Scientists DNA Webquest

DNA Webquest

The History of DNA

Click on each famous scientist to learn about their accomplishments and discoveries.

1952

1952

1953

1868

1928

1950

1944

Rosalind Franklin

Oswald Avery

Friedrich Miescher

Frederick Griffith

Erwin Chargaff

James Watson & Francis Crick

Alfred Hershey & Martha Chase

Friedrich Miescher

(1844-1895)

Friedrich (Fritz) Miescher was born in Basel, Switzerland. Miescher was an excellent student despite his shyness and a hearing handicap. Miescher initially wanted to be a priest, but his father opposed the idea and Miescher entered medical school. When he graduated in 1868, Miescher ruled out specialties where patient interactions were necessary because of his hearing problem. He decided to base his career on medical research. He went to the University of Tübingen to study under Felix Hoppe-Seyler in the newly established faculty of natural science. At a time when scientists were still debating the concept of "cell," Hoppe-Seyler and his lab were isolating the molecules that made up cells. Miescher was given the task of researching the composition of lymphoid cells — white blood cells. These cells were difficult to extract from the lymph glands, but they were found in great quantities in the pus from infections. Miescher collected bandages from a nearby clinic and washed off the pus. He experimented and isolated a new molecule - nuclein - from the cell nucleus. He determined that nuclein was made up of hydrogen, oxygen, nitrogen and phosphorus and there was an unique ratio of phosphorus to nitrogen. Although Miescher did most of his work in 1869, his paper on nuclein wasn't published until 1871. Nuclein was such a unique molecule that Hoppe-Seyler was skeptical and wanted to confirm Miescher's results before publication. Miescher continued to work on nuclein for the rest of his career. It would be years before the role of nucleic acids were recognized. Miescher, himself, believed that proteins were the molecules of heredity. However, Miescher laid the groundwork for the molecular discoveries that followed. Miescher died in 1895 from tuberculosis.

Frederick Griffith

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(1877-1941)

In 1928, British bacteriologist Frederick Griffith conducted a series of experiments using Streptococcus pneumoniae bacteria and mice. Griffith wasn't trying to identify the genetic material, but rather, trying to develop a vaccine against pneumonia. In his experiments, Griffith used two related strains of bacteria, known as R and S.

• R strain. When grown in a petri dish, the R bacteria formed colonies, or clumps of related bacteria, that had well-defined edges and a rough appearance (hence the abbreviation "R"). The R bacteria were nonvirulent, meaning that they did not cause sickness when injected into a mouse.

• S strain. S bacteria formed colonies that were rounded and smooth (hence the abbreviation "S"). The smooth appearance was due to a polysaccharide, or sugar-based, coat produced by the bacteria. This coat protected the S bacteria from the mouse immune system, making them virulent (capable of causing disease). Mice injected with live S bacteria developed pneumonia and died.

As part of his experiments, Griffith tried injecting mice with heat-killed S bacteria (that is, S bacteria that had been heated to high temperatures, causing the cells to die). Unsurprisingly, the heat-killed S bacteria did not cause disease in mice. The experiments took an unexpected turn, however, when harmless R bacteria were combined with harmless heat-killed S bacteria and injected into a mouse. Not only did the mouse develop pnenumonia and die, but when Griffith took a blood sample from the dead mouse, he found that it contained living S bacteria! Griffith concluded that the R-strain bacteria must have taken up what he called a "transforming principle" from the heat-killed S bacteria, which allowed them to "transform" into smooth-coated bacteria and become virulent.

Oswald Avery

(1877-1955)

Oswald Theodore Avery was born in 1877 in Halifax, Nova Scotia. Avery attended Colgate University, where he excelled in literature, public speaking, and debate, earning his BA in the humanities in 1900. Avery moved in 1907 to laboratory work at the Hoagland Laboratory in Brooklyn. Avery initially worked on the bacteriology of yogurt, but soon developed an interest in tuberculosis after his mentor, Benjamin White, contracted a case. It was during this time that Avery’s effort to understand the biological activities of pathogenic bacteria through a knowledge of their chemical composition was set forth. Avery was one of the first molecular biologists and was a pioneer in immunochemistry, but he is best known for his discovery in 1944 with his co-workers Colin MacLeod and Maclyn McCarty that DNA is the material of which genes and chromosomes are made. In a very simple experiment, Oswald Avery's group showed that DNA was the "transforming principle." When isolated from one strain of bacteria, DNA was able to transform another strain and confer characteristics onto that second strain. DNA was carrying hereditary information. With DNA as the hereditary molecule, the stage was set for one of the most exciting periods in DNA science: understanding DNA structure and function. It is said that Avery was the most deserving scientist not to receive the Nobel Prize for his work. Eventually, however, the role of DNA was proved, and Avery’s contribution to genetics was recognized.

Alfred Hershey & Martha Chase

(1908-1997) (1927-2003)

In 1952, Alfred Hershey and Martha Chase took an effort to find the genetic material in organisms. Their experiments led to DNA as genetic material. Bacteriophages (viruses that affect bacteria) were the key element for Hershey and Chase experiment. The virus doesn’t have their own mechanism of reproduction but they depend on a host for the same. Once they attach to the host cell, their genetic material is transferred to the host. Here in case of bacteriophages, bacteria are their host. The infected bacteria are manipulated by the bacteriophages such that bacterial cells start to replicate the viral genetic material. Hershey and Chase conducted an experiment to discover whether it was protein or DNA that acted as the genetic material that entered the bacteria.

Experiment: The experiment began with the culturing of viruses in two types of medium. One set of viruses (A) was cultured in a medium of radioactive phosphorus whereas another set (B) was cultured in a medium of radioactive sulfur. They observed that the first set of viruses (A) consisted of radioactive DNA but not radioactive proteins. This is because DNA is a phosphorus-based compound while protein is not. The latter set of viruses (B) consisted of radioactive protein but not radioactive DNA. The host for infection was E.coli bacteria. The viruses were allowed to infect bacteria by removing the viral coats through a number of blending and centrifugation. Observation: E.coli bacteria which were infected by radioactive DNA viruses (A) were radioactive but the ones that were infected by radioactive protein viruses (B) were non-radioactive. Conclusion: Resultant radioactive and non-radioactive bacteria infer that the viruses that had radioactive DNA transferred their DNA to the bacteria but viruses that had radioactive protein didn’t get transferred to the bacteria. Hence, DNA is the genetic material and not the protein.

Erwin Chargaff

(1905-2002)

Erwin Chargaff was an Austrian-American biochemist born in 1905 and known for pioneering molecular genetics research. He made significant contributions to our understanding of DNA and RNA composition. Chargaff’s work on DNA base composition laid the foundation for Chargaff’s Rules. Chargaff’s observation led to the understanding of complementary base pairing.

• Adenine (A) always pairs with thymine (T)
• Cytosine (C) pairs with guanine (G).
• The amount of A always equalled the amount of T, and the amount of C always equalled the amount of G (A = T and G = C)

This pairing is based on hydrogen bonding: A-T pairs are held together by two hydrogen bonds, while three hydrogen bonds hold C-G pairs together. In contrast, the complementary base pairing is vital for the stability and structure of the DNA double helix.

Rosalind Franklin

(1920-1958)

Photo 51

Rosalind Franklin was born in London, England. Franklin was extremely intelligent and she knew by the age of 15 that she wanted to be a scientist. In 1951, Franklin was offered a 3-year research scholarship at King's College in London. Franklin was to set up and improve the X-ray crystallography unit at King's College. Maurice Wilkins was already using X-ray crystallography to try to solve the DNA problem at King's College. Working with a student, Raymond Gosling, Franklin was able to get two sets of high-resolution photos of crystallized DNA fibers. She used two different fibers of DNA, one more highly hydrated than the other. From this she deduced the basic dimensions of DNA strands, and that the phosphates were on the outside of what was probably a helical structure. In 1962, James Watson, Francis Crick and Maurice Wilkins got the Nobel Prize for the discovery of the shape of DNA. Photo 51 was an X-ray diffraction image that gave them some crucial pieces of information. It was only after seeing this photo that Watson and Crick realized that DNA must have a double helical structure. The problem was that Photo 51 was actually made by Rosalind Franklin. Maurice Wilkins, a colleague, had shown this picture to Watson and Crick without even letting her know. This added to the tension at the time of the discovery of DNA. Unlike her colleagues, Franklin was not awarded a Nobel Prize for her contributions to this important discovery. She died in 1958 and the Nobel Prize cannot be obtained posthumously.

James Watson & Francis Crick

(1928-) (1916-2004)

In the early 1950s, American biologist James Watson and British physicist Francis Crick came up with their famous model of the DNA double helix. Some of their most crucial clues to DNA's structure came from Rosalind Franklin, a chemist working in the lab of physicist Maurice Wilkins. Franklin’s crystallography gave Watson and Crick important clues to the structure of DNA. Some of these came from the famous “image 51,” a remarkably clear and striking X-ray diffraction image of DNA produced by Franklin and her graduate student. To Watson, the X-shaped diffraction pattern of Franklin's image immediately suggested a helical, two-stranded structure for DNA.

The structure of DNA, as represented in Watson and Crick's model, is a double-stranded, antiparallel, right-handed helix. The sugar-phosphate backbones of the DNA strands make up the outside of the helix, while the nitrogenous bases are found on the inside and form hydrogen-bonded pairs that hold the DNA strands together. In the model to the left, the orange and red atoms mark the phosphates of the sugar-phosphate backbones, while the blue atoms on the interior of the helix belong to the nitrogenous bases.

Watson and Crick brought together data from a number of researchers (including Franklin, Wilkins, Chargaff, and others) to assemble their celebrated model of the 3D structure of DNA. In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Medicine. Unfortunately, by then Franklin had died, and Nobel prizes are not awarded posthumously.