Discovery and Structure DNA Discovery and Structure

Timeline Frederick Griffith explores genetic material with mouse experiments 1866 1903 1928 Gregor Mendel experiments with pea plants Sutton develops the chromosome theory of inheritance

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

Griffith’s Experiment (cont.) In the first experiment, he injected the R strain into the mice There was no change in the mice In the second experiment, he injected the S strain into the mice The mice developed pneumonia and died

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’s Conclusion 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 Griffith called this process transformation. Also, since the ability to cause pneumonia was transferred to bacterial offspring, the factor for transformation must be a gene.

Timeline (cont.) Erwin Chargaff discovered that in DNA samples the amount of guanine was always equal to the amount of cytosine and the amount of adenine was always equal to the amount of thymine. 1952 Martha Chase and Alfred Hershey experiment with bacteriophages to prove that DNA is the genetic material

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, so it carries genetic information Protein DNA

They let the virus infect a cell They wanted to find out if the part that entered the cell was DNA or protein They grew viral cultures in substrate containing radioactive phosphate isotopes (phosphate is found in DNA but not protein) They let the virus infect a cell Tests showed that the bacteria became radioactive Radioactive DNA

The bacteriophage infected the bacteria Next, they grew viral cultures in substrate containing radioactive sulfur isotopes (sulfur is found in protein but not DNA) The bacteriophage infected the bacteria Tests showed that the bacteria did not become radioactive These experiments proved that DNA was the genetic material Radioactive Protein

Timeline (cont.) Maurice Wilkins and Rosalind Franklin photographed DNA using X-ray crystallography 1952 Watson and Crick develop the double helix model for the structure of DNA

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 hydrogen bonds

Complementarity Bases that always pair with each other are said to be complementary 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