1. DNA: The Genetic Material
Chapter 9
DNA: The Genetic Material

2. The Griffith Experiments (1928)
Frederick Griffith was working on a vaccine for a pneumonia-causing bacteria.
Vaccine – A substance prepared from killed or weakened microorganisms that creates future immunity.

3. The Griffith Experiments (1928)
He found two strains of bacteria:
Smooth-edged colonies (S) are virulent - able to cause disease.
Rough-edged colonies (R) don’t cause disease.

4. The Griffith Experiments (1928)
Griffith infected numerous mice with varied forms of S and R bacteria.
- Mice infected with S bacteria – DIED
- Mice infected with R bacteria – LIVED
- Mice infected with “heat killed” S bacteria – LIVED
- Mice infected with mixture of “heat killed” S bacteria and live R bacteria – DIED

5. The Griffith Experiments (1928)
Transformation - a change in phenotype caused by introduction of foreign “genetic material”

6. The Avery Experiments (1944)
Oswald Avery wanted to learn how to prevent transformation.
- Protein-destroying enzymes – DID NOT stop transformation
- DNA-destroying enzymes – DID stop transformation
Therefore, DNA is the material responsible for transformation.

7. AT That TIME, SCIENTISTS KNEW:
Viruses are made of DNA and a protein coat.
Some viruses infect bacteria, called bacteriophages
Infected bacterial cells reproduce the virus until the cell bursts.

8. Hershey & Chase Experiments (1952)
Alfred Hershey and Martha Chase traced radioactive molecules in bacteriophage T2.

10. Hershey & Chase Experiments (1952)
Conclusion – The bacteriophage was injecting DNA into the bacteria with the instructions on how to make more viruses.

11. The Structure of DNA
Nucleotides are made of three parts: (1) a phosphate group, (2) a five-carbon sugar, and (3) a nitrogen-containing base.

12. The Structure of DNA
The sugar and phosphate group remain the same, but the nitrogen base can vary.
Four types of nucleotides:
adenine (A)
guanine (G)
thymine (T)
cytosine (C)

13. The Structure of DNA Adenine and Guanine are purines. (two rings)
Thymine and Cytosine are pyrimidines. (one ring)

14. the Structure of DNA
Erwin Chargaff (1949) observed that the amounts of adenine and thymine were equal (A = T).
Guanine and cytosine amounts were equal (G = C).

15. the Structure of DNA
Maurice Wilkins and Rosalind Franklin (1952) made X-ray diffraction photographs of DNA.

16. the Structure of DNA
Their photographs resembled a tightly coiled spiral with 2 or 3 chains.

17. The Structure of DNA
James Watson and Francis Crick (1953) figured out that DNA has the structure of a double helix.

18. The Structure of DNA
They identified base pairing rules – A and T are always partners and C and G are always partners.
They also identified that DNA strands are complementary.

19. The Structure of DNA
The double helix is held together by weak hydrogen bonds between base pairs.

20. Dna Replication
DNA replication – the process of making an exact copy of DNA
One strand serves as a template for building the other strand.
Occurs during the synthesis (S) phase of cell division.

21. Dna Replication 3 main steps:
(1) DNA helicase separates the two strands

22. Dna Replication 3 main steps:
(2) DNA polymerase adds complementary nucleotides to each strand Proofreading as it works.

23. Dna Replication 3 main steps:
(3) Two DNA molecules form that are identical to the original.
Each has one old and one new strand.

24. Dna Replication
The replication of a single human chromosome with one pair of replication forks would take 33 days.

25. Dna Replication
There are about 100 pairs of replication forks, so an entire chromosome can be completely replicated in about 8 hours.