Key Experiments And Identification Of Dna Biology Essay


This essay describes the experiments conducted by Frederick Griffith, Oswald Avery, Alfred D. Hershey and Martha Chase. Also discuss how the findings of their experiments helped to the identification of DNA as the molecular basis of inheritance.

In early 1900's chromosomes discovered in nucleus. The chromosome carries the genetic information which passes from parent to offspring. Chromosomes consist of nucleic acid and protein. At those times many scientists believed that protein was the genetic material. However, later it is found that DNA is the genetic material. Series of experiments carried out by scientists explains how they discover the 'transforming principle', its agent and the heredity material.

In 1928, a British microbiologist Frederick Griffith was made observations with using bacteria called Streptococcus pneumoniae (pneumoccous) which causes pneumonia. Griffith found that, heat killed bacteria could 'transform' to a different living bacteria. Griffith was demonstrated the transformation experiment using two strains of bacterium. One was the S strain a virulent strain surrounded by polysaccharide coat (necessary for infection) produces smooth shiny colonies. The other was R strain a non-virulent strain, produces rough colonies. The R strain was the 'mutation' of S strain which does not have any polysaccharide coat and does not cause any disease. Griffith was worked with IIS and IIIS strains. S cells can mutate into R cells or R cells can mutate into S cells. Griffith was injected to mice with IIR (non-virulent) and IIIS (virulent) strains and was reported whether the mice survived or died. Firstly Griffith was injected smooth virulent strain (IIIS) and mice died. When the mice were injected with rough non-virulent strain (IIR) the mice continued to be healthy. Griffith was injected IIIS bacteria which killed by heat before injection and the mice stayed healthy. These three experiments showed that the bacterium must alive and must have a polysaccharide coat to kill the mice. Griffith's fourth experiment was to inject heat-killed smooth virulent strain mixed with non-virulent rough strain. (IIR and IIIS heat treated) Griffith was injected the mixture to mice and the mice died. Living S bacteria were found in the mice blood. This was the IIIS type of strain. Griffith theorized that transformation takes place from IIR to IIIS cells. Griffith believed a protein could be an agent for the change in genetic material and Griffith called the agent the 'transforming principle'.

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In 1930's and 1940's Oswald Avery with his co-workers, Colin M. MacLeod and Maclyn McCarty found the agent causes genetic transforming is the nucleic acid DNA, not protein. In 1944 they discovered the 'transforming principle'. Avery and his co-workers studied the transformation of R-type bacteria to S-type bacteria. First they broke open heat treated IIIS bacteria with detergent to release the content of cell. Then they treated the bacteria with centrifugation which eliminate cellular components. (Cell extract from cellular debris). The extract mixed with IIR, incubated and plated on peri-dish. The colonies appeared which shows bacteria has transformed. The scientist knew that the one of component (polysaccharides, protein, DNA, RNA) in the extract causes transformation. To find out which macromolecular component causes transformation they did enzyme treatments. They degrade polysaccharides with S111 enzyme and proteins with protease enzyme, where the enzyme digests the polysaccharide and protein respectively. The transformation of IIR to IIIS occurred. Transforming principle was either DNA or RNA so; Avery used the enzyme called nucleases, which degrade nucleic acids. They used ribonuclease (RNase) for RNA, which degrade RNA. The colonies on the plate showed that RNA was not the transforming principle. In the end they treated DNA with deoxyribonuclease (DNase) which degrades the DNA and no transformation occurred. These showed that DNA was the genetic material causes transformation.

Avery's transformation experiment was important because to be able to find the transforming principle Avery discovered isolation of nucleic acid from bacteria contaminated by proteins.

In 1953, Alfred Hershey and Martha Chase discovered that DNA is the genetic material. They studied about bacteriophage (T2).

The T2 phage infects the E.Coli by attaching to the bacterium (host cell). Then phage releases its genetic material to the host cell. Bacterial enzymes provide offspring. The host cell then break open, giving out 100-200 phages. Hershey and Chase knew bacteriophage was a virus that infect bacteria and composed from protein and DNA. They wanted to know which substance either protein or DNA made the genetic material of a bacteriophage so; they designed experiments to find out the true components. Hershey and Chase prepared two T2 phages one containing radioactive DNA and the other containing radioactive proteins. To be able to provide radioactive components they infected E.Coli with T2. Then they grew E.Coli bacteria in two cultures. One media was containing radioactive isotope P-32 and the other containing radioactive isotope S-35. The bacteria which grown in the P-32 media is radioactively labelled DNA because DNA contains phosphorus and the bacteria which grown in the S-35 media is radioactively labelled protein because protein contains sulphur. Each of the two types of radioactively labelled bacteriophage placed into different culture of bacteria. The bacteriophages then infected the bacteria. Hershey and Chase used a blender and a centrifuge to separate the phage's protein parts. (Protein part is called phage ghost) The remaining part is infected cell.

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In the end Hershey and Chase observed the radioactively labelled DNA (P-32) found in host cell and very little found in phase ghost. The radioactively labelled protein (S-35) appeared in phase ghost and none appeared within the host cell. Thus, the radioactive protein had not entered the host cells, but the DNA had. The genetic material passed from parent to offspring. Hershey and Chase proofed that DNA was the genetic material.

In 1969 Alfred Hershey shared a Nobel Prize with Max Delbrück and Salvador E. Luria "for their discoveries concerning the replication mechanism and the genetic structure of viruses". (

At the end of 1920's the transforming experiment is resulted the formation of transforming principle. Transforming principle, which is discovered by Griffith, helped Avery and his co-workers (Colin MacLeod and Maclyn McCarty) to find that the agent of transforming principle is DNA. Hershey and Chase evidenced that DNA was the heredity material and they confirmed DNA's pre-eminent role in genetics. These experiments persuaded the other scientists that DNA alone was the heredity material. This inspired James Watson and Francis Crick to begin to discover the structure of DNA. In 1953 Watson and Crick proofed the physical and chemical structure of DNA.