Zoology
I. Discovery of DNA
 A. Objectives
 i. Relate how Griffith’s bacterial experiments showed
that a hereditary factor was involved in transformation
 Ii. Summarize how Avery’s experiments led his group to
conclude that DNA is responsible for transformation in
bacteria
 Iii. Describe how Hershey and Chase’s experiments led
to the conclusion that DNA, not protein, is the
hereditary molecule in viruses
b. Griffith’s Experiments
 i. 1928, Fredrick Griffith studied Streptococcus
pneumonia, which could cause pneumonia
 Ii., He was trying to develop a vaccine against the
disease – causing or virulent, agent
 Iii. There are two different strains of the disease, one
which caused the disease, the S strain, and one which
did not, the R strain
iv. His Experiment
 1. Inject mouse with live R cells
 A. Result = mouse lived
 B. Conclusion = R cells do not kill the mouse
 2. Inject mouse with S cells
 A. Result = mouse died
 B. Conclusion = S cells kill the mouse
 3. Kill S cells with heat and inject the mouse with heat
killed S cells
 A. Result = mouse lived
 B. Conclusion = heat-killed S cells do not kill mouse
 4. Kill S cells with heat and mix with R cells, Inject
mouse with mixture
 A. Result = mouse died
 B. Conclusion = hereditary material from heat-killed S
cells transforms R cells. The transformed R cells kill the
mouse.
 v. Griffith’s experiments showed that hereditary
material can pass from one bacterial cell to another.
 Vi. The transfer of genetic material from one cell to
another cell from one organism to another organism is
called transformation.
c. Avery’s Experiment
 i. In 1940’s Oswald Avery set out to test whether the
transforming agent in Griffith’s experiment was
protein, RNA, or DNA
 Ii. Avery’s work showed that DNA is the hereditary
material that transfers information between bacterial
cells.
d. Hershey-Chase Experiment
 i. In 1952, Martha Chase and Alfred Avery set out to
test whether DNA or protein was the hereditary
material viruses transfer when viruses enter a
bacterium
 Ii. Viruses that infect bacteria are called
bacteriophages, or just phages
iii. Their Experiment
 1. Step 1 – Hershey and Chase used radioactive
isotopes to label the protein and DNA in the phage
 2. Then they allowed protein labeled and DNA-
labeled phage to separately infect E. coli bacteria
 3. They removed the phage coats from the cells in a
blender
 4. They then used a centrifuge to separate the phage
from the E. coli
 5. They found that all of the viral DNA and little of the
protein had entered E. coli cells
 Hershey and Chase confirmed that DNA, and not
protein, is the hereditary material
II. DNA structure
 A. Objectives
 1. evaluate the contributions of Franklin and Wilkins in
helping Watson and Crick discover DNA’s double helix
structure
 2. describe the three parts of a nucleotide
 3. summarize the role of covalent and hydrogen bonds
in the structure of DNA
 4. relate the role of the base-pairing rules to the
structure of DNA
b. DNA Double Helix
 i. In 1950’s Watson and Crick teamed up to determine
the structure of DNA
 Ii. Proposed that DNA is made of two chains that
wrap around each other in shape of a double helix, a
shape similar to a winding spiral staircase
 Iii. Watson and Crick
created a model of DNA
by using Franklin and
Wilkin’s DNA diffraction
X-rays
 Iv. Received the Nobel
Prize in 1962 for their work
c. DNA Nucleotides
 i. DNA is made of two nucleotide strands that wrap
round each other in the shape of a double helix
ii. A DNA nucleotide is made of a:
 1. 5-carbon deoxyribose sugar
 2. a phosphate group
 3. and one of four nitrogenous bases:
 A. Adenine (A) – purine
 B. Guanine (G) – purine
 C. Cytosine ( C ) – pyrimidine
 D. Thymine (T) – pyrimidine
iii. Bonds hold DNA together
 1. DNA double helix is similar to a spiral staircase
 2. alternating sugar and phosphate molecules form
the side “handrails” of the staircase
 3. Nucleotides along each DNA strand are bonded by
hydrogen bonds
 4. complementary nitrogenous bases are bonded by
hydrogen bonds
 5. each full turn of the DNA helix has 10 nucleotide
pairs
d. Complementary Bases
 i. In 1949, Chargaff observed the base pairing rules of
DNA
 Ii. Hydrogen bonding between the complementary
base pairs
 1. G –C
 2. A-T
 3. Holds t
the two strands
of a DNA
molecule together
iii. Is important because,
 1. the hydrogen bonds between the base pairs help the
two strands of DNA molecule together
 2. The complementary nature of DNA helps explain
how DNA replicates before a cell divides – one strand
of DNA serves as a template