Discovery of DNA

1. History of DNA

2. How do genes work?
How do genes determine the characteristics of an organism?
Are genes single molecules?
Are genes made up of many molecules?

3. In the 1900’s biologists were trying
to find these answers.
In order to understand genetics,
biologists had to figure out the
chemical structure of a gene.
(what are the parts & pieces?)

4. Friedrich Meischer – 1870’s
First to isolate DNA

5. Frederick Griffith - 1928
-Studied two types of bacteria (smooth/rough)
-cause of pneumonia (lung disease)

6. Type 2- (Rough looking)
•Rough looking (non-encapsulated)
•When injected harmless
Type 1- (smooth looking)
•Smooth looking (encapsulated)
•Disease causing
•Killed mice

7. Experiment # 1
Does the disease causing bacteria produce a poison?
Heat Killed
disease causing
bacteria (smooth)
Mouse LivedInjected into
the mouse
Summary: The mouse lived, this suggests that disease
causing bacteria (smooth) does not release a chemical poison.

8. Heat Killed
disease causing
(smooth)
Bacteria
+
live harmless
(rough) bacteria
Injected into
the mouse
Experiment # 2
Summary: By themselves neither should of made the mice sick,
however, many of the mice got pneumonia and died. The lungs
of the mice were filled with the disease causing bacteria.
DEAD

9. These experiments showed that one strain of bacteria
could be transformed into another type of bacteria.
Griffith’s Conclusion
Griffith hypothesized that when the live, harmless
bacteria and heat killed bacteria were mixed, some
factor was transferred from the heat killed cells
into the live harmless cells. This factor may contain
“information” that could transform the harmless
bacteria.
This was the first evidence that DNA stores and
transmits genetic information.

10. Oswald Avery - 1944
Oswald Avery led a group of scientists to further
investigate the work of Griffith. They wanted find out which
molecule in the heat killed bacteria was the most important
for transformation.

11. Summary:
Discovered that DNA stores and transmits the
genetic information from one generation of
organisms to the next generation.
(DNA)

12. Edwin Chargaff – 1947
Studied DNA in various species
Nitrogen bases are proportionate within a species
Ratio of bases are various between species
•Adenine and Thymine were found in proportionate amounts
•Cytosine and Guanine were found in proportionate amounts
Chargaff’s Rules

13. Alfred Hershey and Martha Chase -1952
Their experiments concluded that the genetic material
of the bacteriphage was DNA and not protein.

14. Rosalind Franklin & Maurice Wilkins
Early 1950’s

15. X-ray diffraction showed
•DNA fibers had a twisted pattern
•Large groups of molecules in the fibers
were spaced out in equal intervals

16. James Watson & Francis Crick - 1953
•Watson Crick Model
•“Double Helix”
•Determined the shape and structure
•Adenine bonds with Thymine
•Cytosine bonds with Guanine

18. The
End