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DNA Discovery and Structure Timeline 1866 1903 1928 1952 Gregor Mendel experiments with pea plants Sutton develops the chromosome theory of inheritance Frederick Griffith explores genetic material with mouse experiments Martha Chase and Alfred Hershey experiment with bacteriophages to prove that DNA is the genetic material Timeline (cont.) 1952 1952 1952 Edwin Chargaff discovered that in DNA samples the amount of adenine was always equal to the amount of thymine and the amount of cytosine was always equal to the amount of guanine Maurice Wilkins and Rosalind Franklin photographed DNA using X-ray crystallography Watson and Crick model DNA Griffith’s Experiment He studied Streptococcus pneumoniae, a bacteria that causes pneumonia in mice He found that there were two types: S strain and R strain In the first experiment, he injected the S strain into the mice The mice developed pneumonia and died In the second experiment, he injected the R strain into the mice There was no change in the mice In the next experiment, he took the S strain, heat-killed it, and injected it into the mice There was no change in the mice In the last experiment, he took the heat-killed S strain, combined it with the R strain, and injected it into the mice The mice developed pneumonia and died Griffith concluded that there was some factor that was passed from the S strain to the R strain that enabled the R strain to cause pneumonia Chase and Hershey’s Experiment They worked with bacteriophages, viruses that infect bacteria Bacteriophages are made up of only two substances: DNA and protein When a bacteriophage infects a bacterial cell, part of the phage enters the cell and part of it stays outside The part that enters the cell is what takes control of the bacteria They wanted to find out if the part that entered the cell (the genetic material) was DNA or protein They put radioactive DNA into a bacteriophage and let it infect a cell Tests showed that the bacteria became radioactive Next, they put radioactive protein into a bacteriophage and let it infect a bacterial cell Tests showed that the bacteria did not become radioactive These experiments proved that DNA was the genetic material DNA Structure DNA is made up of nucleotides Nucleotides are made up of three parts: Deoxyribose (a 5-carbon sugar) Phosphate group Nitrogenous base Bases Adenine (A) Guanine (G) Cytosine (C) Thymine (T) Adenine and Guanine are purines Purines have a two-ring structure Cytosine and Thymine are pyrimidines Pyrimidines have a one-ring structure Double Helix The shape of DNA is described as a double helix A double helix resembles a “twisted ladder” The “backbone” of the ladder is made up of alternating sugar and phosphate groups The “rungs” of the ladder are made up of the bases joined by weak chemical bonds Complementarity Bases that always pair with each other are said to be complementary The space between the two sides of the DNA ladder is only large enough to contain 3 rings This means that A (two rings) cannot pair with G (two rings) This also means that C (one ring) cannot pair with T (one ring) because the rings would be too far apart Complementarity (cont.) Chemical bonds formed: A forms two G forms three T forms two C forms three So the only combinations possible are: A pairs with T C pairs with G Complementary Sequence Give the complementary sequence for the following strand of DNA: ATTCGACTGA T AAG C T G AC T Complementary Sequence (cont.) Give the complementary sequence for the following strand of DNA: ATCCGGT TAGGCCA