1. DNA: The Genetic Material

2. Discovery of Genetic Material
Griffith: 1928
Studied streptococcus pneumoniae, which causes pneumonia
Injected smooth and rough strains of the virus into mouse.
Live smooth strain kills mouse, dead smooth strain does not kill mouse, rough strain does not kill mouse, dead smooth strain mixed with live rough strain kills mouse.
Mixed bacteria taken from dead mouse showed live smooth bacteria
This suggests that the disease causing factor was passed from smooth to rough.

4. Cont.
Avery: 1944
Isolated protein, DNA, and lipids from killed smooth cells and exposed them to rough cells.
Found that when rough cells were exposed to smooth cells, they became smooth.
Suggested that DNA was responsible and was the genetic material

5. Hershey and Chase Hershey and Chase: 1952
Provided definitive evidence that DNA is the genetic material.
Used a bacteriophage (a virus that attacks bacteria)
The virus is not a cell and can’t replicate on its own, it injects it’s genetic material into a host cell.
Isolated the two parts of the bacteriophage (protein and DNA) and found that the DNA was injected into the host cell to replicate the virus.

6. Hershey and Chase Radioactive Labeling
They labeled a set of bacteriophages with radioactive phosphorous (no phosphorous in protein, only DNA) so DNA would be radioactive.
They labeled another set of bacteriophages with radioactive sulfur (sulfur is in protein but not DNA)
They then injected the radioactive bacteriophages into the bacteria.
Only the bacteriophages injected with phosphorous (radioactivity on DNA) transferred this radioactivity to the bacteria.

8. DNA Structure
Nucleotides: P.A. Levine discovered that Adenine, Guanine, Cytosine, and Thymine were the nitrogenous “bases” that made up DNA.
Adenine and Guanine are double ringed bases.
Cytosine and Thymine are pyrimidine Single ring) bases

9. Chargaff
Chargaff: Found that the amount of guanine equaled the amount of cytosine, and the amount of adenine equaled the amount of thymine. This gave us Chargaff’s Rule:
A pairs with T, G pairs with C.

10. Double Helix Now, the structure had to be determined.
X-Ray Diffraction
Rosalind Franklin shot X-Rays
at DNA and took a picture.

11. Watson and Crick
Using Franklin’s and Chargaff’s data as well as mathematics (yay math!), Watson and Crick built a model of the double helix that fit the available evidence.
Two outside strands made of dioxyribose (sugar) and phosphate
C and G paired together by three hydrogen bonds
A and T paired together by two hydrogen bonds

12. Orientation
The phosphate backbone strands of DNA run in opposite directions of each other. One runs 5’ to 3’, the other 3’ to 5’.