr himself and had worked as an assistant to Lise Meitner and Otto Hahn, whose work included the first splitting of the atom, which first demonstrated Einstein’s E=mc2. Having worked amidst greatness all his life, he wished to extend the quantum theory to life itself, and to create a quantized model of genes as Bohr had done with hydrogen, and explain genetics as a fully physical phenomenon.
There was, in fact, evidence to support his idea. It was known that radioactivity caused mutations and birth defects, which implied that purely physical phenomena affected biological ones. This only made sense if the underlying biological processes were themselves physical processes. This created the notion of what is now known as molecular genetics.
Delbrück worked with Linus Pauling, who also had similar ideas, having been part of the team that discovered the cause of sickle cell anemia to be genetic in 1949. He, too, thought that the key to understanding the nature of the processes of life was to be found in the molecules.
Pauling and Delbrück searched for the molecular processes that were responsible for the differences in the genes, that were in turn responsible for the differences in the traits. By this time, the entire scientific world was split between those who thought that proteins were the responsible agents and those who thought that deoxyribonucleic acid, or DNA, was the active molecule. Pauling and Delbrück belonged to the latter set and ventured to describe its structure, in the hope that it would reveal the very processes of heredity.
While this was happening in the scientific world, the great British physicist Sir William Lawrence Bragg was a rival to Pauling at the same time.
This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on Wondrium.
The Rivalry of Pauling and Bragg
Bragg, who won the Nobel Prize in Physics, becoming the youngest person ever to bag the coveted prize, shared it with his father, Sir William Henry Bragg, and was constantly plagued with the whisper that it was his father who really deserved the prize, and he was only included because of his shared name.
Naturally annoyed by this whisper, and proud of every breakthrough made using his work on x-ray diffraction, Bragg, while seeking such breakthroughs, locked horns with Pauling. However, Pauling seemed to be winning, always a step ahead of the former in his use of the technology which Bragg had dedicated a lot of his life to.
When Pauling started studying the structure of DNA with Delbrück at the California Institute of Technology, two researchers at the Cavendish Laboratory in Cambridge, led by Bragg, were working on the same research. While Bragg believed in the brotherhood of science as a cooperative and universal endeavor, he also had a desire to overtake Pauling.
Bragg’s team had their worst fears realized when Pauling beat them to come up with a model of DNA, using the best possible x-ray images of the molecule, that clearly showed a helical structure, and proposed three interlocking chains.
However, Bragg’s team had an advantage: Pauling’s son, a student at Cambridge, was able to serve as a mole and provide them with insights on his father’s project. When they found out about the three-chained model, they realized it was wrong, and that Pauling had not caught his error yet.
They had another advantage: better data. Their colleagues at King’s College, London, were doing x-ray crystallography to generate the best images of the DNA molecule taken yet. In a bid to emulate Pauling, they too came up with a flawed model, this time with two chains. After this one was disproved, they were, however, able to come up with a model that worked.
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The Model of DNA
Bragg’s team was able to show that DNA is a double helix—two strands of biopolymers on the outside and pairs of nucleotides on the inside. There are four nucleotides that form base pairs such that each step on the twisted ladder is made up of either adenine and thymine or guanine and cytosine. Since they bond in only those pairs, one could split the ladder, peel the biopolymer chains apart such that one member of the base pair stays bonded to the chain and the other side of the chain can be replicated in a completely unique way, thereby restoring the complete information held by the molecule—the template for protein creation. The encoded sequence then creates proteins that perform the needed functions in the body giving rise to the phenotypic properties, the observable traits. This allowed the replication of the instructions into every cell, thus forming the basis for heredity.
This discovery changed the paradigm in which genetics had hitherto been understood. It proved that heredity was purely a function of chemical composition. It has paved the path for newer understandings of the human body, of biology, and of reality in the biological world as a whole.
Common Questions About the Gene and Its Applications to Genetics
When the gene was established as the seat of heredity within the cell, one of the first problems that stumped biologists was the fact that it was too small to be seen. Physicists and chemists, along with their technology of x-rays, had to be roped in to understand the structure of the gene.
Linus Pauling was a believer in the idea of molecular genetics—that biological processes, at the very core, were physical processes. He teamed up with the German physicist Max Delbrück to form an encompassing model, that could explain the fundamental questions of life as fully physical phenomena.
Bragg and his team were able to come up with a model of DNA that showed it with a double helix, two strands of biopolymers on the outside and pairs of nucleotides on the inside, and four nucleotides that form base pairs such that each step on the twisted ladder is made up of either adenine and thymine or guanine and cytosine. This model changed the paradigm of genetics and allowed a much deeper understanding of heredity.