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Lecture 2

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Biological Sciences
Bebhinn Treanor

Lecture 2 DNA: FINDINGS BY WATSON AND CRICK o When DNA was destroyed, no transformation happened so no smooth colonies Who figured out the structure of DNA: Watson and Crick Historical reasoning behind why DNA is important and a hereditary material: Hershey and Chase: Final Evidence to prove DNA is hereditary molecule 1. Early microscope noticed that when a sperm fertilized an egg only a part of the sperm Transformation: refers to the conversion of a cells hereditary type by the uptake of DNA that entered the egg was the nucleus. Thus, nucleus contains hereditary material 2. Nucleus contains chromosomes and in conjunction wit h cell division the Bacteria Observed: Escheria coli by bacteriophage chromosomes doubled up and split themselves in 2 packages (1 for each cell) o Location of E. coli: bacteria normally found on the intestines of mammals Bacteriophages (AKA phages): viruses that infect bacteria 3. Both nucleus and chromosomes were made up large amounts of proteins and DNA. Thus, scientists were certain that hereditary material was either in protein or nucleic Virus: infectious agent that contains either DNA or RNA surrounded by a protein coat acids o Reproduction: Viruses cannot reproduce in host cells o Virus infects a cell: uses the cells resource to produce more virus particles Scientists favoured proteins as a strong canditiate for hereitdary material due to varied amino acid composition and had such a variety of shapes and known to be important for Phage Cycle: metabolic reactions since DNA and RNA were simple in composition (20 amino acids vs. 4 o Introduction: phage attaches to the surface of a bacterium nucelotides) Some phages stop producing its own molecules and instead produces offspring phases ESTABLISHING DNA AS THE HEREDITARY MOLECULE Examples: T2 phage o After 100-200 phages are lined inside the bacterial cells: viral enzyme Frederick Griffith found a substance that could genetically transform pneumonia bacteria breaks down cell wall Bacteria Observed: Streptococcus pneumonia. Causes severe form of pneumonia in mammals o Result of breaking cell wall: kills the cycle and releases new phases o Duration of life cycle: 90 minutes Griffiths Goal: make a vaccine to prevent pneumonia Question: is DNA or protein the genetic material? Experiment: studied the conversion of nonvirulent R form bacteria to a (infective) S form bacteria Experiment: used phage T2 which only consists of DNA and protein o S form: had a capsule surrounding the cell, having smooth shiny colonies 1. Infected E.coli growing in the presence of radioactive P of S with phage T2. Progeny phases were either labelled in their DNA with P of in their o R form: no capsule, thus rough and non shiny 35 Objective: injected bacteria into mice and determined how they were affected protein S. Results from Experiment: 2. Fresh E.coli cells were infected with the radioactively labelled phages 3. Cells were mixed in a blender to remove the phage coats from the cell o Mice with live infected S cells: mice died, infective S cells in blood thus, S cells are virulent surface. Components were analyzed for radioactivity. o Mice injected with love noninfective R cells: Mice lived, no live R cells in 4. Progeny phases analyzed for radioactivity Results blood, thus nonvirulent o Phage coat lacking DNA: no radioactivity within cell; S in phage coat o heat killed S cells: mice lived, no S cells in blood thus shows S cells are necessary to be virulent o Progeny phages: no radioactivity o Phage coat lacking DNA: P within cell; not in phage coat o Heat killed S cells plus live R cells: mice died, Live S cells in blood, Thus R o Progeny phages: P in progeny phases cells can be converted to virulent S sells via dead S cells Transformation: Conversion of R bacteria to S bacteria Conclusion: 32 Transforming Principle: Agent responsible for transforming R to S bacteria o Radioisotope used to label DNA: P o Location of P: within phage-infected cells and in progeny phases Agent responsible for transforming R to S bacteria: Proteins and Nucleic Acids 32 Avery and Co-Workers identified DNA as the molecule that transforms R bacteria to S bacteria o Significance of P: indicating that DNA35s the genetic material Materials: reproduce transformation using bacteria in culture tubes o Radioisotope used to label proteins: S o Location of S: found in phage coats after infection but not found in Experiment: o Used heat to kill virulent S bacteria and then treated the macromolecules infected cells 35 in progeny phages extracted from the cells with enzymes that break down each of the 3 main o Significance of S: Protein is not the genetic material Nobel Prize for Watson & Crick: Finding out DNAs structure candidate molecules for the hereditary materials: Protein, DNA or other nucleic acids (RNA). How did Watson and Crick find out DNAs structure: analyzed info discovered by other o When they destroyed proteins or RNA, no effect scientists but did not do a single experiment Analyzed info that was discovered by other scientists: o Extract of S bacteria still transformed R bacteria into virulent S o Cells had polysaccharide capsules and produced smooth colonies o Amount of cytosine = amount of guanine 1 | P a g e
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