bio115 note2

bio115 note2 - Bio115 Spring 2009(Hinrich Boeger Notes to...

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Bio115 Spring 2009 (Hinrich Boeger) Notes to Lecture 2 Slide 1 The experiment which eventually convinced most people that indeed DNA was the carrier of genetic information rather than proteins was the “Warring blender experiment” by Alfred Hershey and Martha Chase. Like human beings, bacteria can fall victim to viruses, called bacteriophages, or for short, phages. In the 1940ies it was realized that all phages ecompass a protein shell, the capsid, which contains nucleic acid. After infection the bacterial host produces many new phage particles in its cytoplasm, which are eventually released from the cell by cell lysis. Alfred Hershey and Martha Chase fed infected bacteria with 35S-labeled methionine labeling all proteins and only proteins, including those of the phage capsid. Such radiolabeled phages were used to infect other bacteria. The phage attaches to the outside of the bacterium and releases its genetic information into the host cell. Upon blending the bacteria in a blender, which generates strong shearing forces, shortly after infection, Hershey and Chase found that the radiolabeled viral proteins could be seperated from the bacteria, whereas the infected bacteria contained little or no radioisotope. In contrast, when Hershey and Chase repeated the same experiment with phages whose DNA was labeled with 32P, they found that the radioisotope was efficiently transmitted to the host upon infection, whereas the viral capsids contained little or no radioisotope. Slide 4 Note that the plane of the base is perpendicular to the plane of the deoxyribose ring, which had been determined by X-ray crystallography on single nucleotides. Slide 5
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The four bases found in DNA are either derivatives of purine (adenine and guanine) or pyrimidine (cytosine and thymidine). Memorize these structures. Slide 8 Erwin Chargaff found that the A:T and the G:C ratio in DNAs from different organisms is always close to 1. This is called Chargaff’s rule. It provided a critical hint to Watson and Crick with regard to the structure of DNA. Chargaff determined ratios by paper chromatography after hydrolytic cleavage of DNA. Slide 9 The decisive experiments for solving the structure of DNA were made by Rosalind Franklin, a crystallographer at King’s College in London, where she made her important X-ray pictures of A and B form DNA. She tragically died at young age of ovarian cancer. Slide 10
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bio115 note2 - Bio115 Spring 2009(Hinrich Boeger Notes to...

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