1. DNA Structure and Function

2. Objectives Important experiments Franklin and Wilkins
Griffith
Hershey and Chase
Franklin and Wilkins
Watson and Crick model
Structure of DNA

3. Griffith: a “transforming principle”
1928
Investigated two forms of bacteria by injecting them into mice
“S form” have a smooth coating
“R form” have a rough coating
Only the live S type is virulent (kills mice)
Mice injected with a combination of heat-killed S bacteria and live R bacteria also died

4. Griffith’s Experiment
“Transforming principle”– Griffith found live S bacteria in blood samples of the dead mice
Some material must have transferred from dead S bacteria to live R bacteria

5. Hershey and Chase
1952
Conclusive evidence for DNA as the genetic material
Studied bacteriophages (viruses that take over bacteria cells)
Phage structure:
DNA molecule
Protein coat
This could help answer the DNA vs. protein debate!

6. Hershey and Chase Protein has sulfur but not much phosphorous
DNA has phosphorous but not sulfur
Hershey and Chase grew phages in cultures with radioactive sulfur or phosphorous isotopes
Experiment 1
Bacteria infected with phages that had radioactive sulfur in their protein
Used a blender to separate bacteria from parts of the phages that remained outside the bacteria
No radioactivity found in the bacteria

7. Hershey and Chase Experiment 2
Bacteria infected with phages grown in radioactive phosphorous
Used a blender to separate bacteria from parts of phage that remained outside bacteria
Radioactivity present inside bacteria
These results showed that bacteriophages’ DNA had entered bacteria but protein had not!
Finally convinced scientists that DNA and not protein is the genetic material
Also called the “blender experiment”

8. Hershey and Chase

9. DNA Structure Franklin and Wilkins Watson and Crick
X-ray crystallography photographs of DNA
Watson and Crick
Early 1950s
Described the structure of DNA
Double helix model- two strands twisted
together, antiparallel

10. DNA Structure
DNA molecule is a polymer made of many nucleic acid monomers, called nucleotides
Nucleotide has three parts:
Phosphate group
Deoxyribose (sugar)
Nitrogen-containing base
Adenine
Guanine
Cytosine
Thymine

11. One DNA Strand
The backbone of the molecule is alternating phosphates and deoxyribose sugar
The teeth are nitrogenous bases.

12. Four Nitrogenous Bases
Purines (double ring)
Adenine (A)
Guanine (G)
Pyrimidines (single ring)
Thymine (T)
Cytosine (C)

13. Chargaff’s Rule Ernest Chargaff analyzed DNA of different organisms
Proportions of different bases varied
BUT in ALL organisms, the amount of adenine (A) always equaled the amount of thymine (T)
The amount of cytosine (C) always equaled the amount of guanine (G)
Why would this happen?

14. Base Pairing A always pairs with T (2 hydrogen bonds)
C always pairs with G (3 hydrogen bonds)

15. Antiparallel strands DNA strands are complementary
Fit together (one purine and one pyrimidine)
Opposite of each other
For example, if one strand’s bases are ATCTT, what will the other strand be?
TAGAA