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BIOA01H3 (202)
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Chapter 11

chapter 11

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University of Toronto Scarborough
Biological Sciences

My notes Chapter 11Lecture 6 111 What is the evidence that the gene is DNA y A new dye was developed that could bind specifically to DNA and turned red in direct proportion to the amount of DNA present in a cellThis technique proved circumstantial evidence that DNA was the genetic materialIt was in the right placeDNA was confirmed to be an important component of the nucleus and the chromosomes which were known to carry genesIt varied among speciesWhen cells from different species were stained with the dye and their colour intensity measured each species appeared to have its own specific amount of nuclear DNAIt was present in the right amounts The amount of DNA in somatic cells body cells not specialized for reproduction was twice that in reproductive cells eggs or spermas might be expected for diploid and haploid cells respectively y In the 1920s Griffith was studying the bacterium Streptococcus pneumonia or pneumococcus one of the agents that cause pneumonia in humansHe was trying to find a vaccine against the illnessGriffith was working with two strainsCells of the S strain produced colonies that looked smooth SThe strain was dangerousCells of the R strain produced colonies that looked rough R lacked the protective capsule and were not dangerousGriffith concluded that in the presence of the dead S pneumococci some of the living R pneumococci had been transformed into virulent S strain organisms y Transforming principle an early term for the as yet unidentified chemical substance responsible for bacterial transformationOswald Avery and his colleagues y Avery with Colin MacLeod and Maclyn McCarty isolated virtually pure DNA from a sample containing pneumococcal transforming principle and showed that it cause bacterial transformationIt had little impact becauseMost scientists did not believe that DNA was chemically complex enough to be the genetic material especially given the much greater chemical complexity of proteinsBacterial genetics was a new field of studyit was not yet clear that bacteria even had genes y In 1952 Alfred Hershey and Martha Chase published a paper that had a much greater impact than Averys 1944 paper y The HersheyChase experiment which sought to determine whether DNA or protein was the genetic material was carried out with a virus bacteriophage T2 that infects bacteria y Hershey and Chase deduced that the entry of some viral component affects the genetic program of the host bacterial cell transforming it into a bacteriophage factory y To trace the two components of the virus over its life cycle Hershey and Chase labelled each component with a specific radioisotopeProteins contain some sulfur an element not present in DNAThe deoxyribosephosphate backbone of DNA is rich in phosphorus an element that is not present in most proteins y In one experiment they allowed phosphorus radioisotope bacteriophage to infect bacteria in the other the bacteria were infected by sulfur radioisotope bacteriophageThen they mixed it
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