DNA: The Information Molecule How did scientists use evidence to discover that the DNA molecule contains information?

A Historical Perspective ComplexA few known functions Simple Energy and Structural Unknown

What molecule contains information in cells? Experiment 1: Griffith (1928) Molecules from dead virulent Streptococcus can transform benign Streptococcus into a virulent strain.

There are two strains of Streptococcus pneumoniae. ROUGH COLONY (R)SMOOTH COLONY (S) R strain is benign (Lacking a protective capsule, it is recognized and destroyed by host’s immune system) S strain is virulent (Protective capsule prevents detection by host’s immune system)

Explain each treatment: 1. What data shows the heat- killed S strain did not cause death of the mouse in treatment #4? 2. What data shows S strain molecules cannot kill the mouse alone; the streptococcus must be alive? 3. What data shows that the R strain alone will not kill the mouse? 4. What data shows that the S strain IS virulent?

Griffith’s Conclusion The R strain was able to take in molecules from the dead S strain. These molecules somehow gave the R strain characteristics of the dead S strain. –Ability to make protective coating So, we know that there is a molecule that transfers information… BUT…we still haven’t proved what those molecules are!

What molecule contains information in cells? Experiment 2. (Avery et al., 1942) –Extracted material from dead virulent Streptococcus and treated it with enzymes to destroy either DNA or RNA or proteins.

Sample should Contain NO PROTEIN Add deoxy- ribonuclease Sample should Contain NO DNA Add ribonuclease Sample should Contain NO RNA Add proteinases Sample should contain NO PROTEIN Add deoxy- ribonuclease Sample should contain NO DNA Add ribonuclease Sample should contain NO RNA Add R cells S cells appear S cells appear No S cells appear Transformation occursNo transformation occurs Conclusion: Transformation cannot occur unless __________ is present. Therefore, ______________ must be the hereditary material. Add R cells Add proteinases DETERMINING THAT DNA IS THE HEREDITARY MATERIAL Heat-killed S cells Lipids Carbohydrates 1. Remove the lipids and carbohydrates from a solution of heat-killed S cells. Proteins, RNA, and DNA remain. 2. Subject the solution to treatments of enzymes to destroy either the proteins, RNA, or DNA. 3. Add a small portion of each sample to a culture containing R cells. Observe whether transformation has occurred by testing for the presence virulent S cells.

Are Genes Composed of DNA or Protein? T2 Virus Cell

Virus protein coat Host cell membrane Virus DNA 1. Start of infection. Virus DNA enters host cells. Protein coat does not. 2. Virus DNA directs the production of new virus particles. 3. End of infection. New generation of virus particles burst from host cell.

Host cell genome Virus particle 1. Viral genome enters host cell. Host cell genome Virus particle 1. Viral genome enters host cell. 2. Viral genome is replicated and transcribed. DNA mRNA Host cell genome Virus particle 1. Viral genome enters host cell. 2. Viral genome is replicated and transcribed. DNA mRNA 3. Viral mRNA are translated and proteins processed. Protein HOW DO VIRUSES WORK? Free particles in tissue or enviroment 4. Particles assemble inside host, then burst or bud to exterior.

Hershey Chase Experiment 1952 Two treatments: 1.Radioactive amino acids 2.Radioactive nucleotides

Treatment 1 T2 Phage Bacterium Radioactive protein ( 35 S) Radioactive phage infects bacterial cells Blender separates protein coats from bacterial surface Centrifuge and measure radioactivity in pellet and supernatant Radioactivity in supernatant, but not pellet

Treatment 2 Radioactive DNA ( 32 P) Radioactivity in pellet, but not supernatant Therefore, it is the viral DNA, and not protein, that programs cells to make copies of the virus.

Are Genes Composed of DNA or Protein? T2 virus experiment convinces skeptics that DNA is the genetic material since the material injected by the virus into host cells is DNA, not protein. T2 Virus

What we learned DNA is the molecule that carries information Information in DNA determines which proteins can be made by an organism DNA can be passed between organisms in nature. Bacteria can take in pieces of DNA from their environment. This process is called transformation.