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What is DNA?
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What does DNA look like?
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How do we use DNA in everyday life?
Objectives
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Explain what DNA is and understand it’s
significance
Summarize the experiments of Griffith and
Avery leading to discovery of DNA as genetic
material
Label a diagram of a virus.
Molecular Genetics
Chapter 12
DNA
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DNA stands for:
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DNA is located:
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Deoxyribonucleic acid
In the nucleus of cells
The function of DNA is to:
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Tells the cells how to produce things that make
you up (Blueprint)
When was DNA discovered?
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We are going to study the scientists who
discovered DNA.
1928 Fred Griffith 
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Studied two strains of bacteria,
Streptococcus pneumoniae
Worked with two strains of bacteria: Rough
Strain and Smooth Strain
S strain causes pneumonia and the R strain
does not
1928 Fred Griffith
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Discovered that something from the heat killed
wild type (smooth) bacteria was turning the
mutant (rough) type bacteria into smooth.
Called it the transforming factor
Didn’t really know what it was
1944- Avery, McCarty and
MacLeod
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Purified both protein, RNA, and DNA
separately from Streptococcus pneumonia
bacteria to determine which would transform
the harmless bacteria- How?
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Injected protein into bacteria  no effect
Injected RNA into bacteria  no effect
Injected DNA into bacteria  mice died
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Conclusion: DNA is the genetic material!!
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Nobody believed Avery 
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People thought that Avery had inaccurate
results and contamination and still thought
the transforming factor could be.
Back to square 1!
We know there is a “transforming factor”
Is it protein or DNA?
Lets ask Hershey and Chase!
Hershey and Chase
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Worked with viruses to determine what the
“transforming factor” was.
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*Before we discuss their experiment we must
understand what a virus is and how it works!*
Viruses- 6 Characteristics of Viruses
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1. Have their own genome (genetic make up)
made of either DNA or RNA
2. Does not have enzymes, ribosomes, or
ATP
3. Have external protein shells (capsids)
Viruses- 6 Characteristics of Viruses
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4. Infect only specific cells
5. 2 life cycles (lytic and lysogenic)
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Lytic – breaks out and spreads
Lysogenic- hangs around
6. Smaller than bacteria
Bacteriophage= virus attacking
bacteria…
2 Life Cycles
5 Stages of Attack!!
1) Attachment: virus attaches to it’s host
2) Injection: virus injects it’s DNA into the host
cell.
3) Replication: virus copies it’s genetic material
many times.
4) Assembly: virus assembles more viruses
5) Release: Viruses are release from the cell
ready to attach to new cells!
Identify the Stages
Homework!
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Read pgs. 326-328, Summarize the
experiments of Griffith and Avery
2/23
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DO NOW
Take out your homework!!! 
Which strain of bacteria killed the mice in
Griffith’s experiment?
What is the process that caused rough
bacteria to become smooth?
What did Avery discover?
What are the 5 stages of attack for a virus?
Objectives
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Summarize the experiments of Hershey and
Chase.
Identify the stages of replication of a virus.
Understand the use of viruses in discovering
DNA.
Lets Review…
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Who are the scientists we’ve talked about so
far?
What are the 6 characteristics of a virus?
Viruses!
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5 Stages of Attack animation
1952 Hershey and Chase
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Radioactive Labeling:
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Used radioactive phosphorus (32P) to identify DNA in the
bacteriophages
Used radioactive sulfer (35S) to identify proteins in the
bacteriophagesTracking DNA
Radioactive bacteriophages infected bacteria cells
After review results found radioactive sulfur outside
of bacteria cells and radioactive phosphorus inside
bacteria cells
1952 Hershey and Chase
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Used Bacteriophages
Two radioactively labeled viruses.
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1. Sulfur Protein coat
2. Phosphorous DNA
After infecting cells, only the DNA was still
radioactively labeled
DNA is genetic material
Hershey and
Chase…
Ok… so we KNOW DNA carries
Genetic Material, now what?
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Levene discovered that DNA is made of
nucleotides
What are nucleotides?
DNA STRUCTURE
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Nucleotides:
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5 carbon sugar (Deoxyribose)
Phosphate Group
Nitrogen base
Nitrogenous Bases
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Purines
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Double ring structure
*Adenine and Guanine
Pyrimidines
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Single ring structure
* Thymine, Cytosine, Uracil
Nitrogenous Bases
DNA Structure
Erwin Chargaff
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Chargaff’s rule
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C= G
T=A
Draw in the complimentary sequence
ATGCTAATTCGCATCG
Rosiland Franklin
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X-ray Diffraction use of
photo 51
X-ray diffraction data
helped solve the structure
of DNA
Indicated that DNA was a
double helix
X-Ray Crystallography
Watson and Crick
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Built a model of the double helix that
conformed to the others’ research
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two outside strands consist of alternating sugar
and phosphate molecules
cytosine and guanine bases pair to each other by
three hydrogen bonds
thymine and adenine bases pair to each other
by two hydrogen bonds
DNA Structure
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DNA often is compared to a
twisted ladder.
Rails of the ladder are
represented by the
alternating deoxyribose and
phosphate.
The pairs of bases (cytosine–
guanine or thymine–adenine)
form the steps.
Label the parts of the DNA molecule
Base PairsRungs of the
ladder
-2 Strands of
DNA form a
helix
Orientation of DNA- Anti Parallel
 On the top rail, the strand is said to be
oriented 5′ to 3′.
 The strand on the bottom runs in the
opposite direction and is oriented 3′ to 5′.
Chromosome Structure
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DNA coils around histones (proteins) to
form nucleosomes,
This coils to form chromatin fibers.
The chromatin fibers supercoil to form
chromosomes that are visible in the
metaphase stage of mitosis.