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Transcript
DNA: The Information Molecule How did scientists use evidence to discover that the DNA molecule contains information? A Historical Perspective Type of Molecule Proteins Carbohydrates Fats DNA Structure Function Complex A few known functions Simple Energy and Structural Simple Energy and Structural Simple 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) R strain is benign (Lacking a protective capsule, it is recognized and destroyed by host’s immune system) SMOOTH COLONY (S) S strain is virulent (Protective capsule prevents detection by host’s immune system) Explain each treatment: 1. What data shows the heatkilled 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. DETERMINING THAT DNA IS THE HEREDITARY MATERIAL Heat-killed S cells 1. Remove the lipids and carbohydrates from a solution of heat-killed S cells. Proteins, RNA, and DNA remain. Lipids Carbohydrates Add Add proteinases proteinases Sample should Sample contain should Contain NO PROTEIN NO PROTEIN Add R cells Add Add ribonuclease ribonuclease Sample should Sample contain should Contain NO RNA NO RNA Add R cells Add Add deoxydeoxyribonuclease ribonuclease Sample should Sample contain should Contain NO DNA NO DNA Add R cells 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 S cells S cells No S cells containing R cells. appear appear appear Observe whether transformation has Transformation occurs No transformation occurs occurred by testing for the presence Conclusion: Transformation cannot occur unless __________ is present. virulent S cells. Therefore, ______________ must be the hereditary material. Are Genes Composed of DNA or Protein? T2 Virus Cell Virus protein coat 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. HOW DO VIRUSES WORK? Virus Virus particle Virus particle particle Host Host cell genome Host cell cell genome genome Free particles in tissue or enviroment 4. Particles assemble inside host, then burst or bud to exterior. 1. 1. Viral genome 1. Viral Viral genome genome enters enters host cell. enters host host cell. cell. Protein 3. Viral mRNA are translated and proteins processed. DNA DNA mRNA mRNA 2. 2. Viral Viral genome genome is is replicated replicated and and transcribed. transcribed. Hershey Chase Experiment 1952 Two treatments: 1. Radioactive amino acids 2. Radioactive nucleotides Treatment 1 Radioactive protein (35S) Centrifuge and measure radioactivity in pellet and supernatant T2 Phage Bacterium Radioactivity in supernatant, but not pellet Radioactive phage infects bacterial cells Blender separates protein coats from bacterial surface Treatment 2 Radioactive DNA (32P) 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.