The Discovery Of The Structure Of DNA

Deoxyribonucleic acid (DNA), a self-replicating material which is present in nearly all living organisms as the main constituent of chromosomes. It is the carrier of genetic information. DNA was discovered in 1860. The molecule now known as DNA was first identified in the 1860s by a Swiss chemist named Johann Friedrich Miescher. Johann set out to research the key components of white blood cells, part of our body’s immune system. The main source of these cells was pus-coated bandages collected from a nearby medical clinic.

Johann carried out experiments using salt solutions to understand more about what makes up white blood cells. He noticed that when he added acid to a solution of the cells, a substance separated from the solution. This substance then dissolved again when an alkali was added. When investigating this substance he realised that it had unexpected properties different to those of the other proteins? he was familiar with. Johann called this mysterious substance ‘nuclein’, because he believed it had come from the cell nucleus. Unbeknown to him, Johann had discovered the molecular basis of all life – DNA. He then set about finding ways to extract it in its pure form.

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Johann was convinced of the importance of nuclein and came very close to uncovering its elusive role, despite the simple tools and methods available to him. However, he lacked the skills to communicate and promote what he had found to the wider scientific community. Ever the perfectionist, he hesitated for long periods of time between experiments before he published his results in 1874. Before then he primarily discussed his findings in private letters to his friends. As a result, it was many decades before Johann Friedrich Miescher’s discovery was fully appreciated by the scientific community.For many years, scientists continued to believe that proteins were the molecules that held all of our genetic material. They believed that nuclein simply wasn’t complex enough to contain all of the information needed to make up a genome. Surely, one type of molecule could not account for all the variation seen within species.

Albrecht Kossel was a German biochemist who made great progress in understanding the basic building blocks of nuclein.In 1881 Albrecht identified nuclein as a nucleic acid and provided its present chemical name, deoxyribonucleic acid (DNA). He also isolated the five nucleotide? bases that are the building blocks of DNA and RNA?: adenine (A), cytosine (C), guanine (G), thymine (T) and uracil (U).This work was rewarded in 1910 when he received the Nobel Prize in Physiology or Medicine.

In the early 1900s the work of Gregor Mendol was rediscovered and his ideas about inheritance began to be properly appreciated. As a result, a flood of researchers began to try and prove or disprove his theories of how physical characteristics are inherited from one generation to the next.

In the middle of the nineteenth century, Walther Flemming, an anatomist from Germany, discovered a fibrous structure within the nucleus of cells. He named this structure ‘chromatin’, but what he had actually discovered is what we now know as chromosomes?. By observing this chromatin, Walther correctly worked out how chromosomes separate during cell division, also known as mitosis?.

The chromosome theory of inheritance was developed primarily by Walter Sutton and Theodor Boveri. They first presented the idea that the genetic material passed down from parent to child is within the chromosomes. Their work helped explain the inheritance? patterns that Gregor Mendel had observed over a century before.

Interestingly, Walter Sutton and Theodor Boveri were actually working independently during the early 1900s. Walter studied grasshopper chromosomes, while Theodor studied roundworm embryos. However, their work came together in a perfect union, along with the findings of a few other scientists, to form the chromosome theory of inheritance.

Building on Walther Flemming’s findings with chromatin, German embryologist Theodor Boveri provided the first evidence that the chromosomes within the egg and sperm cells are linked to inherited characteristics. From his studies of the roundworm embryo he also worked out that the number of chromosomes is lower in egg and sperm cells compared to other body cells.

American graduate, Walter Sutton, expanded on Theodor’s observation through his work with the grasshopper. He found it was possible to distinguish individual chromosomes undergoing meiosis? in the testes of the grasshopper and, through this, he correctly identified the sex chromosome?. In the closing statement of his 1902 paper he summed up the chromosomal theory of inheritance based around these principles:

• Chromosomes contain the genetic material.
• Chromosomes are passed along from parents to offspring.
• Chromosomes are found in pairs in the nucleus of most cells (during meiosis these pairs separate to form daughter cells).
• During the formation of sperm and eggs cells in men and women, respectively, chromosomes separate.
• Each parent contributes one set of chromosomes to its offspring.

DNA, contains the patterns for constructing proteins in the body, including the various enzymes. A new understanding of heredity and hereditary disease was possible once it was determined that DNA consists of two chains twisted around each other, or double helixes, of alternating phosphate and sugar groups, and that the two chains are held together by hydrogen bonds between pairs of organic bases—adenine (A) with thymine (T), and guanine (G) with cytosine (C). Modern biotechnology also has its basis in the structural knowledge of DNA—in this case the scientist’s ability to modify the DNA of host cells that will then produce a desired product, for example, insulin.

The background for the work of the four scientists was formed by several scientific breakthroughs: the progress made by X-ray crystallographers in studying organic macromolecules; the growing evidence supplied by geneticists that it was DNA, not protein, in chromosomes that was responsible for heredity; Erwin Chargaff’s experimental finding that there are equal numbers of A and T bases and of G and C bases in DNA; and Linus Pauling’s discovery that the molecules of some proteins have helical shapes—arrived at through the use of atomic models and a keen knowledge of the possible disposition of various atoms.

Of the four DNA researchers, only Rosalind Franklin had any degrees in chemistry. She was born into a prominent London banking family, where all the children—girls and boys—were encouraged to develop their individual aptitudes. She attended Newnham College, one of the women’s colleges at Cambridge University. She completed her degree in 1941 in the middle of World War II and undertook graduate work at Cambridge with Ronald Norrish, a future Nobel laureate. She resigned her research scholarship in just one year to contribute to the war effort at the British Coal Utilization Research Association. There she performed fundamental investigations on the properties of coal and graphite. She returned briefly to Cambridge, where she presented a dissertation based on this work and was granted a PhD in physical chemistry. After the war, through a French friend, she gained an appointment at the Laboratoire Centrale des Services Chimiques de l’Etat in Paris, where she was introduced to the technique of X-ray crystallography and rapidly became a respected authority in this field. In 1951 she returned to England to King’s College London, where her charge was to upgrade the X-ray crystallographic laboratory there for work with DNA.

Already at work at King’s College was Maurice Wilkins, a New Zealand–born but Cambridge-educated physicist. As a new PhD he worked during World War II on the improvement of cathode-ray tube screens for use in radar and then was shipped out to the United States to work on the Manhattan Project. Like many other nuclear physicists, he became disillusioned with his subject when it was applied to the creation of the atomic bomb; he turned instead to biophysics, working with his Cambridge mentor, John T. Randall—who had undergone a similar conversion—first at the University of St. Andrews in Scotland and then at King’s College London. It was Wilkins’s idea to study DNA by X-ray crystallographic techniques, which he had already begun to implement when Franklin was appointed by Randall. The relationship between Wilkins and Franklin was unfortunately a poor one and probably slowed their progress.

Meanwhile, in 1951, 23-year-old James Watson, a Chicago-born American, arrived at the Cavendish Laboratory in Cambridge. Watson had two degrees in zoology: a bachelor’s degree from the University of Chicago and a doctorate from Indiana University, where he became interested in genetics. He had worked under Salvador E. Luria at Indiana on bacteriophages, the viruses that invade bacteria in order to reproduce—a topic for which Luria received a Nobel Prize in Physiology or Medicine in 1969. Watson went to Denmark for postdoctoral work, to continue studying viruses and to remedy his relative ignorance of chemistry. At a conference in the spring of 1951 at the Zoological Station at Naples, Watson heard Wilkins talk on the molecular structure of DNA and saw his recent X-ray crystallographic photographs of DNA. He was hooked.

Watson soon moved to the Cavendish Laboratory, where several important X-ray crystallographic projects were in progress. Under the leadership of William Lawrence Bragg, Max Perutz was investigating hemoglobin and John Kendrew was studying myoglobin, a protein in muscle tissue that stores oxygen. (Perutz and Kendrew received the Nobel Prize in Chemistry for their work in the same year that the prize was awarded to the DNA researchers—1962.) Working under Perutz was Francis Crick, who had earned a bachelor’s degree in physics from University College London and had helped develop radar and magnetic mines during World War II. Crick, another physicist in biology, was supposed to be writing a dissertation on the X-ray crystallography of hemoglobin when Watson arrived, eager to recruit a colleague for work on DNA. Inspired by Pauling’s success in working with molecular models, Watson and Crick rapidly put together several models of DNA and attempted to incorporate all the evidence they could gather. Franklin’s excellent X-ray photographs, to which they had gained access without her permission, were critical to the correct solution. The four scientists announced the structure of DNA in articles that appeared together in the same issue of Nature.

Bibliography

1. https://academic-eb-com.content.elibrarymn.org/levels/collegiate/article/heredity/111157
2. Britannica Academic-I used this source to get some basic information on heredity,genes and DNA. https://academic-eb-com.content.elibrarymn.org/levels/collegiate/article/DNA/30730
3. Britannica Academy-i used this article to get some basic info on the general topic, like who discovered DNA, What is DNA, when did all this happen and how it happened. https://academic-eb-com.content.elibrarymn.org/levels/collegiate/article/Friedrich-Miescher/5260
4. Britannica Academy-I used this short article to get some basic info on Frederich Meischer, the first person to isolate neuclein and discover DNA, http://www.dnaftb.org/15/bio.html