1. How do genes control what you look like?
The transition...
To truly understand genetics, biologists first had to discover the chemical nature of the gene.
How do genes control what you look like?

2. DNA’s “Experiment” History
Griffith: How do certain types of bacteria cause pneumonia?
The experiment that tested this question led to new knowledge.
Genetic information could be transformed from one bacterium to another.

3. Genetic Info. Transferred

4. Transformation
Heat killed bacteria had passed their disease causing ability to the harmless strain
Griffith called this transformation
One strain of bacteria (harmless) had changed into the other (disease causing)
Some factor was transferred from the heat killed cells to the live cells
This factor might contain a gene with information that could change harmless bacteria into disease causing ones!

5. Avery & DNA
Avery and a group of scientists decided to repeat Griffith’s experiment
determine which molecule in the heat killed bacteria was needed for transformation
Made an extract (juice) from heat killed bacteria and added enzymes that destroy proteins, carbohydrates, & lipids
Transformation still occurred!
Repeated the experiment but added enzymes that destroy DNA
Transformation DID NOT occur!!

6. Avery’s Conclusion What was the “transforming factor”? DNA
Conclusion: DNA is the nucleic acid that stores and transmits genetic info from generation to generation

7. Hershey-Chase Experiment
Studied viruses; specifically bacteriophages
Bacterio = bacteria
Phage = eater
Bacteria eater
Viruses—made up of:
DNA + protein coat
Research Question: Which part of the virus enters the infected bacteria cell?

8. Grew viruses in the presence of radioactive markers 32P and 35S.
Sulfur is found in protein coat
Phosphorus is found in DNA

9. No Sulfur!
Phosphorus!

11. Hershey-Chase Conclusion
Genetic information of the bacteriophage that they infected with bacteria was DNA
NOT protein

12. What DNA Looks Like
Rosalind Franklin: used X-Ray diffraction patterns to discover that DNA twisted around each other like a helix

13. Watson & Crick: while trying to build a 3-D model of DNA, Watson was shown Franklin’s photograph and within weeks, figured out the structure of DNA