DeoxyriboNucleic Acid

Transcription

1 12-1 DeoxyriboNucleic Acid There are 2 molecules shown in this image. The structure of one molecule is a double helix, or twisted ladder. functions to store and copy the genetic information of HEREDITY! We know this due to a series of 3 experiments performed by 4 scientists... 1

2 Griffith and Transformation Experiment 1 Fredrick Griffith (1928) was a British scientist who studied diseases. His specific focus of study was the bacteria that cause pneumonia (Streptococcus pneumoniae) Experiment 1 was to inject mice with the smooth S-strain of S. pneumoniae. The control was mice injected with distilled water. The independent variable was whether or not mice receive bacteria, and the dependent variable was whether or not the mice die from pneumonia. The results were that the experimental mice died, indicating that the S-strain was pathogenic and would cause pneumonia. No surprise! Experiment 2 was the same as Experiment 1, except using a rough R-strain of S. pneumoniae. The results were that the experimental mice lived indicating the R-strain was nonpathogenic. Interesting! Experiment 2 Harmless bacteria (rough colonies) Lives 2

3 Griffith and Transformation Experiment 3 The cells of heat-killed bacteria are still intact but their enzymes have been denatured so they no longer live and reproduce. Experiment 3 was conducted the same as Experiments 1, however the normally pathogenic S-strain bacteria were heat-killed prior to their injection into mice. The results were that the mice lived, indicating that heatkilling rendered the bacteria unable to cause disease (non-pathogenic). No surprise! Heat-killed disease-causing bacteria (smooth colonies) Lives 3

4 Griffith and Transformation Experiment 4 was conducted just like Experiment 1, except Griffith injected the experimental mice with a combination of heat-killed S-strain, normally pathogenic bacteria and the non-pathogenic Rstrain. The results were that the experimental mice died, indicating that the combination was pathogenic. SURPRISE! In addition, bacteria cultured from the blood of dead mice were the living S-strain pathogenic bacteria. SURPRISE! Griffith conclude that the normally nonpathogenic R-strain had somehow transformed into the pathogenic S-strain by taking up SOMETHING from the killed s-strain! Dead Living Living R-strain Heat-killed diseasecausing bacteria (smooth colonies) Harmless bacteria (rough colonies) How???? S-strain Experiment 4 S-strain Live disease-causing bacteria (smooth colonies) Griffith s Transformation What??? Dies of pneumonia 4

5 Avery and Oswald Avery et al. (1940) experimented to answer Griffith s WHAT? question by repeating his Experiment 4. The difference was that Avery used an extract of the heat-killed S-strain bacteria that he treated with various hydrolytic Protein Camp enzymes. Hydrolytic enzymes break down organic biological compounds. In Avery's first experiment, he treated But... the extract with all the hydrolytic enzymes; proteases, lipases, carbases, and nucleases. Transformation did not occur, indicating that the genetic material was a known organic biological compound. In Avery's second experiment, he used...so it took one more experiment!... all the same enzymes except the nuclease, DNase. Transformation did occur, indicating that the genetic material was! (see cartoon) 5?

6 The Hershey-Chase Experiment TEM of T4 Bacteriophage Alfred Hershey and Martha Chase (1952) experimented to determine once and for all whether the genetic material was protein or? Bacteriophages are viruses that infects bacteria. A bacteriophage was perfect for answering this question because they are made of nothing but protein and. 6

7 The Hershey-Chase Experiment Viruses are non-living obligate intracellular parasites! A bacteriophage first attaches to a bacterium with its tail fibers. It then injects its genetic material into the bacterium. The viral genetic material is replicated and expressed into viral proteins using the host bacterium s metabolic processes. Viral proteins and genetic material self-assemble into new virus particles. Viral enzymes cause the bacterial plasma membrane and cell wall to rupture, releasing the new viruses which can go on to infect more cells to replicate more viruses.? Hershey and Chase understood this process. What they did not know and experimented to find out was the identity of the genetic material, either protein or! 7

8 Bacteriophage with sulfur-35 in protein coat The Hershey-Chase Experiment Phage infects bacterium No radioactivity inside bacterium Hershey and Chase prepare the 35S-labeled bacteriophages for their first experiment by culturing bacteriophages and their host bacteria in media containing 35S-labeled amino acids, so they would incorporate them into their proteins. Bacteriophages with radioactively labeled proteins were allowed to infect host bacteria, injecting their genetic material into the cell. A blender and cetrifuge was used to separate the bacteriophages from the bacterial cells, which were then scanned for radioactivity. There was none. These results indicated that viral proteins did not enter host bacterial cells, REFUTING the hypothesis that protein is the genetic material. 8

9 Bacteriophage with phosphorus-32 in The Hershey-Chase Experiment Phage infects bacterium (click) Radioactivity inside bacterium Hershey and Chase prepare the 32P-labeled bacteriophages for their second experiment by culturing bacteriophages and their host bacteria in media containing 32P-labeled deoxyribo nucleotides, so they would incorporate them into their. Bacteriophages with radioactively labeled were allowed to infect host bacteria, injecting their genetic material into the cell. A blender and cetrifuge was used to separate the bacteriophages from the bacterial cells, which were then scanned for radioactivity. They were radioactive! These results indicated that viral entered host bacterial cells, SUPPORTING the hypothesis that is the genetic material! The scientific community widely accepted these results and conclusions which continue to be supported by all evidence. 9