1. DNA: The Hereditary Material
Wednesday, April 10th, 2018
Mr. Andy Fuchigami

2. Chapter 4 (DNA – The Molecular Basis of Life)
Reading: Pages ,

3. Why study molecular genetics?

4. What is DNA?

5. Key Questions to Answer
Why is DNA the hereditary material? 1
How did we discover the structure of DNA? 2

6. Discovery of DNA’s Existence
Friedrich Miescher (1869)
WBCs
nucleus
Slightly acidic,  [P],  [N], and unlike protein
NUCLEIN

7. RECAP: Friedrich Miescher (1869)
Extracted nuclei of white blood cells (WBCs) from pus-filled bandages.
Identified non-protein substance that was acidic and high in P and N = nuclein.
Discovered the existence of the key substance which later turned out to be DNA.
RECAP:

8. Where is the Hereditary Material?
Cap
Stalk
Foot
(contains nucleus)
Joachim Hammerling (1930s)
Acetabularia (one-celled green alga)

9. Hammerling’s Experiment (1)
Cap regenerates
Cut off cap
Cut off foot
Foot does not regenerate
Hypothesis: Hereditary information driving regeneration is found within the foot of Acetabularia and possibly in the nucleus.

10. RECAP: Joachim Hammerling (1930s)
Experimented using Acetabularia. Observed that the regeneration of new appendages (ie: cap) was determined by the nucleus-containing “foot” of the alga.
Hypothesized that the hereditary material was stored in the nucleus.
RECAP:

11. The Transforming Principle
Frederick Griffith (1920s):
-Studied Streptococcus pneumoniae, the bacterium which causes pneumonia.
R-strain (rough):
Non-pathogenic
S-strain (smooth):
-Polysaccharide capsule
-Pathogenic
-Non-pathogenic when heat-killed

12. Griffith’s Experiment
Write down the results:
Living or dead mouse?
Living or non-living S-strain?
Conclusion: something from heat-killed S-strain transformed the living, non- pathogenic R-strain to make them pathogenic.
A chemical substance from one cell is genetically transforming another cell.

13. RECAP: Frederick Griffith (1920s)
Experimented using mice and two different strains of S. pneumoniae bacteria.
Observed when heat-treated S-strain was mixed with non-pathogenic R-strain, mouse died and living S-strain reappeared.
Discovered the process of transformation.
RECAP:

14. DNA – The Transforming Principle
Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944) conducted experiments in test tubes with S. pneumoniae.
Experiment:
1. DNA from S-strain + R-cells = transformation.
2. RNA / protein / polysaccharide from S-strain + R-cells = no transformation.
3. Heat-treated S-strain mixed with DNA-digesting enzyme + R-cells = no transformation.

15. RECAP Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944)
Conducted experiments with Streptococcus pneumoniae bacteria (S- strain and R-strain).
Demonstrated that DNA was the transforming principle of the bacteria (not proteins).
RECAP

16. DNA – The Hereditary Material
Alfred Hershey and Martha Chase (1952)
Bacteriophage: virus that infects bacterial cells.
Viral DNA
Protein coat

17. Hershey-Chase Experiment
Proteins contain S but not P.
DNA contains P but not S.
Use radioactively-labelled isotopes of P and S on the bacteriophage to follow DNA and protein, respectively.
Observation: only radioactive P is found inside bacterium.

18. RECAP: Alfred Hershey and Martha Chase (1952)
Used radioactively labelled viruses to infect bacterial cells.
Infected bacterial cells contained radioactive P from DNA of the virus.
Concluded that DNA is the hereditary material.
RECAP:

19. Chemical Composition of DNA
Erwin Chargaff (1949)
Discovered that in many organisms:
The amount of A = amount of T.
The amount of G = amount of C.
*Total amount of purines = total amount of pyrimidines.
CHARGAFF’s RULE

20. Building a Model for DNA Structure
Rosalind Franklin(1953)
Used X-ray diffraction analysis of DNA to determine its structure. In X- ray diffraction, a molecule is bombarded with a beam of X-rays.
Pattern is analyzed and 3D structure is determined.
Observations: Pattern of DNA appeared to be in the shape of a double helix.

21. Taking All the Credit! James Watson and Francis Crick (1953)
Deduced the double helix structure of DNA using information from the work of Chargaff, Franklin, and Wilkins.
Received the Nobel Prize (but not Rosalind Franklin).