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8.1 Identifying DNA as the Genetic Material KEY CONCEPT DNA was identified as the genetic material through a series of experiments.

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Presentation on theme: "8.1 Identifying DNA as the Genetic Material KEY CONCEPT DNA was identified as the genetic material through a series of experiments."— Presentation transcript:

1 8.1 Identifying DNA as the Genetic Material KEY CONCEPT DNA was identified as the genetic material through a series of experiments.

2 8.1 Identifying DNA as the Genetic Material 10.1 DNA: The Hereditary Material 1868 Miescher found material in nucleus half protein and half something else 1890’s learned more about the unknown and named it deoxyribonucleic acid or DNA 1902 Walter Sutton proposed hereditary material is in the chromosomes Chromosome have equal parts of protein and DNA Scientists still had not determined whether DNA or protein was the one that made up genes

3 8.1 Identifying DNA as the Genetic Material Griffith finds a ‘transforming principle.’ Griffith experimented with the bacteria that cause pneumonia. He used two forms: the S form (deadly) and the R form (not deadly). A transforming material passed from dead S bacteria to live R bacteria, making them deadly.

4 8.1 Identifying DNA as the Genetic Material Genetic Material and Transformation Frederick Griffith Frederick Griffith, British microbiologist, made a series of unexpected observations while performing an experiment with the disease-causing bacteria pneumococcus and laboratory mice. Griffith's experiment dealt with two strains of the bacteria pneumococcus. One was a virulent strain with a smooth polysaccharide coat necessary for infection and colonies of this strain appear smooth. The other was a non-virulent strain with a rough coat that could not cause infection and colonies of the strain appear rough. Griffith injected one group of mice with the smooth virulent strain and these mice died after a few days. He then injected another group with the rough non-virulent strain and these mice continued to be healthy. Griffith took a heat-killed strain of the virulent bacteria and injected it into mice and observed that they did not die. Griffith's fourth experiment was to inject heat treated, killed, smooth virulent strain mixed with the non virulent rough strain. He injected this mixture and found that after a few days the mice died. The blood of the dead mice showed high levels of virulent pneumococcus. Griffith theorized that some type of transformation takes place from the virulent to the non- virulent strain for it to synthesize a new polysaccharide coat. He did not know if it was DNA or protein that was being transferred.

5 8.1 Identifying DNA as the Genetic Material Avery identified DNA as the transforming principle. Avery isolated and purified Griffith’s transforming principle. Avery performed three tests on the transforming principle. –Qualitative tests showed DNA was present. –Chemical tests showed the chemical makeup matched that of DNA. –Enzyme tests showed only DNA-degrading enzymes stopped transformation.

6 8.1 Identifying DNA as the Genetic Material Transformation- the process during which bacteria are changed by absorbing genetic material from an outside source Oswald Avery, Canadian physician and bacteriologist, found that the agent responsible for genetic transferring is the nucleic acid DNA and not protein as most biochemists theorized at the time. In 1944 Avery and his coworkers, McCarty and MacLeod, discovered the "transforming principle.“ The Experiment  First they treated the bacteria with centrafugation, which eliminates large cellular pieces. The result: bacteria still transformed  Added protease, which removes all proteins The result: bacteria still transformed  Treated the bacteria with deoxyribonuclease, which eliminates all DNA The result: no transformation in the bacteria  Conclusion: the trio concluded that DNA is the cause of transformation, where in this experiment virulence is inherited. There was not enough evidence to convince everyone

7 8.1 Identifying DNA as the Genetic Material Hershey and Chase confirm that DNA is the genetic material. Hershey and Chase studied viruses that infect bacteria, or bacteriophages. Tagged DNA was found inside the bacteria; tagged proteins were not. – They tagged viral DNA with radioactive phosphorus. – They tagged viral proteins with radioactive sulfur.

8 8.1 Identifying DNA as the Genetic Material DNA and Bacterial Viruses 1952: Geneticists Alfred Hershey and Martha Chase publish the findings of their so- called blender experiments, which conclude that DNA is where life's hereditary data is found. Prior to these experiments, so named because they were conducted using a regular kitchen blender, it was generally believed that proteins -- not DNA -- were the genetic stuff of life. Using the blender, Hershey and Chase separated the protein coating from the nuclei of bacteriophages, the viruses that infect bacteria. Injecting nucleic acid into the bacterial cell, they found that it was the acid itself, and not the protein, that caused the transmission of genetic information. Their conclusion was that genes are made of the nucleic acid DNA. Hershey would subsequently share the 1969 Nobel Prize in Physiology or Medicine for his work in discovering the properties of DNA. But Chase, who served as Hershey's lab assistant during his experiments and whose name appears on the paper, was snubbed

9 8.1 Identifying DNA as the Genetic Material


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