DNA

Griffith’s Experiment Fredrick Griffith 1928 British scientist Wanted to see why people got sick from bacteria (pneumonia) Used mice and a strain of disease causing bacteria Harmless strain had rough edges when it was grown on a petri dish Disease –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 Griffith’s Experiment Proved Griffith’s 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 bacteria Transforming Factor This factor was probably a gene because he noticed that the offspring inherited the disease as well

Avery’s Experiments: DNA is the Transforming Factor American biologist Oswald Avery, 1944 Was transforming factor DNA or Protein? Took Griffith’s heat-killed bacteria and good bacteria and made an extract (aka a juice) Treated this extract “protein-destroying enzymes” Injected mice with the treated extract Did the bacteria still function and kill the mice? Yes, the bad bacteria still transformed the harmless bacteria…therefore protein did NOT contain the “transforming factor” Treated Griffith’s deadly bacteria and harmless bacteria with “DNA-destroying enzymes” Did they still function? No, the bacteria was not transformed, therefore, DNA had to be the transforming factor

Avery’s Conclusion DNA is the cell’s genetic material Scientists were still skeptical Protein is made of 20 a.a. and DNA is only 4 nucleotides…didn’t make sense They thought….”DNA is too simple!”

Hershey and Chase’s Experiments 1952, American biologists Alfred Hershey and Martha Chase Which is the hereditary material: Protein or DNA ??? Conducted Experiments using viruses Viruses Package of nucleic acid wrapped in a protein coat Not made of cells Can only reproduce by infecting living cell with its genetic info Genetic info of virus then tells the cell’s organelles to make more viruses Bacteriophage (phage)-virus that infects bacteria…literally means “bacteria eater” Virus 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 Chase Experiment 1 Treated virus with radioactive sulfur-35 isotope (sulfur is in protein but not DNA) Sulfur would attach to protein of virus If rad. sulfur was found in bacteria, that means it was the protein coat of virus that contained hereditary information Experiment 2 Treated virus with radioactive phosphorus-32 isotope (phosphorus is in DNA but not protein) Phosphorus would attach to DNA of virus If rad. phosphorus was found in bacteria, that means it was the protein coat of virus that contained hereditary information Used blender to mix up bacteria and viruses Experiment 1=radioactive material was found out side the bacteria cells Experiment 2=radioactive material was found inside the bacterial cells

Hershey and Chase’s Experiments

Hershey and Chase Conclusion The Phage’s (virus’) DNA entered the bacteria during infection but the protein did not DNA must carry the genetic information of the virus DNA is the hereditary material

DNA Structure Deoxyribonucleic Acid DNA is a polymer made up of many monomers called nucleotides Nucleotide contains: 5-carbon sugar called deoxyribose RNA contains RIBOSE sugar instead Phosphate group One Nitrogenous base (there are 4 types)

Sugar and phosphate make up backbone (sides of ladder) Nitrogenous bases make up steps of ladder Bases are always paired (Chargaff’s rule)

What are these Nitrogenous bases??? Make up the “steps” of the DNA ladder One Step= A Purine + A Pyrimidine Purines Double ring structure Adenine Guanine Pyrimidines Single-ring structure Cytosine Thymine (in DNA only) Uracil (in RNA only)

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

Chargaff’s Rule Erwin Chargaff, Am. biochemist Years before Watson and Crick Percentage of guanine and cytosine in DNA sample were about equal…the same with adenine and thymine Chargaff’s Rule: A=T and G=C Rule for “base-pairing”

Rosalind Franklin and Maurice Wilkins Rosalind Franklin, 1950s, Brit scientist who studied DNA Used X-ray diffraction technique to learn about structure of DNA She noticed an X-shaped pattern This indicated that DNA had a helical structure She did not get a chance to publish her research…and her idea was basically stolen 

Watson and Crick 23-year old American Biologist James Watson went to work with English Physicist Francis Crick Used information from Rosalind Franklin and Maurice Wilkins (1950’s) These two scientists used Rosalind Franklin’s x-ray crystallography images to begin to piece together the DNA structure Showed that there were 2 strands twisted around each other

Watson and Crick DNA is a double helix Created the first accurate model of DNA using wire Bases paired up specifically with each other b/c of hydrogen bonding Complementary base-pairing A purine with a pyrimidine A=T G=C

DNA Structure Chromatin wrapped around proteins called histones Chromatin condenses to form chromosomes during replication

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