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DNA Structure
Frederick Griffith
• In 1928, Frederick Griffith wanted to learn how
certain types of bacteria produce pneumonia
• Griffith injected mice with disease-causing
bacteria, the mice died of pneumonia
• Griffith injected mice with harmless bacteria, the
mice didn’t get sick
• Griffith thought that the disease-causing
bacteria might produce a poison
Frederick Griffith
• Griffith mixed heat-killed disease-causing
bacteria with harmless bacteria and injected the
mixture into mice
• To his amazement, the mice developed bacteria
and many died
• He found that the lungs of those mice who died
were not filled with harmless bacteria, but of
disease-causing bacteria
Frederick Griffith
• The heat-killed bacteria had passed their
disease-causing ability to the harmless strain
• Griffith coined this process transformation
because one strain of bacteria had been changed
into another
• Griffith hypothesized that when the two types of
bacteria were mixed, some factor was
transformed from the heat-killed cells into the
live harmless bacteria
Frederick Griffith
• Griffith discovered that the ability to cause
disease was inherited by the transformed
bacteria’s offspring, the transforming factor
was a gene
Oswald Avery
• In 1944, a group of scientists repeated Griffith’s
• Avery and his colleagues made an extract from
the heat killed bacteria and treated the extract
with enzymes that destroyed proteins,
carbohydrates, lipids, and other molecules
• Since transformation still occurred, these
molecules were not responsible for the
Oswald Avery
• Avery and other scientists repeated the
experiment, this time using enzymes that
targeted DNA
• When they destroyed the nucleic acid DNA,
transformation did not occur
• Avery and other scientists discovered that
the nucleic acid DNA stores and transmits
the genetic information from one
generation of an organism to the next
The Hershey-Chase Experiment
• In 1952, American scientists Alfred Hershey and
Martha Chase collaborated in studying viruses
• One type of virus that infects bacteria is known
as a bacteriophage
• Bacteriophages are composed of a DNA or RNA
core and a protein coat
• When a bacteriophage enters a
bacterium, the virus attaches to the
surface of the cell and injects its genetic
information into it
The Hershey-Chase Experiment
• Hershey and Chase reasoned that if they could
determine which part of the virus entered the
infected cell, they would learn whether genes
were made of protein or DNA
• Viruses were grown in cultures containing
radioactive isotopes of phosphorus-32 (32P) and
sulfur-35 (32S)
• Proteins contain almost no phosphorus and
DNA contains no sulfur
The Hershey-Chase Experiment
• The radioactive substances were used as
• If 32S was found in the bacteria, it would mean
that the viruses’ bacteria had been injected into
the bacteria
• If 32P was found in the bacteria, then it was the
DNA that had been injected
The Hershey-Chase Experiment
• Marked viruses were mixed with bacteria, after a
few minutes viruses injected their genetic
• Viruses were separated were from bacteria,
bacteria was tested for radioactivity
• Nearly all the radioactivity in the bacteria was
from phosphorus, the marker found in DNA
• Hershey and Chase demonstrated that
DNA, not protein, functions as the
phage’s genetic material
The Components and Structure of DNA
• DNA is made up of nucleotides
• Nucleotides are made up of three basic
components: a 5-carbon sugar called
deoxyribose, a phosphate group, and a
nitrogenous base
The Components and Structure of DNA
• There are four kinds of nitrogenous bases in
• Adenine and Guanine are two
nitrogenous bases that belong in a group
of compounds known as purines
• Cytosine and Thymine are known as
• Purines have two rings in their
structures, whereas pyrimidines have one
• Adenine,
cytosine, and
thymine are
four kinds of
bases in DNA.
The Components and
Structure of DNA
• The phosphate of one
nucleotide is attached to the
sugar of the next nucleotide
in line
• The result is a “backbone” of
alternating phosphates and
The Components and Structure of DNA
• Erwin Chargaff discovered that the percentages
of guanine and cytosine bases are almost equal
in any sample of DNA
• The same thing is true for the other two
nucleotides, adenine and thymine
• The observation that [A]=[T] and [G]=[C]
became known as Chargaff’s rules
The Components and Structure of DNA
• According to Chargaff’s rules, the percentages of
equal to thymine and the
percentages of ______
cytosine are equal to guanine in
the DNA molecule.
• Describe the
components and
structure of a DNA
• DNA is made of
nucleotides. Each
nucleotide has three
parts: a 5-carbon
sugar, a phosphate
group, and a
nitrogenous base
• The four nitrogenous
bases are adenine and
guanine (purines),
cytosine and thymine
The Components and Structure of DNA
• Francis Crick and James
Watson were trying to
understand the structure of
DNA by building 3
dimensional models of the
• Watson was shown a copy of
Rosalind Franklin’s X-ray
pattern of DNA
The Components and Structure of DNA
• Using clues from Franklin’s pattern, Watson and
Crick had built a structural model that explained
the puzzle of how DNA could carry information,
and how it could be copied
• Watson and Crick’s model of DNA was a
double helix in which two strands were
wound around each other
The Components and Structure of DNA
• Explain how Chargaff’s rules helped Watson and
Crick model DNA.
• The two stands of DNA are held together by
hydrogen bonds between certain bases-A and T,
and G and C-which explained Chargaff’s rules.
The Double Helix
• The two strands are held together by hydrogen
bonds between the nitrogenous bases, which are
paired in the interior of the double helix
• Hydrogen bonds can form only between certain
base pairs-adenine and thymine, and guanine
and cytosine.
• Base pairing principle that bonds DNA can
form only between adenine and thymine and
between cytosine and guanine
DNA and Chromosomes
• Eukaryotic chromosomes contain both DNA and
proteins, packed together to form chromatin
• Eukaryotic chromosomes contain DNA wrapped
around proteins called histones (globular
proteins that DNA tightly coils around to form
DNA Replication
• The parent molecule has two complimentary
strands of DNA
• The first step in replication is separation of the
two DNA strands
• Each parent strand now serves as a template that
determines the order of nucleotides along a new
complementary strand
• Each parent stand produces two new
complimentary strands following the rules of
base pairing
DNA Replication
• What is the complimentary strand of bases for a
strand with the bases TACGTT?
DNA Replication
DNA Replication
• The replication of a DNA molecule begins at
special sites called origins of replication
• Proteins recognize a stretch of DNA having a
specific sequence of nucleotides
• These proteins attach to the DNA and separate
the two strands and open up a replication
DNA Replication
• Once the two strands of the double helix have
separated, two replication forks (a Y-shaped
where the new strands of the DNA are
elongating) form at the end of a replication
• As each new strand forms, new bases are added
following the rules of base pairing
DNA Replication
• Elongation of new DNA at the replication fork is
catalyzed by enzymes called DNA
polymerases (enzymes that catalyzes the
elongation of new DNA at a replication fork by
the addition of nucleotides to the existing chain)
DNA Replication
• The DNA molecule _______,
separates or unzips, into
two strands
Each strand of the DNA molecule serves
as a _______,
template or model, to produce the
new strands
Two new ___________
complimentary strands are
produced, following the rules of
DNA Replication
DNA Replication
• The two strands of DNA are anti-parallel
(their sugar phosphate backbones run in
opposite directions)
• In the double helix, the two sugar-phosphate
backbones are upside down relative to each
DNA Replication
• The 5’ to 3’ direction
of one strand runs
counter to the 5’ to 3’
of the other
DNA Replication
• DNA polymerases only add nucleotides to the
free 3’ end of a growing DNA strand, never to the
5’ end
• A new DNA strand can only elongate in the 5’ to
the 3’ direction
DNA Replication
• DNA polymerase adds nucleotides to a
complimentary strand as the replication fork
• The DNA strand made by this mechanism is
called the leading strand
DNA Replication
• To elongate the other new strand of DNA, DNA
polymerase works along the other template
strand in the direction away from the replication
• DNA synthesized in this direction is called the
lagging strand