Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
DNA Deoxyribonucleic Acid Frederick Griffith • • • • • worked with pneumonia causing bacteria two strains Smooth (S) strain Rough (R) strain Used four different trials Group 1 • He injected the first group of mice with the “S” Strain. • Those mice died Figure 16.1 Transformation of bacteria Group 2 • He injected the second group with the “R” strain. • Those mice lived. Figure 16.1 Transformation of bacteria Group 3 • He heated up a sample of “S” bacteria to kill them and injected the mice in Group 3 with this. • Those mice lived. Figure 16.1 Transformation of bacteria Group 4 • He mixed some of the “heat-killed ‘S’” bacteria with some of the “R” bacteria and injected the mice with that mixture. • Those mice died. Figure 16.1 Transformation of bacteria Figure 16.1 Transformation of bacteria Results • He took a sample of the blood from the mice in group 4 and found that the harmless “R” bacteria had been “transformed” into fatal “S” bacteria. • What was this genetic material that transformed the bacteria? Proteins or Nucleic Acids? • We knew both existed, but it was unknown which was responsible for this genetic transformation Avery • 2 samples • 1. Avery took the “heat-killed ‘S’ and ‘R’” mixture and injected those bacteria with protein digesting enzymes. • 2. He took the same mixture and injected those bacteria with nucleic acid digesting enzymes. • He assumed that the one that was digested and did not infect would be the genetic material. Avery • Sample 1: DNA present, proteins digested • Sample 2: DNA digested, proteins present Avery continued • He found that sample 1 infected and killed the mice, but sample 2 did not. • He concluded that DNA must be the genetic material. Skeptics • Most people still did not believe DNA could be responsible for genetics because there are only 4 different bases (nucleotides) while proteins are made of 20 different bases (amino acids). • How could a 4 letter “alphabet” account for the differences between living things? Hershey-Chase Experiment Bacteriophage-virus that infects bacteria Hershey and Chase (1952) Used viruses to prove that DNA is the genetic material. – Nucleic Acid + Protein coat – bacteriophages or phages. Figure 18.4 The lytic cycle of phage T4 Figure 16.2a The Hershey-Chase experiment: phages Figure 16.2ax Phages Building Blocks Of DNA Nucleotides • Deoxyribose sugar • A phosphate group • A nitrogenous base How many different nucleotides are there? 2 Samples • Sample One: Radioactively labeled the protein coat of the virus. • Sample Two: Radioactively labeled the DNA in the virus. Hershey and Chase • Figured that whichever sample made radioactive bacteria would tell him which was the genetic material (DNA or proteins). Protein Labeled DNA Labeled Protein Labeled DNA Labeled Structure of DNA Building Blocks Of DNA Nucleotides • Deoxyribose sugar • A phosphate group • A nitrogenous base How many different nucleotides are there? Figure 16.3 The structure of a DNA stand ! " " ! ! ! # # Bases Franklin • Rosalind Franklin was able to use x-ray crystallography to make a diffraction image of a DNA molecule Figure 16.4 Rosalind Franklin and her X-ray diffraction photo of DNA Watson and Crick • used Chargaff’s base pair rules and Franklin’s images to make a 3-D model of the DNA double helix • never really did any experiments themselves, just used other peoples’ results • they received the Nobel Prize and all the credit… hardly acknowledged Franklin Figure 16.0 Watson and Crick Figure 16.5 The double helix DNA Structure • • • • Double stranded HELIX Like a “twisted ladder” Uprights of ladder are S-P “Rungs” are nitrogen bases