The History of DNA

• Gregor Mendel-1866

– Determined “Unit characters” were the method of passing on traits for inheritance

• Friedrich Meischer - 1868

– Studied nuclei of pus cells obtained from discarded surgical bandages

– Detected a phosphorus-containing substance that he named nuclein.

• Frederick Griffith 1928 : Work with Bacteria– Found that DNA taken from a virulent

(disease-causing) strain of bacteria (Streptococcus pneumoniae)

– Transformed a non-virulent form of the bacterium into a virulent form.

Oswald Avery, Colin MacLeod, and Maclyn McCarty 1943 Continued the study of “Transformation” principle

Mendel Meischer Griffith

Avery McCartyMacLeod

Transformation Of BacteriaTwo Strains Of Streptococcus

Smooth Strain w/Capsule(Virulent)

Rough Strain(Harmless)

Capsule

Experimental

Transformation Of Bacteria -Griffith’s Experiment

Control

Control

Control

OUCH!

Rough

Smooth (virulent)

The History of DNA• Alfred Hershey and Martha

Chase - 1952– radioactive isotope tracer

experiment

– bacterial virus (bacteriophage T2) infects a host cell ( bacterium Escherichia coli)

– found that T2 virus DNA, not its protein coat, enters the host cell

– genetic information for replication of the virus

Using SUsing S3535Bacteria grown in normal non-radioactive media

T 2 grown in media containing S35 incorporate S35 into their proteins

Blending causes phage protein coat to fall off

T2 attach to bacteria and inject genetic material

Is protein the genetic material?

When centrifuged, phage protein coats remain in the supernatant while bacteria form a pellet.

The supernatant is radioactive, but the pellet is not.

Did protein enter the bacteria?

Using P32Bacteria grown in normal non-radioactive media

T2 grown in P32 containing media incorporate P32 into their DNA

Blending causes phage protein coat to fall off

T2 attach to bacteria and inject genetic material

Is DNA the genetic material?

When centrifuged, phage protein coats remain in the supernatant while bacteria form a pellet

The pellet is radioactive, but the supernatant is not.

Did DNA enter the bacteria?

The History of DNA Structure

• Erwin Chargaff- 1940 - “Chargaff’s rule”

– four bases may occur in varying proportions in DNA of different organisms

– # of A = # of T, w/ two hydrogen bonds

– = # of G and C are present w/ 3 hydrogen bonds

• Rosalind Franklin and Maurice Wilkins

– X-ray diffraction study concluded DNA fibers have two strands.

The History of DNA Structure

James Watson and Francis Crick - University of Cambridge -1953

Worked on problem of making a DNA molecule model that was double stranded but also had the specific A - T and G - C base equivalencies Solution-double helical structure for DNA.

Chargaff

Franklin

Wilkins

Watson and Crick