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Guided Notes: Chapter 9 - DNA: The Genetic Material
Section 1 Identifying the Genetic Material
Objectives
 Relate Griffith’s conclusions to the observations he made during the transformation
experiments.
 Summarize the steps involved in Avery’s transformation experiments, and state the
results.
 Evaluate the results of the Hershey and Chase experiment.
Transformation : Griffith’s Experiments

1928, Frederick Griffith, a bacteriologist, tried to find a vaccine for pneumonia.

Vaccine

Prepared from – killed or weakened disease-causing agents, including certain bacteria.

Purpose – introduced into the body to protect the body against future infections by the
disease-causing agent.

Virulent – able to cause disease.

Transformation – a change in genotype caused when cells take up foreign genetic
material.

Griffith’s Experiments:
o
R bacteria – rough bacteria that did not have an outer capsule
o
S bacteria – smooth bacteria had an outer capsule that protected the bacteria
from the body’s defenses.
Injected mice with:
Result:
S bacteria
Mice die
R bacteria
Mice live
Heat-killed S bacteria
Mice live
Heat-killed S bacteria & R bacteria
Mice die
What useful application is there for humans in regards to bacterial transformation? Bacteria can
be transformed with a gene from humans that will allow the bacteria to produce human insulin.
Transformation: Avery’s Experiments – He wanted to determine what molecule was
responsible for transformation.
Result:
Enzyme used
Protein destroying enzyme
Transformation continued
DNA destroying enzyme
Transformation stopped
Conclusion: DNA is the material responsible for transformation.
Viral Genes and DNA: DNA’s Role Revealed
Hershey’s and Chase’s Experiments: They wanted to prove that DNA carried genetic
material.

Bacteriophage or phage – a virus that infects bacteria.
Radioactive isotopes can be used to tag a molecule so that you can determine where it goes by
tracing the radioactive isotope.
 They tagged the protein coat of the bacteriophage with radioactive sulfur (35S) because
proteins contain sulfur, but DNA does not.
 They tagged the DNA of the bacteriophage with radioactive phosphorous (32P) because
DNA contains phosphorous, but protein does not.
 They carried out 2 separate experiments.
Virus’s protein coat labeled with (35S)
Phage infects E. coli bacteria
Bacterial cells were spun to remove the
virus’s protein coats.
Result: Radioactive sulfur remained in
the phages.

Virus’s protein coat labeled with (32P)
Phage infects E. coli bacteria
Bacterial cells were spun to remove the
virus’s protein coats.
Result: Radioactive phosphorus moved
into the bacterial cells.
Conclusion: DNA, rather than proteins, is the hereditary material, at least in viruses.
Section 2 The Structure of DNA
Objectives
 Describe the three components of a nucleotide.
 Develop a model of the structure of a DNA molecule.
 Evaluate the contributions of Chargaff, Franklin, and Wilkins in helping Watson and Crick
determine the double-helical structure of DNA.
 Relate the role of the base-pairing rules to the structure of DNA.
DNA Overview (Video Clip)
DNA is made up of subunits called nucleotides, each made of 3 parts: sugar, a phosphate
group, and a nitrogen base. In DNA, the sugar is deoxyribose.

Watson and Crick determined that a DNA molecule is a double helix — two strands
twisted around each other, like a winding staircase.

Nucleotides are the subunits that make up DNA. Each nucleotide is made of three parts:
a phosphate group, a five-carbon sugar molecule, and a nitrogen-containing base.

The five-carbon sugar in DNA nucleotides is called deoxyribose.
Nucleotides (Video clip) What portion of the ladder do the sugars and phosphate groups of
a nucleotide form? Sides The nitrogen bases face each other and bond to form what
portion of the ladder? Rungs

The nitrogen base in a nucleotide can be either a bulky, purine, or a smaller, pyrimidine.
Discovering DNA’s Structure:
Chargaff’s Observations
What is Chargaff’s rule, based on his observations? Amount of A = amount of thymine; C = G
Wilkins and Franklin’s Photographs
 What did the X-ray diffraction photographs suggest about the structure of DNA? The
DNA molecule resembled a tightly coiled helix and was composed of two or three chains
of nucleotides.
Watson and Crick’s DNA Model
 Whose ideas did Watson & Crick incorporate into their model of DNA? It took into
account Chargaff’s observations and the patterns on Franklin’s X-ray diffraction
photographs.
Pairing Between Bases
 The strictness of base-pairing results in two strands that contain complementary base
pairs.

3 things that determine why the bases pair:
1. Size
2. Shape
3. Number of hydrogen bonds
Section 3 The Replication of DNA
Objectives
 Summarize the process of DNA replication.
 Describe how errors are corrected during DNA replication.
 Compare the number of replication forks in prokaryotic and eukaryotic DNA.
Roles of Enzymes in DNA Replication

The process of making a copy of DNA is called DNA replication.

It occurs during the synthesis (S) phase of the cell cycle, before a cell divides.

DNA replication occurs in three steps: See page 198 Figure 9.
o
Step 1 DNA helicases open the double helix by breaking the hydrogen bonds that
link the complementary nitrogen bases between the two strands. The areas where
the double helix separates are called replication forks.
o Step 2 At the replication fork enzymes known as DNA polymerases move along
each of the DNA strands. DNA polymerases add nucleotides to the exposed nitrogen
bases, according to the base-pairing rules.
o Step 3 Two DNA molecules form that are identical to the original DNA molecule.
(Video clip) DNA Replication
What is the end result of DNA replication? Two identical DNA molecules each made up of an
original strand and a new strand.
Checking for Errors

An important feature of DNA replication is that DNA polymerases have a “proofreading”
role.

This proofreading reduces errors in DNA replication to about one error per 1 billion
nucleotides.

The Rate of Replication See page 200 Figure 10.

The circular DNA molecules found in prokaryotes usually have two replication forks that
begin at a single point.

The replication forks move away from each other until they meet on the opposite side of
the DNA circle.

In eukaryotic cells, each chromosome contains a single, long strand of DNA.

Each human chromosome is replicated in about 100 sections that are 100,000
nucleotides long, each section with its own starting point. With multiple replication forks
working in concert, an entire human chromosome can be replicated in about 8 hours.

Video Clip: Multiple replication forks in eukaryotes reduce the replication time from?
From 33 days to 8 hours.