Background Info • Once scientists showed that genes control the heritability of traits, and that they are located on chromosomes, the two chemical components of chromosomes, DNA and protein, were studied as beging carriers of genetic material • Bacteria and viruses were the simplest organisms to study DNA and proteins and their involvement with genetic material
Frederick Griffith • A British medical officer • Studied a bacterium called Streptococcus pneumoniae • Can cause pneumonia in mammals • He was trying to develop a vaccine against the virulent strain, disease causing strain, of the bacterium Target #1- I can describe the Griffith experiment
Each virulent bacterium is surrounded by a capsule made of polysaccharides that protects it from the body’s defense system • S strain: a virulent strain of bacteria that grows into smooth-edged colonies • R strain: a non-virulent strain of bacteria that grows into rough colonies • Griffith used the two strains of bacteria in a series of 4 experiments • Provide insight about the nature of the hereditary material. Target #1- cont.
Griffith concluded that heat-killed virulent bacterial cells release a hereditary factor that transfers the disease-causing ability to the live harmless cells • Transformation: the transfer of genetic material from one cell to another cell or from one organism to another organism Target #2- I can explain Griffith’s conclusions
Target #3- I can describe the Avery-McCarty-MacLeod experiment • Oswald Avery • An American researcher who wanted to test whether the transforming agent in Griffith’s experiment was protein, RNA, or DNA • Used enzymes to separately destroy each of the three molecules in heat-killed S cells • Protease enzymekilledthe protein in the S cells • Dnaseenzymekilled the DNA in the S cells • Rnaseenzymekilledthe RNA in the S cells • Injected mice with each of the three types of heat-killed S cell batches with live R cells
Target #4- I can explain the conclusions of the Avery experiment • Conclusions • The cells missing protein and RNA were able to transform R cells into S cells and kill the mice • Cells missing DNA did not transform R cells into S cells • Mice survived • Concluded that DNA is responsible for transformation in bacteria
Hershey-Chase Experiment • Martha Chase & Alfred Hershey • Set out to test whether DNA or protein was the hereditary material viruses transfer when viruses enter a bacterium • Viruses that infect a bacterium are known as bacteriophages • Bacteriophage studied T2 • Caused a lytic response in the host cell Target #5- I can describe the Hersey-Chase Experiment
Step 1: radioactive isotopes were used to label the protein and DNA in the bacteriophages • Sulfur: protein label • Phosphorus: DNA label • Step 2: Allowed protein-labeled and DNA-labeled bacteriophages to infect E. Coli bacteria • Step 3: removed the bacteriophage coats from the cells in a blender • Step 4: used a centrifuge to separate the bacteriophage from the E. Coli • Conclusions: found that all of the viral DNA and little of the protein had entered E. Coli cells • DNA is the hereditary molecule in viruses Target #6- I can state the steps of the Hershey-Chase experiment