Griffith’s Experiment

Griffith was searching for a vaccine for pneumonia.

1. Injection mice with live encapsulated bacteria -- mice contracted pneumonia and died

2. Injection mice with live naked (rough) bacteria -- mice lived, immune system destroyed the bacteria

3. Injection mice with heat killed encapsulated bacteria -- mice remained healthy

4. Injection mice with dead encapsulated bacteria and live naked bacteria -- mice contracted pneumonia and died

What conclusions can be drawn from the experiment?

Griffith’s Experiment• Living bacteria acquired genetic information from dead

bacteria - particularly the instructions for making capsules, thus transforming the naked bacteria into incapsulated bacteria.

• The Transforming agent was discovered to be DNA.

Avery, McCarty & MacLeod

• purified both DNA & proteins separately from Streptococcus pneumonia bacteria

• added protein into new bacteria– no effect

• added DNA into new bacteria– transformed harmless bacteria into

virulent bacteria

The Hershey-Chase Experiment1. They grew one population of phages in a medium with radioactive

phosphorous. 2. They grew another group of phages in a medium with radioactive

sulfur.

What occurred in each of the types of phages above?

3. They infected bacteria with each type of phage.– Bacteria infected by phages containing radioactive protein did not show any

radioactivity.– Bacteria infected by phages containing radioactive DNA became radioactive.

This illustrated that it was the DNA, not the protein that was the molecule of heredity.

Hershey-Chase

• http://highered.mcgraw-hill.com/olc/dl/120076/bio21.swf

Chargaff’s RuleChargaff noticed a pattern in the 4 nucleotides of DNA.1. The amount of A, T, G, C varies from species

to species2. In each species, the amount of A = T, and the

amount of G = C ---- Base Pair Rule

Two Types of Bases

Compare and contrast these bases.

Franklin and Wilkins

Worked with x-ray crystallography of DNA molecules.

What does the x-ray crystallography illustrate?

Watson and Crick

Watson and Crick

• Watson and Crick are credited with finally piecing together all the information previously gathered on the molecule of DNA.

• They established the structure as a double helix –– like a ladder that is twisted. – The two sides of the ladder are held together by

hydrogen bonds between the bases.

Watson and Crick Model• The sugar (deoxyribose) and

phosphates make up the "backbone" of the DNA molecule.

• Explain the 5’ and 3’ notations.

Compare and contrast these diagrams.Which one is not accurate?

What does this diagram illustrate?

The Question:• How does DNA make copies (replicate)?

Meselson and Stahl’s Experiment

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio22.swf::Meselson%20and%20Stahl%20Experiment

• They determined that the semi-conservative model hypothesis was most probable.

How does DNA replicate?

• http://highered.mcgraw-hill.com/olc/dl/120076/bio23.swf

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA%20Replication%20Fork

Other Proteins

• Which proteins are not illustrated in the animations?

• What untangles the DNA double helix?

http://www.biologycorner.com/quiz/DNA1_qz.html