Frederick Griffith conducted an experiment in 1928 using two strains of bacteria that cause pneumonia - a severe type (S) and a relatively harmless type (R). He found that injecting a mixture of live R bacteria and dead S bacteria into rats resulted in the rats contracting the lethal S infection, showing that some "transforming principle" from the dead S bacteria allowed the R bacteria to take on the S phenotype. This transforming principle was later identified as DNA.

1. Frederick Griffith
• British bacteriologist
• 1928 = designed and performed experiment
on rats and bacteria that causes pneumonia.
• 2 strains of the bacteria
• Type S = causes severe pneumonia
• Type R = relatively harmless

2. Griffith’s Rats
1. First he injected living Type S bacteria
into rats:

3. • Second he injected dead Type S into the
rats.

4. • Next he injected living type R bacteria

5. • Finally he injected a mixture of living Type
R and dead Type S :

6. Results of experiments:
• Because the dead rat tissue showed living
Type S bacteria, something “brought the
Type S back to life”
• Actually one bacterial type incorporated the
DNA, or instructions, from the dead
bacteria into its own DNA
• Known as transformation. Confirmed by
Avery, MacLeod, and McCarty in 1944

7. Oswald Avery
• Canadian
biologist (1877-
1955)
• Discovered
DNA in 1944
with a team of
scientists.

8. Hershey and Chase
• 1952
• Attempted to solve the debate on whether
DNA or proteins are responsible for providing
the genetic material.

9. • They used a
bacteriophage (a
virus which
attacks bacteria)
to prove that
DNA was
definitely the
genetic material.

11. Phoebus A. Levene
• Russian born; immigrated to America,
moves to Europe.
• 1920’s discovered nucleotides
(building blocks of DNA)
1. Sugar
2. Phosphate group
3. Nitrogenous base

12. Composition of DNA

13. Components and structure of
DNA
• A very long molecule. 4 nitrogenous bases:

14. Chargaff’s rules
• The relative amounts of adenine and
thymine are the same in DNA
• The relative amounts of cytosine and
guanine are the same.
• Named after Erwin Chargaff

15. Rosalind Franklin
• Used X-Ray
diffraction to get
information about the
structure of DNA:

16. Structure of DNA
• Discovered in 1953
by two scientists:
• James Watson
(USA)
• Francis Crick
(GBR)
• Known as the
double-helix model.