210_trans.pdf - Genetic Material Objectives Translating the Code DNA is the code Transcription Translation Codons mRNA tRNA rRNA ribosome

210_trans.pdf - Genetic Material Objectives Translating the...

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1 Objectives: Translating the Code DNA is the code Transcription Translation Codons mRNA, tRNA, rRNA, ribosome Post-translation modification Genetic Material When Morgan’s group showed that genes are located on chromosomes, the two components of chromosomesDNA and proteinbecame candidates for the genetic material The key factor in determining the genetic material was choosing appropriate experimental organisms The role of DNA in heredity was first discovered by studying bacteria and the viruses that infect them Evidence That DNA Can Transform Bacteria The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 Griffith worked with two strains of a bacterium, a pathogenic “S” strain and a harmless “R” strainWhen he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA Living S cells (control) Living R cells (control) Heat-killed S cells (control) Mixture of heat-killed S cells and living R cells Mouse dies Living S cells are found in blood sample Mouse healthy Mouse healthy Mouse dies RESULTS In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming substance was DNA Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria Many biologists remained skeptical, mainly because little was known about DNA Transformation Evidence That Viral DNA Can Program Cells More evidence for DNA as the genetic material came from studies of a virus that infects bacteria Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research
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2 Bacterial cell Phage head Tail Tail fiber DNA 100 nm In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2 To determine the source of genetic material in the phage, they designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. colicell during infection They concluded that the injected DNA of the phage provides the genetic information Genetic material Bacterial cell Phage DNA Radioactive protein Empty protein shell Phage DNA Radioactivity (phage protein) in liquid Batch 1: Sulfur (35S) Radioactive DNA Centrifuge Pellet (bacterial cells and contents) Pellet Radioactivity (phage DNA) in pellet Centrifuge Batch 2: Phosphorus (32P) Additional Evidence That DNA is the Genetic Material In 1947, Erwin Chargaff reported that DNA composition varies from one species to the next This evidence of diversity made DNA a more credible candidate for the genetic material By the 1950s, it was already known that DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group Sugarphosphate backbone
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  • Spring '16
  • Jon Lecanda

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