1. DNA EXPERIMENTS
Chapter 12.1

2. Target Learning Students should be able to:
Describe the experiments that lead to the discovery of DNA as the molecule of heredity and to the discovery of the structure of DNA.
Describe the overall structure of the DNA molecule.
Identify the components of a nucleotide.

3. Review What organelle is known as the “control center” of the cell?
Nucleus
What structures are found in the nucleus?
Chromosomes
What are chromosomes composed of?
DNA wound around proteins (called histones)
How do genes and chromosomes control the activity of a cell?
By producing proteins that regulate cellular functions or become part of the cell structure

4. 1928 Experiment: Frederick Griffith
Injected 4 different samples of bacteria.
When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria caused fatal pneumonia.
These two types injected together caused fatal pneumonia.
Conclusion: Genetic information can be transferred from one bacterium to another.

5. Griffith and Transformation
Because one harmless strain of bacteria had been changed permanently into another disease-causing strain, Griffith called this process transformation.
Griffith hypothesized that when the live, harmless bacteria and the heat-killed bacteria were mixed, some factor was transferred from the heat-killed cells into the live cells.
Since the ability to cause disease was inherited by the transformed bacteria’s offspring, the transforming factor might be a gene.

6. 1944 Experiment: Oswald Avery
Avery made an extract from Griffith’s heat-killed bacteria and treated it with enzymes that destroyed proteins, lipids, carbohydrates, and RNA: Transformation still occurred.
Avery repeated the above with an enzyme that destroyed DNA: No transformation.
Conclusion: The nucleic acid DNA stores and transmits the genetic information from one generation to the next.

7. 1952 Experiment: Alfred Hershey & Martha Chase
A bacteriophage is a virus that infects bacteria.
Bacteriophages are composed of a DNA or RNA core and a protein coat.
When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it.
The viral genes produce many new bacteriophages until they gradually destroy the bacterium and 100s of viruses burst out.

8. 1952 Hershey-Chase Experiment
Used different radioactive markers to label the DNA and proteins of bacteriophages.
Bacteriophages injected only DNA into bacteria not proteins.
Conclusion: Genetic material of the bacteriophage is DNA.

9. Components and Structure of DNA
DNA is a long molecule made up of nucleotides.
Nucleotides are composed of a 5-C sugar called deoxyribose, a phosphate group, and a nitrogenous base.
The nitrogenous bases are adenine and guanine (purines) and cytosine and thymine (pyrimidines).
Adenine and thymine always bond together (A-T), and cytosine and guanine always bond together (C-G).

10. Chargaff’s Rule
Erwin Chargaff discovered that the percentages of G and C bases were almost equal in any DNA sample.
The same was true for A and T.
No one knew why.
This became known as Chargaff’s Rule.
DNA Source A T G C
Streptococcus
29.8
31.6
20.5
18.0
Yeast
31.3
32.9
18.7
17.1
Herring
27.8
27.5
22.2
22.6
Human
30.9
29.4
19.9
19.8

11. 1952: Rosalind Franklin Experiments
Rosalind Franklin, through her ingenious and meticulous use of X-ray crystallography, managed to produce clear X-ray diffraction patterns of DNA.
She discovered that DNA is in the form of a double helix and that the nitrogenous bases are near the center of the molecule.
She died of cancer at age 37, possibly as a result of exposure to the X-rays she used in her research.

12. : DNA Experiments
Rosalind Franklin's work was pivotal in the development of an understanding of how DNA works.
She died not knowing that the results of her work had been "informally communicated" to James Watson and Francis Crick (probably by Maurice Wilkins).
Crick (a British physicist) and Watson (an American biologist) were trying to understand the structure of DNA by building 3-D models using cardboard and wire.
In early 1953, Watson was shown a copy of Franklin’s X-ray pattern.
Within weeks, Watson and Crick had built a structural model that explained how DNA could carry information and how it could be copied.
Photo 51

13. Watson, Crick and Wilkins were awarded the Nobel Prize in Medicine (1962) "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material.”
While Watson and Crick later credited Franklin's contributions as pivotal to the development of an accurate model of DNA, Franklin was never nominated or awarded the Nobel Prize as she had already succumbed from cancer and Nobel Prizes are not awarded posthumously.
James Watson & Francis Crick, 1953
Maurice Wilkins with X-ray crystallography equipment, 1954.

14. DNA Structure
DNA is a double helix in which two strands are wound around each other like a twisted ladder.
Each strand is made up of a chain of nucleotides.
The two strands are held together by hydrogen bonds between adenine and thymine and between guanine and cytosine.
A and T are double-bonded; C and G are triple-bonded.

15. DNA Double Helix A? nitrogenous bases hydrogen bond B? phosphate groupC?
phosphate group D?
deoxyribose

16. Review Questions
Who conducted an experiment with bacteriophages that used radioactive isotopes of sulfur and phosphorus to identify DNA, and not protein, as the molecule of heredity?
Griffith
Chargaff
Avery
Watson & Crick
Franklin
Hershey & Chase
Who hypothesized that the “transforming factor” that changed harmless bacteria into pathogens was a gene?
Griffith
Chargaff
Avery
Watson & Crick
Franklin
Hershey & Chase

17. Review Questions
Who took X-ray diffraction photos of the DNA molecule?
Griffith
Chargaff
Avery
Watson & Crick
Franklin
Hershey & Chase
Who discovered that the DNA molecule is a double helix (or twisted ladder)?
Griffith
Chargaff
Avery
Watson & Crick
Franklin
Hershey & Chase

18. Review Questions
Who used enzymes to digest proteins, lipids, RNA, carbohydrates and DNA and found that transformation does NOT take place when DNA is destroyed, therefore DNA is the heredity molecule?
Griffith
Chargaff
Avery
Watson & Crick
Franklin
Hershey & Chase
Who conducted experiments on the DNA of dozens of organisms and determined that adenine occurs in approximately equal amounts as thymine within a species but not across species?
Griffith
Chargaff
Avery
Watson & Crick
Franklin
Hershey & Chase

19. Review Questions
Who received the Nobel Prize in 1962 for the discovery of the structure of DNA?
Griffith
Chargaff
Avery
Watson & Crick
Franklin
Hershey & Chase
Who used heat-killed harmful bacteria combined with live harmless bacteria to show that traits can be passed from one organism to the next?
Griffith
Chargaff
Avery
Watson & Crick
Franklin
Hershey & Chase