DNA Structure and Gene Function Lecture PDF

DNA Structure and Gene Function Lecture 2 week 1 Rosalind Franklin’s Experiments on DNA crystals Extracted DNA from cells, crystallised it Exposed crystal DNA to X-rays – X ray diffraction/crystollagraphy Resulted in an X pattern caught on X ray film, showing DNA had a hel...

DNA Structure and Gene Function Lecture 2 week 1 Rosalind Franklin’s Experiments on DNA crystals Extracted DNA from cells, crystallised it Exposed crystal DNA to X-rays – X ray diffraction/crystollagraphy Resulted in an X pattern caught on X ray film, showing DNA had a helix structure. Watson and Crick’s DNA Model Made the DNA model based on the results of all these scientists.  Watson and Crick’s DNA Model Used Rosalind Franklin’s X-ray data & other scientists’ information to construct DNA model Had no idea how nucleotides were arranged Deduced DNA is a double helix, sugar –phosphate backbone, bases paired on the inside – spacing between bases 0.34 nm Complete turn of helix is 3.4nm Two DNA strands run antiparallel – 5’-3’ and 3’-5’ At 5’ end is a Phosphate on 5th carbon atom of sugar molecule; at 3’ end is hydroxyl group on 3rd carbon atom of sugar molecule Model agrees with Chargaff’s rule: A = T and G = C, held together by hydrogen bonds Complementary base pairing occurs – purine bonds with a pyrimidine  In summary: DNA structure discoveries (1950s) Geneticists discovered DNA to be the genetic material of all organisms, while Biochemists frantically studied and described the structure of DNA Chargaff’s rule: Composition of DNA % of A, T, G and C in DNA varies from Showed that DNA molecule is helical species. through X-ray diffraction of DNA crystals Used RF’s data and other scientists, such as EC, to construct DNA model and show complementary base-pairing NATURE OF SCIENCE: DNA DISCOVERIES IN MID 20TH CENTURY Why experiment on mice? Martha Chase & Alfred Hershey Geneticists (Mendel) discovered DNA to be the genetic material of all organisms before these scientists did. Discovery of genetic material Frederick Griffith, 1928 Studied Streptococcus pneumoniae there are 2 strains of Streptococcus: S strain is virulent (ability to kill) R strain is nonvirulent Griffith infected mice with these strains hoping to understand the difference between the strains Griffith’s results Live S strain killed mice Live R strain did not kill mice Heat-killed S strain cells did not kill the mice Heat-killed S strain + live R strain cells killed the mice Griffith’s conclusion: Information specifying virulence passed from the dead S strain cells into the live R strain cells Griffith called the transfer of this information transformation What is a vaccine? a biological substance that improves immunity for a particular disease contains a virus or bacterium or its toxins or proteins in a form that is not harmful (killed or weakened), is given to a person or animal to prevent them from getting the disease that the virus or bacterium causes The animal or person develops antibodies against this pathogen (disease-causing) enabling the body to destroy it, hence no infection. From Griffith’s results Scientist then thought Was this transform material DNA or protein? Avery, MacLeod, & McCarty, 1944 Repeated Griffith’s experiment using purified cell extracts and discovered: Removal of all protein from the transforming material did not destroy its ability to transform R strain cells DNA-digesting enzymes destroyed all transforming ability Conclusion: transforming material is DNA From Griffith’s and Avery et al. results Scientist then Knew transform material was DNA BUT what carries the genetic information? Protein or DNA? Hershey & Chase, 1952 Investigated bacteriophages: viruses that infect bacteria The bacteriophage was composed of only DNA and protein They wanted to determine which of these molecules is the genetic material that is injected into the bacteria Genetic Material 1.Bacteriophage DNA was labelled with radioactive phosphorus (32P) 2.Bacteriophage protein was labelled with radioactive sulphur (35S) 3.Radioactive molecules were tracked 4.Only the bacteriophage DNA (as indicated by the 32P) entered the bacteria and was used to produce more bacteriophage Conclusion: DNA is the genetic material Video bacterial transformation