1. Deoxyribonucleic Acid or DNA
Notes - DNA (Chapter 12-1)
Deoxyribonucleic Acid or DNA
Chapter 12-1

2. How is genetic information passed from cell to cell?
By early 1900, scientists knew that…
genetic information is passed from parents to offspring
genetic information is passed in the form of genes
But in early 1900, scientists did not understand…
what a gene actually was
how a gene was responsible for carrying genetic information
More research and experimentation was necessary

3. Griffith & Transformation
1928 – Frederick Griffith wanted to learn how certain types of bacteria cause pneumonia
He worked with two strains (types) of pneumonia bacteria
Disease-causing bacteria (smooth colony)
Harmless bacteria (rough colony)
He injected mice with the different strains to see what would happen

4. Griffith & Transformation
Results – Inject mouse with…
disease-causing bacteria = mouse dies
harmless bacteria = mouse lives
heat-killed, disease-causing bacteria = mouse lives
mixture of heat-killed, disease-causing bacteria and live harmless bacteria = mouse dies
Conclusion
Transformation – something is transferred from dead, harmful bacteria to live, harmless bacteria, which changes them into disease-causing bacteria
Could this “something” be a gene? - Probably

5. Avery and DNA
1944 – Oswald Avery repeats Griffith’s work to see which molecule was most important in transformation
Treated heat-killed, disease-causing bacteria with different enzymes that destroyed different types of molecules (lipids, proteins, carbs, nucleic acids)

6. Avery and DNA Results – Destroy the… Conclusion
proteins = transformation occurs
carbs = transformation occurs
lipids = transformation occurs
RNA = transformation occurs
DNA = transformation DOES NOT occur
Conclusion
DNA is responsible for transformation
Genes are probably made of DNA

7. The Hershey-Chase Experiment
1952 – Alfred Hershey & Martha Chase try to figure out which part of a virus, protein or DNA, enters a bacteria cell to infect it
Grow viruses in culture containing radioactive phosphorous (32P) and radioactive sulfur (35S)
Protein contains sulfur but little phosphorous
DNA contains phosphorous but no sulfur
Allow viruses to infect bacteria

8. The Hershey-Chase Experiment
Results – When the viruses infected the bacteria…
nearly all the radioactivity in the bacteria was from 32P, the DNA marker
Conclusion
Genetic material of a bacteriophage virus is DNA
Genes are made of DNA

9. Structure of DNA Genes are made of DNA, but how can DNA…
carry information?
determine the characteristics of an organism?
be replicated easily?

10. What did scientists already know about DNA in 1952?
DNA is a polymer
A polymer is a large molecule made up of many individual, smaller molecules called monomers, that are bonded together
The monomer of the DNA polymer is the nucleotide
DNA is a long chain of many nucleotides bonded together

11. Nucleotides: A Closer Look
A nucleotide is made up of 3 basic parts:
A phosphate group
A deoxyribose sugar molecule
A nitrogen containing base molecule (either adenine, thymine, cytosine, or guanine)
All DNA nucleotides have the same phosphate group, deoxyribose sugar molecule, and a nitrogen base
Since there are 4 different bases (A, T, C, G), DNA nucleotides come in 4 varieties
These nucleotides can be strung together in any order

12. It took even more research and experimentation to figure out what DNA actually does and how it does it
Erwin Chargaff
DNA from any organism contains equal amounts of A & T, and of C & G
Rosalind Franklin & Maurice Wilkins
Take X-Ray diffraction pictures of DNA
“X” pattern indicates that DNA molecules are helical – they contain 2 strands of nucleotides that are twisted around one another

13. Chargaff’s Ratios

14. The Discovery of DNA’s Structure
1953 – James Watson & Francis Crick
Used Chargaff’s information & Franklin’s images to inform their model building
Built a double helix model with two strands wound around one another
Model paired up “A” nucleotides with “T” nucleotides, and “C” nucleotides with “G” nucleotides

15. Watson Builds a Model

16. Base Pairing in DNA
DNA strands are held together by weak hydrogen bonds between the nitrogen bases of each strand
A always bonds with T
C always bonds with G

18. The Double Helix

19. Watson & Crick’s Model Answers the Crucial Questions
The genetic information is carried by DNA and is determined by the order of the A,T,C,G nucleotides
DNA can be copied easily by splitting in half, and each half forming new, complimentary pairs