1. Bellwork: What is a bacteriophage
Bellwork: What is a bacteriophage? What happens when a a bacteriophage infects a bacterium? What did Hershey & Chase do with bacteriophages?

2. Identifying the substance of genes
Section 12.1

3. Bacterial transformation
In 1928, Frederick Griffith tried to work out how bacteria made people sick
Specifically how bacteria lead to the lung disease pneumonia
Two similar types of bacteria were isolated from mice
Two different varieties (strains) of the same bacteria species
One was harmless, one caused pneumonia
They were easy to tell apart on the streak plate

4. The results
Heat killed s bacteria mixed with harmless r strain bacteria
A big surprise that the mouse dies and that the lungs were filled with harmful bacteria

5. Bacterial transformation
The heat killed bacteria passed their disease causing ability to the harmless bacteria
Griffith figured that some chemical factor was transferred
This factor contained information that could change harmless bacteria to disease causing ones
He named this process transformation
He figured that the transforming actor was a gene

6. What causes transformation ?
In 1944, a group of American scientists (lead by oswald avery) tried to understand the cause of transformation
They extracted molecules, and treated the heat treated bacteria with enzymes that destroyed proteins, lipids, carbs and other molecules including RNA.
Transformation still occurred
In one final attempt, they destroyed DNA, and no transformation occurred
This showed that the nucleic acid DNA stores and transmits genetic information from one generation of bacteria to the next

7. Why were bacterial viruses important
Hershey and Chase in 1952 (part of Avery’s team) studied viruses – tiny, non living particles that can infect living cells
Bacteriophage: Virus that infects a bacteria . Attaches to the surface and injects genetic information
Hershey and Chase used bacteriophage to study DNA

8. Hershey and Chase experiment

9. The role of DNA DNA that makes up genes must be able to do 3 things
Store genetic material
Genes control patterns of development
Cells have a predetermined fate
Copy genetic material
Cells make a complete copy of their genetic material before dividing
Transmit genetic material
As shown by Mendel’s work

10. The structure of DNA
Section 12.2

11. What are the components of DNA
Nucleotide joined into long strands
Joined by covalent bonds
Nitrogenous base and covalent bonds
Bases that contain nitrogen
DNA has 4 types
Adenine
Guanine
Cytosine
Thymine
Covalent bond forms between sugar of one nucleotide and phosphate group of the next
Nitrogenous bases stick out sideways
Scientists in 1950s couldn’t understand why DNA was so special

12. Chargraff’s rule
Curious relationship existed between numbers of nucleotides
Adenine and thymine numbers shown to be equal
Cytosine and Guanine also equal
Every DNA sample obeyed this rule, but nobody understood why!

13. Franklin’s X-rays
X-ray diffraction used to get information about DNA molecule
Large amount of DNA purified
Stretched fibers into long glass tube so strands were parallel
Aimed powerful X-ray beam and measured scattering pattern
Not definitive, but showed X shaped pattern, DNA strands are twisted - helical
Also noted nitrogenous bases towards the center

14. Watson and Crick Made 3-D models out of cardboard and wire
Work was pointless until they saw Franklin’s X-ray pattern
Light bulb moment
Model built to explain DNA’s structure and properties
Key breakthrough – double helix
Two strands of nucleotide sequence wind around each other

15. The double helix model Looks like a twisted ladder
Explains chargraff’s base pairing law and how two stands are held together
Two strands of DNA run in different directions – antiparallel
Nitrogenous base pairs on each strand come into contact in the center of the molecule
Nucleotides arranged like letters in a 4 letter alphabet
Hydrogen bonds exist between the base pairs
Bonds only form between specific base pairs – base pairing