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Section 12-1
Discovery of DNA
Griffith’s Transforming Factor
(1928)
Griffith’s Transforming Factor
(1928)
 Harmless R strain + killed S strain did not
cause pneumonia on their own; but did
when mixed together.
 The harmless bacteria must have picked
up the disease-causing trait from the killed
strain transformation.
Avery continues the work (1944)
 Used enzymes to
break down proteins,
lipids, carbs, RNA,
and DNA in bacteria.
 Transformation
stopped only when
DNA was broken
down  DNA had
genetic info.
Hershey and Chase and
Bacteriophages (1952)
 Debate continued on whether protein or
DNA was the hereditary info of living
things.
 Hershey and Chase used a simple virus
(made of only protein and DNA) and
radioactive markers to trace genetic
material.
 Convinced the world that DNA (not
protein) was genetic material.
DNA Structure Discovered (1953)
 J. Watson and F.
Crick (with help of R.
Franklin) describe
DNA as a chain of
nucleotides arranged
in a “double helix”.
 (Watson, Crick, and
M. Wilkins share
Nobel Prize in 1962.)
Nucleotide Structure
 Nucleotides: small




units that make up a
chain of DNA.
Include:
1) phosphate group
2) sugar
(deoxyribose)
3) nitrogen base (A, T,
C, G)
The Double Helix
 Two nucleotide chains
connect with
hydrogen bonds
between the nitrogen
bases; then twist into
a spiral.
 Chargaff’s Rule:
Adenine pairs with
Thymine; Cytosine
pairs with Guanine.