Even though researchers discovered that the factor responsible for the inheritance of traits comes from within the organisms; they failed to identify the hereditary material. The chromosomal components were isolated but the material which is responsible for inheritance remained unanswered. Griffith’s experiment was a stepping stone for the discovery of genetic material. It took a long time for the acceptance of DNA as genetic material. Let’s go through the discovery of DNA as genetic material. We know about Griffith’s experiment and experiments that followed to discover the hereditary material in organisms. Based on Griffith’s experiment, Avery and his team isolated DNA and proved DNA to be the genetic material. But it was not accepted by all until Hershey and Chase published their experimental results. In 1952, Alfred Hershey and Martha Chase took an effort to find the genetic material in organisms. Their experiments led to an unequivocal proof 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. For more video lessons on DNA as genetic material, visit BYJU’S.
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Who first identified DNA?
Although James Watson and Francis Crick determined the double-helical structure of DNA, DNA itself was identified nearly 90 years earlier by Swiss chemist Friedrich Miescher. While studying white blood cells, Miescher isolated a previously unknown type of molecule that was slightly acidic and contained a high percentage of phosphorus. Miescher named this molecule "nuclein," which was later changed to "nucleic acid" and eventually to "deoxyribonucleic acid," or DNA. Interestingly, Miescher did not believe that nuclein was the carrier of hereditary information, because he thought it lacked the variability necessary to account for the incredible diversity among organisms. Rather, like most scientists of his time, Miescher believed that proteins were responsible for heredity, because they existed in such a wide variety of forms.
Who linked DNA to heredity?
For multiple decades following Miescher's discovery, most scientists continued to believe that protein, not DNA, was the carrier of hereditary information. This changed in 1944, when biologist Oswald Avery performed a series of groundbreaking experiments with the bacteria that cause pneumonia. At the time, scientists knew that some types of these bacteria (called "S type") had an outer layer called a capsule, but other types (called "R type") did not. Through a series of experiments, Avery and his colleagues found that only DNA could change R type bacteria into S type. This meant that something about DNA allowed it to carry instructions from one cell to another. This was not true of any other substances within the bacteria, including protein. This result highlighted DNA as the "transforming factor," thereby making it the best candidate for the hereditary material.
Who confirmed Avery's findings?
But just how did the injection of viral DNA into a bacterium create new viruses? Hershey and Chase admitted that they were unsure of the answer to this question; however, they knew it didn't have anything to do with protein, but did have something to do with DNA. Thanks to additional research, scientists now know that the DNA in a virus can take over a bacterial cell, causing it to replicate only the viral DNA and to create new viruses (Figure 2). This process is a form of hijacking, wherein the viral life-form takes over the regular machinery inside another life-form (in this case, a single bacterial cell). Under normal circumstances, a bacterial cell will reproduce by a form of cell division called binary fission. When Avery, MacLeod, McCarty, Hershey, and Chase performed their experiments, scientists knew that binary fission involved the copying of the hereditary substance and the redistribution of this substance into two new cells. So, when DNA was proven to be the material responsible for controlling the operations inside a single cell, it became easier to understand how the process of cell division and the transfer of the DNA could control the characteristics of newly born cells. Therefore, although they did not state it explicitly, Hershey and Chase had presented experiments that clearly suggested that DNA controls the production of more DNA, and that DNA itself was the substance that directed the construction and function of living things. Only one year after Hershey and Chase performed these experiments, James Watson and Francis Crick determined the three-dimensional structure of DNA. This discovery enabled investigators to put together the story of how DNA carries hereditary information from cell to cell. Indeed, the experiments connecting heredity and the structure of DNA were happening in parallel, so the next few years would be an exciting time for the discovery of DNA function.
Alloway, J. L. The transformation in vitro of R pneumococci into S forms of different specific types by the use of filtered pneumococcus extracts. Journal of Experimental Medicine 55, 91–99 (1932) Avery, O. T., & Goebel, W. F. Chemoimmunological studies on the soluble specific substance of pneumococcus I: The isolation and properties of the acetyl polysaccharide of pneumococcus type I. Journal of Experimental Medicine 58, 731–755 (1933) Avery, O. T., MacLeod, C. M., & McCarty, M. Studies on the chemical nature of the substance inducing transformation of pneumococcal types: Induction of transformation desoxyribonucleic acid fraction isolated from pneumococcus type III. Journal of Experimental Medicine 79, 137–157 (1944) Griffith, F. The significance of pneumococcal types. Journal of Hygiene 27, 113–159 (1928) McCarty, M. Discovering genes are made of DNA. Nature 421, 406 (2003) doi:10.1038/nature01398 (link to article) National Library of Medicine. "Profiles in Science: Oswald T. Avery Collection." (accessed on September 30, 2008) Sia, R. H. P., & Dawson, M. H. In vitro transformation of pneumococcal types II: The nature of the factor responsible for the transformation of pneumococcal types. Journal of Experimental Medicine 54, 701–710 (1931) Steinman, R. M., & Moberg, C. L. A triple tribute to the experiment that transformed biology. Journal of Experimental Medicine 179, 379–384 (1994) |
Who demonstrated that DNA is the genetic material?
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