DNA crash course

DNA

Griffiths Experiment

Fredrick Griffith1928 British scientistWanted to see why people got sick from bacteria (pneumonia)Used mice and a strain of disease causing bacteriaHarmless strain had rough edges when it was grown on a petri dishDisease causing strain had rough edges when grown on a petri dish

S-strain of bacteria killed mouse (smooth)R-strain does NOT kill mouse (rough)Heat kills the bacteria

What Griffiths Experiment ProvedGriffiths hypothesis:When live, harmless bacteria and heat-killed bacteria are mixed, the heat-killed bacteria passed on disease-causing information to the live cells of the harmless bacteria, causing harmless bacteria to transform into bad bacteriaTransforming FactorThis factor was probably a gene because he noticed that the offspring inherited the disease as well

Averys Experiments: DNA is the Transforming Factor American biologist Oswald Avery, 1944Was transforming factor DNA or Protein?Took Griffiths heat-killed bacteria and good bacteria and made an extract (aka a juice)Treated this extract protein-destroying enzymesInjected mice with the treated extractDid the bacteria still function and kill the mice?Yes, the bad bacteria still transformed the harmless bacteriatherefore protein did NOT contain the transforming factorTreated Griffiths deadly bacteria and harmless bacteria with DNA-destroying enzymesDid they still function?No, the bacteria was not transformed, therefore, DNA had to be the transforming factor

Averys ConclusionDNA is the cells genetic materialScientists were still skepticalProtein is made of 20 a.a. and DNA is only 4 nucleotidesdidnt make senseThey thought.DNA is too simple!Hershey and Chases Experiments1952, American biologists Alfred Hershey and Martha ChaseWhich is the hereditary material: Protein or DNA ???Conducted Experiments using virusesVirusesPackage of nucleic acid wrapped in a protein coatNot made of cellsCan only reproduce by infecting living cell with its genetic infoGenetic info of virus then tells the cells organelles to make more virusesBacteriophage (phage)-virus that infects bacterialiterally means bacteria eaterVirus attaches to the surface of bacteria, injects its genetic material into the bacteria, the viral genes cause many more viruses to be made inside the bacteria until the bacteria burst and hundreds of new viruses are released

Hershey and ChaseExperiment 1Treated virus with radioactive sulfur-35 isotope (sulfur is in protein but not DNA)Sulfur would attach to protein of virusIf rad. sulfur was found in bacteria, that means it was the protein coat of virus that contained hereditary informationExperiment 2Treated virus with radioactive phosphorus-32 isotope (phosphorus is in DNA but not protein)Phosphorus would attach to DNA of virusIf rad. phosphorus was found in bacteria, that means it was the protein coat of virus that contained hereditary informationUsed blender to mix up bacteria and virusesExperiment 1=radioactive material was found out side the bacteria cellsExperiment 2=radioactive material was found inside the bacterial cells

Hershey and Chases Experiments

Hershey and ChaseConclusionThe Phages (virus) DNA entered the bacteria during infection but the protein did notDNA must carry the genetic information of the virusDNA is the hereditary material

DNA StructureDeoxyribonucleic AcidDNA is a polymer made up of many monomers called nucleotidesNucleotide contains:5-carbon sugar called deoxyriboseRNA contains RIBOSE sugar insteadPhosphate groupOne Nitrogenous base (there are 4 types)

Sugar and phosphate make up backbone (sides of ladder)Nitrogenous bases make up steps of ladderBases are always paired (Chargaffs rule)

What are these Nitrogenous bases???Make up the steps of the DNA ladderOne Step= A Purine + A Pyrimidine PurinesDouble ring structureAdenineGuaninePyrimidinesSingle-ring structureCytosineThymine (in DNA only)Uracil (in RNA only)

Adenine (Purine) binds with Thymine (pyrimidine)TWO hydrogen bondsCytosine (Purine) binds with Guanine (pyrimidine)THREE hydrogen bonds

Chargaffs RuleErwin Chargaff, Am. biochemistYears before Watson and CrickPercentage of guanine and cytosine in DNA sample were about equalthe same with adenine and thymineChargaffs Rule: A=T and G=CRule for base-pairing

Rosalind Franklin and Maurice WilkinsRosalind Franklin, 1950s, Brit scientist who studied DNAUsed X-ray diffraction technique to learn about structure of DNA She noticed an X-shaped patternThis indicated that DNA had a helical structureShe did not get a chance to publish her researchand her idea was basically stolen

Watson and Crick23-year old American Biologist James Watson went to work with English Physicist Francis CrickUsed information from Rosalind Franklin and Maurice Wilkins (1950s)These two scientists used Rosalind Franklins x-ray crystallography images to begin to piece together the DNA structureShowed that there were 2 strands twisted around each other

Watson and CrickDNA is a double helixCreated the first accurate model of DNA using wireBases paired up specifically with each other b/c of hydrogen bondingComplementary base-pairingA purine with a pyrimidineA=TG=C

DNA StructureChromatin wrapped around proteins called histonesChromatin condenses to form chromosomes during replication

This is what they already knew from the work of many scientists, about the DNA molecule:

DNA is made up of subunits which scientists called nucleotides. Each nucleotide is made up of a sugar, a phosphate and a base. There are 4 different bases in a DNA molecule: adenine (a purine) cytosine (a pyrimidine) guanine (a purine) thymine (a pyrimidine) The number of purine bases equals the number of pyrimidine bases The number of adenine bases equals the number of thymine bases The number of guanine bases equals the number of cytosine bases The basic structure of the DNA molecule is helical, with the bases being stacked on top of each other