Major Points
1. Evidence that DNA is the genetic material
2. Structure of DNA
3. Replication of DNA
Molecular Biology
Seeks to trace molecular causes of biological phenomena
* Has focused on nature of heredity
- Structural basis of heredity
- Transfer of the instructions by a messenger
- Translation into proteins-the chemical products of the instructions
* Recent work has focused on modifying the instructions to study cells
Transformation (Griffith)
Streptococcus pneumoniae cells cause pneumonia
* They come in two forms-rough (R) and smooth (S)
* Only S cells are lethal
* R cells are harmless
Griffith managed to "transform" an R strain into an S strain
Transformation
* Injected S cells into mice --> dead
* Injected R cells into mice --> live
* Injected heat-killed S cells into mice --> live
* Injected heat-killed S cells and live R cells --> dead [bacteria rescued
from the mice were S type]
Therefore, the hereditary instructions of the S cell were transfered into
the R cell
Avery, MacLeod & McCarty
* They repeated Griffith's experiments with purified DNA
* They directly tested the R --> S phenotype change
* They conducted critical controls
Avery, MacLeod & McCarty
* Purified DNA from S type
* Added DNA to R type --> transformed to S type
* Pre-treating the DNA with protease had no effect
* Pre-treating it with DNAse eliminated transformation
Therefore, DNA is the transforming principle
Hershey-Chase Experiment
* Bacteriophage are viruses that attack bacteria (literally "bacteria
eaters")
* They consist of only protein and DNA
* Most of the bacteriophage never enters the cell-only the genetic material
does
Hershey-Chase Experiment
* Labeled bacteriophage T4 with 35S (protein) and 32P (DNA)
* Infected cells with either
* Removed T4 from cell surface
* All of the 35S was outside the cells
* Most of the 32P was inside the cells
Therefore, DNA is the genetic material