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DNA: The Stuff of Life
Griffith and Transformation
• Griffith and Transformation
• In 1928, British scientist Fredrick Griffith was trying to
learn how certain types of bacteria caused pneumonia.
• He isolated two different strains of pneumonia bacteria
from mice and grew them in his lab.
Griffith and Transformation
 Performed the first major experiment that led to the
discovery that genetic information could be passed
between organisms.
Griffith and Transformation
– Transformation
• Griffith determined that bacteria could pass genetic
information from one to another.
• Griffith called this process transformation because
one strain of bacteria (the harmless strain) had
changed permanently into another (the diseasecausing strain).
• Transformation = the exchange of genetic
information between bacterial organisms.
Oswald Avery
 1931 - Oswald Avery repeated Griffith’s work
to determine which molecule was most
important for transformation.
 Identified the molecule that transformed the
R strain of bacteria into the S strain
 Concluded that when the S cells were killed,
DNA was released
 R bacteria incorporated this DNA into their
cells and changed into S cells.
The Hershey-Chase
Experiment
• The Hershey-Chase Experiment
– Alfred Hershey and Martha Chase studied
viruses—nonliving particles smaller than a
cell that can infect living organisms.
The Hershey-Chase
Experiment
• Bacteriophages
• A virus that infects bacteria is known as a
bacteriophage.
• Bacteriophages are composed of a DNA or RNA core
and a protein coat.
The Hershey-Chase
Experiment
• If Hershey and Chase could determine which part of
the virus entered an infected cell, they would learn
whether genes were made of protein or DNA.
• They grew viruses in cultures containing radioactive
isotopes of phosphorus-32 (32P) and sulfur-35 (35S).
The Hershey-Chase
Experiment
Hershey and Chase
 Used radioactive labeling to
trace the DNA and protein
 Concluded that the viral DNA
was injected into the cell and
provided the genetic
information needed to produce
new viruses
Chargaff
 Chargaff’s rule: C = G and T = A
•The percentages of
guanine [G] and
cytosine [C] bases are
almost equal in any
sample of DNA.
•The percentages of
adenine [A] and thymine
[T] bases are almost
equal in any sample of
DNA.
Two Groups of Bases in DNA
• Pyrimidines are
single ring bases.
– Thymine & Cytosine
N
N C
O C
C
N C
N
• Purines are double
ring bases.
– Adenine & Guanine
N C
C
CN
N C
NC
X-ray Diffraction
 Rosalind Franklin’s X-ray diffraction data
helped solve the structure of DNA
 Indicated that DNA was a double helix
Watson and Crick
 Built a model of the double
helix that conformed to the
others’ research
1. Model was a double helix
2. Backbone made of sugars and
phosphates
3. Base pairs attached to deoxyribose with A
& T in equal amounts and C & G in equal
amounts.
DNA Structure
 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.
The Components and
Structure of DNA
• Watson and Crick discovered that hydrogen
bonds can form only between certain base
pairs—adenine and thymine, and guanine
and cytosine.
• This principle is called base pairing.
Copyright Pearson Prentice Hall
• DNA Double Helix
DNA Structure
 Nucleotides
 Consist of a five-carbon sugar, a phosphate
group, and a nitrogenous base
DNA Replication
DNA  DNA
DNA Replication
– Duplicating DNA
• Before a cell divides, it
duplicates its DNA in a
process called
replication.
• Replication ensures that
each resulting cell will
have a complete set of
DNA.
• Occurs in the S phase of
Interphase.
DNA Replication
• DNA must be copied in order for cells to
multiply in number and maintain genetic
information.
• The DNA molecule produces 2 IDENTICAL
new complementary strands following the
rules of base pairing:
A-T, G-C
•Each strand of the
original DNA serves as a
template for the new
strand
DNA Replication
• Semiconservative
Model:
1. Watson and Crick
showed: the two strands
of the parental molecule
separate, and each
functions as a template
for synthesis of a new
complementary strand.
.
Parental DNA
DNA Template
New DNA
DNA Replication
New Strand
Original strand
Nitrogen Bases
Replication Fork
The sites where separation
and replication occur are
called replication forks.
Replication Fork
DNA Polymerase
DNA Replication
• DNA must first unwind and “unzip”
• DNA helicase, an enzyme, is responsible for
unwinding and unzipping the double helix.
– Single stranded binding proteins hold the DNA open
DNA Replication
• DNA polymerase continues adding appropriate
nucleotides to the chain by adding to the open end
of the new DNA strand.
DNA Replication
• Okazaki fragments, sections of nucleotides
created by replication in small segments, must be
joined together.
• Finally, DNA ligase links the fragments together
to form the full complementary strand.
DNA Replication
• Give the complementary sequence for the
following strand of DNA:
– DNA 5’ A T C C G A A G C T T 3’
– DNA 3’ T A G G C T T C G A A 5’