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Chap 13: DNA - Genetic Material
QUESTION:
Where is genetic information stored?
a)
b)
c)
d)
in the ribosomes of cells
within the proteins of cells
within the DNA of cells
within the membrane of cells
*It is important to remember that scientists
didn’t always have the answers,
experiments and research must be
performed and consensus reached.
Science is a process!
The answer is here
somewhere!
Finding the molecule of heredity
The material must be…
1.Able to store information that pertains to
the development, structure and metabolic
activities of the cell
2.Stable so that it can be replicated
3.Able to undergo changes (mutations)
1869 – Discovering Nucleic Acids
Swiss Physician, Johannes Friedrich
Miescher isolated the chemical he
called “nuclein” from the nuclei of white
blood cells
Now called nucleic acids
DNA (deoxyribonucleic acid)
RNA (ribonucleic acid)
Miescher used pus and bloodstained
bandages from a hospital to study
“nuclein”
NUCLEOTIDES (found in nuclein)
Contain a sugar, phosphate and a nitrogen base
Four types of Nitrogen Bases
Adenine
Guanine
Cytosine
Thymine
***THE BIG QUESTION***
Is it nucleic acids that contain the genetic
code or is it proteins?
a) Proteins contain 20 amino acids that can be
organized in countless ways to determine traits
b) Nucleic acids only contained 4 different nucleotides
20 amino acids
As a storage molecule amino
acids would have a lot of
options, you could combine
these in different ways
Only 4 bases would seem to
not have as much versatility
It’s like have an alphabet of
20 letters versus and
alphabet of 4.
TRANSFORMATION OF BACTERIA
Frederick Griffith
attempted to find a
vaccine against
pneumococcus
He found that one type
of bacteria could turn
into another
An overview of the transformation experiment.
Can you summarize this in your own words?
DNA WAS DETERMINED TO BE THE
TRANSFORMING SUBSTANCE
●DNA from S strain bacteria causes R strain to
be transformed
●Enzymes that degrade proteins will not stop
the transformation
●Enzymes that degrade DNA does stop the
transformation
Transformation Animation Activity
Quick Recap:
1) What caused scientists to believe that proteins contained the
genetic code?
2) What was Miescher’s contribution to genetic studies?
3) How were the nonvirulent R strain bacteria transformed into a
virulent strain?
4) Griffith’s experiment resulted in which conclusion?
a) DNA contains the genetic code; it is the transforming agent
b) Proteins contain the genetic code, they are not destroyed during heating
c) Bacteria can spontaneously come to life after being heat
killed.
Alfred Hershey and Martha Chase Experiments
Remember the bacteriophage!!? It’s back!
What do you remember about phages?
Bacteriophages
–
viruses that infect bacteria
●Consist of a protein capsid
●A core of DNA (or RNA)
●Experiments used radioactive sulfur to tag
the protein
●And radioactive phosphorous to tag the DNA
●The goal was to see which substance (protein or DNA)
moved into the infected cell
Conclusion: The radioactive tag on the DNA went into the bacteria, not
the tagged proteins
Animation of the Hershey and Chase Experiment
1. What part of the phage entered the bacterial cell following infection?
A)DNA
B)RNA
C)protein coat
D)the entire phage
E)no part
2. If 35S was found in progeny phages rather than 32P, Hershey and Chase
would have concluded that
A)proteins contain phosphorus.
B)DNA contains sulfur.
C)phage DNA enters the host cell.
D)phage protein enters the host cell.
E)phage can kill the E. coli cell.
3
3. In the Hershey and Chase experiment, radioactively-labeled
A)32P did not enter the cell.
B)32P remained inside the cells after vigorous shaking.
C)32P was removed from the cells by vigorous shaking.
D)32P and 35S remained inside the cells after vigorous shaking.
E)32P and 35S were removed from the cells after vigorous shaking.
Lecture Reflection
1) How was DNA established as the molecule that contains the genetic code?
2) Now is a good time to mention that at this point, no one knew the structure of DNA, they
only had a basic idea of what it was made of, but not its shape. Every basic biology student
knows the shape of DNA now, can you sketch it?
THE RACE IS ON!
Who will be the first to discover the structure of DNA?