Name:ANSWER KEY
12.1 Reading Guide – DNA: The Genetic Material
Use the following questions to guide you as you read about the discovery of DNA as the
hereditary molecule of all living things in pages 326-332 of your textbook.
p. 326
1.) List two things that scientists knew about genetic information at the start of the 1900s.

genetic information was carried on the chromosomes in eukaryotic cells

the two main components of chromosomes are DNA and protein
2.) What types of Streptococcus pneumoniae bacterium did Frederick Griffith use in his
experiment? Fill in the chart to summarize two differences between these types.
Types of Streptococcus pneumoniae bacteria
Strain
R- strain
S-strain
Appearance
Bacteria colonies have rough
edges.
Bacteria colonies have smooth
edges.
Does not cause pneumonia.
Causes pneumonia.
Effect on organisms
3.) Read the account of Griffith’s experiment and examine Figure 12.2 on page 327. Fill in the
chart to summarize Griffith’s results.
Summary of Griffith’s Experiment
Type(s) of bacteria injected
Observed result
Conclusion
Live S-strain
Mouse dies.
S-strain causes pneumonia.
Live R-strain
Mouse lives
R-strain does not cause
pneumonia.
Heat-killed S-strain
Mouse lives.
Heat-killed S-strain bacteria
cannot cause disease.
Mouse dies
Somehow the live R-strain
had been changed into living
S-strain bacteria.
Heat-killed S-strain &
live R-strain
4.) Griffith was able to take blood from the mice that were injected with heat-killed S-strain and
live R strain together. He isolated bacteria in this blood and concluded that some had been
transformed. What did Griffith mean by this?
He concluded that there was something contained in the remains of the heat-killed S-strain
bacteria that could transform, or change, the live R-strain into living S-strain bacteria.
p. 327
5.) How did Oswald Avery modify Griffith’s experiment?
Avery and his colleagues separated the heat-killed S-strain into its component parts DNA,
proteins, and lipids. When the live R-strain cells were exposed to the S-strain DNA, they were
transformed. When they were exposed to proteins or lipids, they were not. This showed that it
was the DNA and not other parts of the bacteria that were capable of doing the transforming.
6.) Avery concluded that DNA was the transforming substance because when he exposed live R
cells to the S-strain DNA, they were changed into S cells.
7.) What is a bacteriophage?
A bacteriophages is a type of virus that attacks bacteria.
8.) Give two reasons why bacteriophages were ideal for an experiment to confirm DNA as the
genetic material?
 The bacteriophages is simple. It consists of only DNA and protein.

The viruses cannot replicate themselves, but must inject their genetic material into a
living cell.
p. 328
9.) Examine Figure 12.3 and the text account of Hershey & Chase’s experiment on this page.
Use the scientific method (outlined below) to organize this experiment.
What was Hershey & Chase’s hypothesis?
Answers may vary. Essentially, Hershey & Chase hypothesized that DNA, not proteins was
the genetic material of living things.
Give a step by step description of Hershey & Chase’s procedure/ experimental method.
1. They labeled one set of bacteriophages with radioactive phosphorus. Phosphorus is only
found in DNA so only the DNA part of the bacteriophages would be radioactive.
2. They labeled another set of bacteriophages with radioactive sulfur. Sulfur is only found in
proteins so only the protein part of the bacteriophages would be radioactive.
3. They infected separate groups of bacteria with viruses from the two groups.
4. The infected bacteria were separated from the viruses.
5. They found that the radioactive proteins remained outside the bacteria cells, while the
radioactive phosphorus could be found inside the affected bacteria cells.
What did Hershey & Chase conclude from their experiment?
Hershey & Chase concluded that DNA, not protein, was the genetic material.
p. 329
10.) Nucleotides are the subunits of nucleic acids.
11.) Draw a sketch of a nucleotide. Label the phosphate group, the 5-carbon sugar, and the
nitrogenous base.
12.) The two nucleic acids found in living cells are DNA and RNA.
13.) Complete the chart:
Nitrogenous Bases in Nucleic Acids
DNA
RNA
Purines
1.adenine
1.adenine
(double ring)
2.guanine
2.guanine
Pyrimidines
1.cytosine
1.cytosine
(single ring)
2.thymine
2.uracil
14.) What did Edwin Chargaff discover when he analyzed the amount of adenine, guanine,
cytosine, and thymine in the DNA of various organisms?
Chargaff found that the amount of guanine nearly equals the amount of cytosine, and the
amount of adenine nearly equals the amount of thymine within a species.
p. 330
15.) Rosalind Franklin’s Photo 51 revealed that the structure of DNA was a double helix.
16.) Watson and Crick are responsible for building the first accurate model of DNA.
17.) The structure of DNA resembles a twisted ladder. The rails (sides) of this ladder are
composed of alternating deoxyribose and phosphate. The rungs of the ladder are composed
of pairs of bases.
p. 331
18.) The DNA molecule is said to be antiparallel. What does this mean?
One strand of DNA is oriented 5’ to 3’. The complementary strand of DNA runs in the
opposite direction from the 3’ to 5’.
p. 332
19.) Complete the graphic organizer that describes the structure of the chromosome.
DNA
tightly
coils
around
histones
which due to
positive/negative
attracts wrap up
into
nucleosomes
which
group
together
to form
chromatin
fibers