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Ch. 12 Notes: DNA Structure Chromosomes are made of DNA and proteins in approximately equal amounts originally it was believed that proteins carried the information because they are more complex o 20 types of amino acids exist, while DNA only has 4 types o Believed to be like the alphabet and could be arranged in a variety of ways. DNA Trail Genetic material had to meet 4 criteria: 1) It must carry the genetic information from parent to daughter cell and from generation to generation and carry a great deal of information 2) It must contain information for producing a copy of itself 3) It must be chemically stable 4) It must be capable of mutation (change) Frederick Griffith (1928) – bacteriologist looking for vaccine for S. pneumoniae (pneumonia) Two forms: virulent (disease-causing) – polysaccharide capsule nonvirulent (harmless) – no capsule Tested to see if heat–killed (harmless) virulent could be used as vaccine (See Fig. 14-3) o Injected mouse with live virulent mouse dies o Injected mouse with live non-virulent mouse lives o Injected mouse with heat-killed virulent mouse lives o Injected mouse with live nonvirulent and heat-killed mixture mouse dies (both harmless) o Blood sample removed showed live virulent How? o Transformation = when a cell incorporates DNA from surroundings o Transforming Factor = the “something” that causes the conversion (was DNA) Miescher (1869) isolated DNA eventually named DeoxyRibonucleic Acid Feulgen – DNA has high affinity for fuchsin stain showed DNA exists in ALL CELLS, in their nuclei Levene (1920’s) – showed DNA could be broken down into units made of a 5-C sugar, a phosphate, and one of four N-bases = nucleotide Purines Pyrimidines Adenine Thymine Guanine Cytosine Avery, et al. showed took up where Griffith left off. They showed that if: the DNA from the virulent strain was added to nonvirulent bacteria, it became virulent they added DNAase (an enzyme that digests DNA), it prevents the transformation from happening they added enzymes that degrade proteins and RNAase, but the transformation still happened (showing RNA nor protein is the genetic material) BACTERIOPHAGE EXP/s Delbruick and Luria (1940) Bacteriophage – viruses that attack bacteria (“bacteria eaters”) ex. E-coli phages: T1 – T7 life cycle o Upon infection- disappear except for a few fragments o After 10 minutes – virus detectable in large numbers MORE PROOF THAT GENES ARE ON DNA Alfred D. Hershey and Martha Chase (1952) Two samples: one where DNA was traced with P35 (which is radioactive) other where protein was traced with S34 (also radioactive) Left to multiply Put in blender to separate intra- and extra-cellular material and examined for isotopes Found: S34 remained outside but P35 was inside cells PROVED: GENETIC MATERIAL IS DNA (NOT protein)!!! (We now know phages inject their DNA into the cell and leave protein coats outside) Transformation of Organisms Today Knowledge of DNA and genetics has led to our ability to take select genes of one organism and put them into another, where they are expressed = Genetically Modified Organisms (GMOs) Possible because ALL LIVING THINGS USE THE SAME GENETIC CODE !!! Watson – Crick Model James Watson and Francis Crick (1950s) Did not do much research themselves, but based their theory on Known Data: o DNA molecules known to be very large – very long and thin, and made of nucleotides containing N-bases (A, G, T, C) o Linus Pauling (1950) had shown that a protein’s chains of amino acids are often arranged in the shape of a helix and are held in their form by H-bonds between successive turns. He suggested that DNA might be the same. o X-ray diffraction photos of DNA (Maurice Wilkins and Rosalind Franklin) showed patterns that reflected the turns of a giant helix o Chargaff’s data showed that the amount of T = A and C = G. Building the Model THE BIG BREAKTHROUGH: postulated that DNA is a DOUBLE helix (like twisted ladder) “Rungs” = N-bases o 2 per rung – held together by H-bonds o each base bonded to sugar – phosphate unit (to complete the nucleotide) o each base pair was a purine – pyrimidine combination Nucleotides along one side can be arranged in any order o Allowed for a possibility of great variation Each strand has direction o each phosphate group is attached to one sugar at 5’ position and to the next sugar at the 3’ position o There is a 5’ end and a 3’ end to each strand Strands are antiparallel = run in opposite directions one 5’ → 3’ other 3’ ← 5’ (opposite direction) Because of structures A - T (A = T) form 2 H bonds C - G (C ≡ G) form 3 H bonds These “complementary bases” are what allow DNA to replicate itself so precisely