DNA
How was DNA discovered?
• There were 3 major experiments that led
to the discovery of DNA as the genetic
material.
– Griffiths Transformations
– Avery
– The Hershey-Chase Experiment
Griffiths
• Griffith’s injected
bacteria into
mice.
• He showed that
something in the
smooth bacteria
TRANSFORMED
the non-deadly
rough.
Avery and DNA
• Avery wanted to figure out what exactly
transferred from the smooth strain to the
rough.
– One at a time he used enzymes to destroy
the:
•
•
•
•
•
Proteins
Lipids
Carbohydrates
RNA
Still the smooth bacteria tranferred something
All that was left was the DNA!!!!
Hershey and Chase
• Studied viruses that infect bacteria –
called bacteriophage.
• Bacteriophages are simple: just DNA or
RNA and a protein coat.
• The labeled the DNA OR protein coat with
a a radioactive marker.
• What they found was the DNA entered the
cells, not the proteins.
• Only the
DNA
entered the
cell
• Therefore
DNA must
be the
genetic
material
DNA
• Deoxyribonucleic Acid
• Made up of units called nucleotides
– Each nucleotide has 3 parts:
• A) 5-Carbon sugar
• B) Phosphate group
• C) nitrogenous base
Nuclotides – differ in the Nitrogen Base!
• Nucleotides belong to 1 of 2 groups:
• Purines: Have 2 rings in their structure
– Adenine
– Guanine
• Pyrimidines: 1 ring
– Cytosine
– Thymine
Structure of DNA
• These 4 nucleotides are
linked in various ways and
order – creating DNA.
• There are 2 strands of DNA
• The sugar and the
phosphate make up the
backbone of the DNA.
• One nucleotide from each
strand come bond together
to form what looks like
ladder rungs.
Chargaff
• Chargaff’s Rules
– Erwin Chargaff
showed that in
most samples of
DNA the
Percentages of
Cytosine and
Guanine were
equal.
– Likewise, the
Adenine and
Thymine
Percentages were
also very close.
Percentages of Bases in
Organisms
Organism
A
T
C
G
Human
30.9
29.4
19.9
19.8
Chicken
28.8
29.2
20.5
21.5
Yeast
31.3
32.9
18.7
17.1
E. coli
24.7
23.6
26.0
25.7
Shape of DNA
• In the 1950’s Rosalind Franklin looked at
the shape of DNA.
• Used technique called X-Ray Diffraction
• Her work showed that the strands of DNA
wrap around each other  called a
Double Helix
• Watson and Crick used Franklin’s X-Ray
diffraction to build a model of DNA
DNA
• DNA is a long chain of
repeating nucleotides.
• DNA has 2 strands to it
• The phosphate group of
one nucleotide is bonded
to a sugar group of the
next nucleotide.
• The nitrogenous bases
of one strand extends
out and bonds with a
complementary
nitrogenous base on the
other DNA strand.