1. Discovery and Structure
DNA
Discovery and Structure

3. 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

4. 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

5. 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

6. 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

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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

13. DNA Structure DNA is made up of nucleotides
Nucleotides are made up of three parts:
Deoxyribose (a 5-carbon sugar)
Phosphate group
Nitrogenous base

14. 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

15. 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

16. 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