1. The Genetic Code Chapter 8 in your textbook
DNA
The Genetic Code
Chapter 8 in your textbook

2. Discovery of DNA
Experiments that identified DNA as the genetic material & determined its structure:
Griffith
Avery
Hershey – Chase
Chargaff
Franklin & Wilkins
Watson & Crick

3. Griffith’s Experiment
Used mice and pneumonia bacteria
One strain, “S”, was harmful (pathogenic)
Other strain, “R”, was not harmful (non-pathogenic)
Discovered transformation – one bacteria was genetically changed by another bacteria

4. Griffith, cont’d Animation
The harmless strain (R) mixed with the heat-killed pathogenic strain (S) killed the mice.

5. Griffith, cont’d
Some “factor” was transferred from the S to the R bacteria, turning the harmless R into something deadly
At this point, scientists weren’t sure whether the transforming factor was DNA or protein

6. Avery’s Experiment
Piggy-backed off of Griffith’s experiment to figure out what the “transforming factor” was
Used enzymes to isolate various components of the bacteria: DNA, proteins, lipids, and carbohydrates
Transformation only took place when the harmless bacteria was treated with DNA from the harmful bacteria

7. Avery, cont’d
Discovered that DNA stores and transmits genetic information

8. Hershey & Chase’s Experiment
Used viruses that infect bacteria (bacteriophages)
Used radioactive phosphorus to mark the DNA
Used radioactive sulfur to mark the proteins
This allowed them to trace where these molecules went during the viral infection process
Demonstrated that DNA was the genetic material -- video (2:05)

9. Hershey / Chase, cont’d

10. Chargaff
Chargaff determined that the amount of adenine is the same as the amount of thymine in all the organisms he studied (and that C = G)

11. Franklin & Wilkins
Franklin & Wilkins did X-ray crystallography pictures of the DNA molecule that showed it to be a helical structure with at least 2 helices

12. Watson & Crick Using Chargaff’s rules, they knew that A=T and C=G
Using the X-ray pictures from Franklin & Wilkins, they knew it was a helical shape
They knew the structure had to provide a way to replicate very accurately
Video (4:04)

13. Watson & Crick, cont’d Determined the structure of DNA in 1953
Double helix with bases on the inside and sugar and phosphate alternating on the outside

14. DNA Basics Deoxyribonucleic Acid Double helix (twisted ladder)
Located in the nucleus of eukaryotic cells
Prokaryotes lack a nucleus, but still have DNA in their cells; prokaryotic (bacteria) DNA hangs out in the nucleoid region

15. DNA is a Nucleic Acid
Organic compound made up of carbon, hydrogen, nitrogen, oxygen, and phosphorus.
Made of repeating subunits (monomers) called nucleotides

16. The Nucleotide
Made of a sugar (deoxyribose), a phosphate group and one of 4 nitrogen bases

17. Types of Nitrogen Bases
Pyrimidines (1 ring)
Cytosine
Thymine
Purines (2 rings)
Adenine
Guanine

18. DNA Structure Sides of the ladder are the sugar and phosphate group
The rungs (part you’d step on) are 2 of the nitrogen bases
Each rung is one purine paired with one pyrimidine – always C-G and A-T

19. DNA Structure

20. DNA Structure

21. DNA Replication The process of making identical copies of DNA
It is semiconservative – in each molecule, there is one old and one new strand
Occurs during the S phase of the cell cycle
It is necessary for cell division

22. Steps of DNA Replication
Step 1: DNA helicase (an enzyme) unwinds the molecule
Step 2: DNA polymerase binds free-floating nucleotides to the exposed bases on the original strands
Step 3: The 2 new molecules wind up again
Animation