CH 12.1 DNA
The Genetic Material
Learning Objectives:
1. Summarize the experiments leading
to the discovery of DNA as the
genetic material.
2. Diagram and Label the basic
structure of DNA
Scientific History
• The march to understanding that DNA is
the genetic material
– T.H. Morgan (1908)
– Frederick Griffith (1928)
– Avery, McCarty & MacLeod (1944)
– Erwin Chargaff (1947)
– Hershey & Chase (1952)
– Watson & Crick (1953)
– Meselson & Stahl (1958)
1908 | 1933
Chromosomes related to phenotype
• T.H. Morgan
– working with Drosophila
• fruit flies
– associated phenotype with
specific chromosome
• white-eyed male had specific X
chromosome
1908 | 1933
Genes are on chromosomes
• Morgan’s conclusions
– genes are on chromosomes
– but is it the protein or the DNA
of the chromosomes that are
the genes?
• initially proteins were thought to
be genetic material…
What’s so impressive
about proteins?!
The “Transforming Principle”
Frederick Griffith – 1928
-- Studied bacteria to find a cure for pneumonia
live pathogenic
strain of bacteria
A.
mice die
live non-pathogenic heat-killed
strain of bacteria
pathogenic bacteria
B.
C.
D.
mice live
mice live
mix heat-killed
pathogenic &
non-pathogenic
bacteria
mice die
Transforming Principle – a substance passed from
dead bacteria to live bacteria to change their phenotype
DNA is the “Transforming
Principle”
• Avery, McCarty & MacLeod
– purified both DNA & proteins separately from
Streptococcus pneumonia bacteria
• which will transform non-pathogenic bacteria?
– injected protein into bacteria
• no effect
– injected DNA into bacteria
• transformed harmless bacteria into
virulent bacteria
mice die
What’s the
conclusion?
1944
1944 | ??!!
Avery, McCarty & MacLeod
• Conclusion
– first experimental evidence that DNA
was the genetic material
Oswald Avery
Maclyn McCarty
Colin MacLeod
Confirmation of DNA
• Hershey & Chase
– classic “blender” experiment
– worked with bacteriophage
• viruses that infect bacteria
– grew phage viruses in 2 media,
Why use radioactively labeled with either
Sulfur
vs.
Phosphorus?
• Sulfur in their proteins
• Phosphorous in their DNA
– infected bacteria with
labeled phages
1952 | 1969
Hershey
Protein coat labeled
with 35S
Hershey
&
Chase
T2 bacteriophages
are labeled with
radioactive isotopes
S vs. P
bacteriophages infect
bacterial cells
bacterial cells are agitated
to remove viral protein coats
Which
radioactive
marker is found
inside the cell?
Which molecule
carries viral
genetic info?
DNA labeled with 32P
35S
radioactivity
found in the medium
32P
radioactivity found
in the bacterial cells
Blender experiment
• Radioactive phage & bacteria in blender
– Sulfur labeled Protein
• radioactive proteins stayed in supernatant
• therefore viral protein did NOT enter
bacteria
– Phosphorous labeled DNA
• radioactive DNA stayed in pellet
• therefore viral DNA did enter bacteria
– Confirmed DNA is “transforming factor”
Taaa-Daaa!
Hershey & Chase
Martha Chase
1952 | 1969
Alfred Hershey
Hershey
Structure of DNA
Now scientists agreed that DNA was in
fact the genetic material….
But what did it look like?
Let’s take a closer look at DNA.
Chargaff
• DNA composition: “Chargaff’s rules”
– varies from species to species
– all 4 bases not in equal quantity
– bases present in characteristic ratio
• humans:
A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
Rules
A = T
C = G
1947
Structure of DNA
1953 | 1962
• Watson & Crick
– developed double helix model of DNA
• Used the findings of other leading scientists working on
question:
– Rosalind Franklin
– Maurice Wilkins
– Linus Pauling
Franklin
Wilkins
Pauling
1953 article in Nature
Watson and Crick
Watson
Crick
Rosalind Franklin (1920-1958)
DNA
Stands for
Deoxyribonucleic Acid
Nucleic Acids
• Function:
– genetic material
• stores information
–genes
–blueprint for building proteins
»DNA  RNA  proteins
DNA
• transfers information
– blueprint for new cells
– blueprint for next generation
proteins
G
C
T
A
A
C
G
T
A
C
G
T
A
Nucleic Acids
• Examples:
– RNA (ribonucleic acid)
• single helix
– DNA (deoxyribonucleic acid)
• double helix
• Structure:
– monomers = nucleotides
DNA
RNA
Nucleotides
• 3 parts
– nitrogen base (C-N ring)
– pentose sugar (5C)
• ribose in RNA
• deoxyribose in DNA
– phosphate (PO4) group
Are nucleic acids
charged molecules?
Nitrogen base
I’m the
A,T,C,G or U
part!
Types of nucleotides
• 2 types of nucleotides
– different nitrogen bases
– purines
• double ring N base
• adenine (A)
• guanine (G)
– pyrimidines
•
•
•
•
single ring N base
cytosine (C)
thymine (T)
uracil (U)
Purine = AG
Pure silver!
Nucleic polymer
• Backbone
– sugar to PO4 bond
– phosphodiester bond
• new base added to sugar of
previous base
• polymer grows in one direction
– N bases hang off the
sugar-phosphate backbone
Dangling bases?
Why is this important?
Pairing of nucleotides
• Nucleotides bond between
DNA strands
– H bonds
– purine :: pyrimidine
– A :: T
• 2 H bonds
– G :: C
• 3 H bonds
Matching bases?
Why is this important?
DNA molecule
Shape = Double helix
– H bonds between
bases join the 2
strands
• A :: T
• C :: G
H bonds?
Why is this important?
Time for Questions!!!
Learning Objectives:
1.
Summarize the experiments leading to the discovery of DNA as
the genetic material.
Name that Scientist(s)….
1.
The double helix structure of DNA was first described by
________.
2.
The first major experiment that led to the discovery of DNA as
the genetic material was conducted by ______. He used heatkilled bacteria in mice.
3.
The scientist who identified the transforming agent in Griffith’s
famous experiment as DNA was _______.
4.
These scientists preformed the famous “blender experiment”
to demonstrate that DNA is the genetic material in viruses.
5.
This scientist’s X-ray diffraction data helped Watson and Crick
solve the structure of DNA.
Answers: 1. Watson and Crick 2. Griffith 3. Avery
4. Hershey and Chase 5. Rosalind Franklin
Learning Objective 2:
Diagram and Label the basic structure of DNA.
1. Use the following words to label this piece of
DNA:
– Deoxyribose
– Phosphate
– Adenine
– Thymine
– Cytosine
– Guanine
2.
3.
4.
Circle a nucleotide
Put a star by the purines
Underline the pyrimidines