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7.1
Isolating the Material of Heredity
E X P E C TAT I O N S
Explain the roles of evidence, theories, and paradigms in the development
of scientific knowledge about genetics.
Demonstrate an understanding of the process of discovery that led to the
identification of DNA as the material of heredity.
Interpret the findings of key experiments that contributed to this process.
In 1865, the Austrian monk Gregor Mendel
presented the results of his research on patterns of
inheritance in garden peas to the Natural Science
Society in Brunn, Austria. He proposed a number
of hypotheses that challenged much of the thinking
of his day about heredity. He argued, for example,
that the maternal and paternal gametes contributed
equally to the development of the offspring. He
also held that the information contributed by each
parent was not blended but rather passed on to the
offspring as discrete bits of information or “factors
of inheritance.” He went on to state that, while two
factors will exist for any one visible trait, one of
them (known as the recessive factor) might not
be expressed.
Mendel’s findings on the transmission of
hereditary information were not widely recognized
at the time. This was partly due to the strong
divisions that existed among scientific disciplines
then, which meant that the work of a botanist was
not likely to be noticed by zoologists or by medical
doctors. The apparently fixed nature of Mendelian
factors of inheritance also seemed to be at odds
with the newly emerging theory of evolution. Over
the next few decades, however, scientists began to
recognize the many similarities among cellular
processes in bacterial, plant, animal, and human
cells (including the processes you studied in Unit 1).
They also found that Mendel’s principles were
consistent with the idea that species change and
evolve over time. Today, Mendel’s work is
recognized as the foundation of modern genetics.
Only four years after Mendel’s presentation in
Brunn, and less than 300 km away, the young
Swiss physician and scientist Friedrich Miescher
isolated a substance he called “nuclein” from the
nuclei of white blood cells. Miescher, shown in
Figure 7.1, determined that nuclein was made up
of an acidic portion (which he termed “nucleic
acid”) and an alkaline portion (which was later
shown to be protein). Shortly thereafter Miescher
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MHR • Unit 3 Molecular Genetics
turned to the study of chemical properties of other
cellular structures. Almost a century passed before
scientists established the connection between the
nucleic acid isolated by Miescher and Mendel’s
factors of inheritance.
Figure 7.1 Friedrich Miescher was 25 years old when he
isolated nucleic acids from the nuclei of white blood cells
in 1869. He was working in a hospital treating wounded
soldiers, and he was able to collect white blood cells from
their bandages.
The Components of Nucleic Acids
Following the work of Miescher, Phoebus Levene
studied nucleic acid in more detail. During a career
that stretched from the early 1900s to the 1930s,
Levene isolated two types of nucleic acids that
could be distinguished by the different sugars
involved in their composition. One acid contained
the five-carbon sugar ribose, so Levene called it
“ribose nucleic acid” (ribonucleic acid or RNA).
The other acid contained a previously unknown
five-carbon sugar molecule. Since this sugar was
similar in structure to ribose but lacked one oxygen
molecule, Levene called it deoxyribose. He went on
to call the nucleic acid containing this sugar
“deoxyribose nucleic acid” (deoxyribonucleic acid
or DNA). Figure 7.2 shows the structures of ribose
and deoxyribose sugars. Levene is pictured in
Figure 7.3.
RNA
5′
HO CH2
4′
DNA
O
H
3′
H
2′
OH
5′
HO CH2
1′
4′
H
ribose
H
3′
OH OH
A
O
OH
H
2′
1′
of a five-carbon sugar, a phosphate group, and one
of four nitrogen-containing (nitrogenous) bases.
The bases found in DNA nucleotides are adenine
(A), guanine (G), cytosine (C), and thymine (T). In
RNA, the base uracil (U) is found instead of thymine.
The only difference between the nucleotides in
each nucleic acid is in their bases. As a result,
scientists studying nucleic acids soon began to
identify the nucleotides simply by their bases or,
more commonly, by their initials: A, G, C, T, and U.
H
phosphate
P
OH H
B
C
5′
O
deoxyribose
S
4′
Figure 7.2 The structure of (A) ribose, found in RNA, and
2′
3′
pentose sugar
(B) deoxyribose, found in DNA. In ribose, the 2′ carbon is
bonded to a hydroxyl group. In deoxyribose, this carbon
is bonded to a single hydrogen molecule.
Figure 7.3
Phoebus Levene
made some
important
discoveries about
the properties of
nucleic acids.
After distinguishing between DNA and RNA,
Levene went on to show that nucleic acids are
made up of long chains of individual units he
termed nucleotides. Both DNA and RNA contain
a combination of four different nucleotides. As
shown in Figure 7.4, each nucleotide is composed
O
O
H
During the period when Levene was conducting
his studies on nucleic acids, other experimenters
demonstrated that Mendel’s factors of inheritance
were associated with the nuclein substance first
isolated by Miescher. By that time, nuclein had
been shown to be made up of individual structures
known as chromosomes, strand-like complexes of
nucleic acids and protein tightly bound together.
Thus, the finding that the factors of inheritance
were associated with nuclein drew increased
attention to both the protein component and the
properties of nucleic acids.
nitrogencontaining
base
1′
H
O
CH3
H
N
N
N
H
uracil
H
O
N
H
H
thymine
Figure 7.4 The general structure of a nucleotide. In DNA,
the sugar is deoxyribose, and the nitrogenous base is one
of the following: adenine (A), guanine (G), cytosine (C), or
thymine (T). In RNA, the sugar is ribose and the nitrogenous
base uracil (U) appears instead of thymine.
While A, G, C, T, and U are the major bases
found in nucleic acids, there are also some minor
ones. These are usually slightly altered forms of the
major bases. In many cases the minor bases serve
as specific signals involved in programming or
protecting genetic information.
At this point, the results of Levene’s work led
him to conclude incorrectly that nucleic acids
contained equal amounts of each of these
nucleotides. Based on this finding, he suggested
that DNA and RNA were made up of long chains
in which the nucleotides appeared over and over
again in the same order; for example, ACTGACTG
ACTG and so on. This, in turn, caused most
scientists to conclude that DNA could not be the
material of heredity because it was not complex
enough to account for the tremendous variation in
inherited traits. It was generally accepted that DNA
could be a structural component of hereditary
material, but scientists thought the primary
instructions for inherited traits must lie in the
proteins that are also found in chromosomes.
Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR
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Several decades passed before Levene’s conclusion
was finally corrected.
Mounting Evidence for the
Role of DNA in Heredity
One important piece of evidence that DNA was, in
fact, the material of heredity came in 1944, when
the team of Oswald Avery, Colin MacLeod, and
Maclyn McCarty published the results of their
experiments with bacteria. These experiments,
which built on the 1928 work of British researcher
Fred Griffith, were conducted over a period of nearly
15 years. As illustrated in Figure 7.5, Griffith showed
that when a heat-killed, pathogenic (diseasecausing) strain of the bacterium Streptococcus
pneumoniae was added to a suspension containing
a non-pathogenic strain, the non-pathogenic strain
was somehow transformed to become pathogenic.
Avery and his colleagues undertook several
important steps to isolate the agent behind this
transformation, which Griffith had called the
transforming principle. When they treated a
suspension of the pathogenic bacteria with a
protein-destroying enzyme, they noticed that the
transformation of non-pathogenic bacteria into a
pathogenic strain still took place. When the
pathogenic bacteria were treated with a DNAdestroying enzyme, however, the transformation
did not take place. Finally, when the bacteria were
treated with an enzyme that destroyed RNA but
not DNA, the transformation occurred again. This
demonstrated that the substance responsible for
the transformation of the non-pathogenic bacteria
into a pathogenic strain was DNA.
Although the work of Avery, MacLeod, and
McCarty provided strong evidence for the role of
DNA in determining cell function, the results were
mice live
heat-killed
pathogenic strain
of S. pneumoniae
A When a heat-killed, pathogenic strain of
Streptococcus pneumoniae is injected
into mice, the mice live.
mice die
mixture of heat-killed
pathogenic and live
+
nonpathogenic strain
of S. pneumoniae
mice die
live pathogenic
strain of
S. pneumoniae
B When live, pathogenic S. pneumoniae
bacteria are injected into mice, the
mice die.
mice live
live nonpathogenic
strain of
S. pneumoniae
C When a live, non-pathogenic mutant
strain of the same S. pneumoniae
bacteria is injected into mice, the
mice live.
Figure 7.5 Griffith’s discovery of the “transforming
principle” in 1928 was accidental. He employed heat-killed,
pathogenic bacteria as a control in an experiment on
infection, but did not treat the cells at a high enough
temperature to denature their DNA. In so doing, he
discovered that the dead cells’ pathogenic properties
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D When heat-killed, pathogenic bacteria are
added to a suspension containing the live,
non-pathogenic strain of bacteria,
transformation occurs and the colony
of non-pathogenic bacteria become
pathogenic. When these bacteria are
injected into mice, the mice die.
MHR • Unit 3 Molecular Genetics
could be passed on to living bacterial cells. Griffith died of
injuries suffered in an air raid during World War II before he
could discover what caused this transformation. In 1944,
Avery and his team were the first to demonstrate that the
transforming principle was DNA.
not widely accepted. Many scientists who had
accepted Levene’s theory of the structure of nucleic
acids simply refused to believe that the apparently
simple, repetitive DNA molecule could play a key
role in heredity. Others maintained that while
DNA might be an agent of heredity in bacteria,
prokaryotes were not a reliable model for genetic
mechanisms in more complex organisms. It was not
until many years later that scientists determined
that the encoding of genetic information works in
very similar ways in all living cells.
During the same years that Avery and his team
were trying to pin down the identity of the
transforming principle, other experimental evidence
for the role of DNA in heredity began to accumulate.
One key discovery was that in any given species, the
quantity of DNA in somatic cells is both constant
and double the quantity of DNA in gametes. Since
at each mating two gametes come together to produce
a zygote with a full complement of hereditary
material, you would expect reproductive cells to
have only half as much hereditary material as the
cells of the body. However, it was found that the
amount of protein varies widely from the cells of
one tissue to another, and is not necessarily any
lower in reproductive cells.
In the late 1940s, Erwin Chargaff, shown in
Figure 7.6, revisited the results of Levene’s
experiments on the nucleotide composition of DNA.
A more careful study, made possible in part by
more advanced equipment, led Chargaff to overturn
one of Levene’s main conclusions. Chargaff argued
that the four nucleotides were not present in equal
quantities, but rather were found in varying but
characteristic proportions. Chargaff demonstrated
that although the nucleotide composition of DNA
varies from one species to another, DNA specimens
taken from different animals of the same species (or
from different tissues collected from one animal)
have the same nucleotide composition. He also
found that this base composition remains consistent
despite changes in the age of the specimen, its
physical state (including nutrition and health), or
its environment. Perhaps the most significant of
Chargaff’s findings was his discovery that, in any
sample of DNA, the amount of adenine present is
always equal to the amount of thymine, and the
amount of cytosine is always equal to the amount
of guanine. This constant relationship is known as
Chargaff’s rule.
Figure 7.6 Erwin Chargaff clearly refuted the theory that
DNA was made up of a single sequence of nucleotides
repeated over and over again. The possibility that DNA had
a more complex structure helped scientists accept that DNA
could play a role in heredity and development.
Further, and largely conclusive, evidence that
DNA and not protein is the genetic material
emerged in 1952. In an experiment, Alfred Hershey
and Martha Chase used radioactive labelling
techniques to follow the process of a virus known as
T2 infecting a bacterial host. The T2 virus, which
infects the bacteria Escherichia coli, is made up of
a protein coat housing a strand of DNA. As shown
in Figure 7.7, when the virus infects a bacterium,
it first attaches to the wall of the bacterium and
Figure 7.7 Looking somewhat like space capsules, these
T2 phages use leg-like structures to bind to the cell wall of
a bacterium.
Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR
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then uses a tail-like projection to inject its genetic
material into the bacterial cell. The genetic material
reprograms the cell, causing it to produce new
viruses. These new viruses accumulate within the
bacterial cell until the cell ruptures and releases
the viruses to infect nearby cells.
WEB LINK
www.mcgrawhill.ca/links/biology12
Griffith’s discovery of the transforming principle led some
researchers to propose that transformation could occur in
eukaryotes as well. What effect would this viewpoint have on
theories about inheritance and development? Use the Internet
to research the history of scientific thought from 1900 to 1950.
What other world events might have influenced how scientific
theories were applied during these years? Go to the web site
above, and click on Web Links. Write down some ideas to
discuss with your class.
Sample 1
Figure 7.8 illustrates the Hershey-Chase
experiment. The scientists knew that virtually all
of the phosphorus present in the T2 virus was in
its DNA, while sulfur was found only in its protein
coat. Consequently, they prepared two different
samples of the T2 virus, one tagged with radioactive
phosphorus and the other tagged with radioactive
sulfur. Each sample was then added to a separate
suspension of non-radioactive E. coli. After a
period of growth, the two resulting mixtures were
individually agitated in a blender to shake off the
part of the virus that remained attached to the
exterior cell wall of the bacterium after the cell was
infected. Finally, the infected bacteria were separated
from each mixture, leaving the viral protein coats
in suspension. The results were then analyzed.
In both cases, the bacteria became infected by
the phage. In the first sample, in which the viral
DNA was radioactive, the infected bacteria were
Sample 2
radioactive
phosphorus in
DNA
non-radioactive
DNA
non-radioactive
protein coat
radioactive
sulphur in
protein coat
A Two batches of phages are cultured. One has
radioactively tagged DNA, while the other has a
radioactively tagged protein coat. A sample of
each type of virus is added to a separate
suspension of non-radioactive E. coli.
B The viruses inject their DNA into the bacteria.
“ghosts” sheared
off by blender
C Each suspension is shaken in a blender to
separate the virus heads or “ghosts” from the
outside of the cell walls of the infected bacteria.
“ghosts” and bacteria
separated by centrifuge
radioactive
non-radioactive
non-radioactive
radioactive
D The bacterial cells infected by the virus with
radioactive DNA are found to be radioactive,
indicating that the viral DNA entered the host
cell. In contrast, the bacterial cells infected by
the virus with radioactive protein are found to be
non-radioactive, indicating that no viral protein
entered the host cell.
Figure 7.8 The experiments conducted by Hershey and Chase demonstrated that
when a virus infects a bacterium, only the DNA of the virus enters the host cell.
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MHR • Unit 3 Molecular Genetics
found to be radioactive while the fluid containing
the separated viral protein coats was not. In the
second sample, in which only the virus protein
coat was labelled, the reverse occurred — the fluid
containing the protein coats was radioactive while
the infected bacteria were not. Hershey and Chase
concluded that only the DNA from the virus
entered the bacterial cell; the protein coat remained
outside the cell wall. Therefore, the transmission of
genetic information from the virus to the metabolic
machinery of the bacterium could only take place
as a result of the injection of DNA into the bacterium.
Through the 1940s and into the early 1950s,
convincing evidence mounted to support the
SECTION
REVIEW
1.
K/U What is the relationship between nuclein and a
chromosome?
2.
K/U Identify the five different nucleotides. Which one
is only found in RNA?
3.
C Draw the general structure of a DNA nucleotide
and label each of its components.
4.
MC Mendel’s findings involved plants, which is
perhaps why biologists whose investigations lay in
different fields largely ignored them for some four
decades after Mendel presented his findings. With a
partner or in a small group, discuss other factors that
might have contributed to the relative obscurity of
Mendel’s discoveries during this period. If you had
had the opportunity to work with Mendel, what
advice would you have given him to help ensure that
his discoveries received more timely recognition?
5.
central role of Miescher’s nucleic acids — and
specifically DNA — in the mechanisms of heredity.
Scientists from a variety of fields began to devote
more and more attention to the problem of
determining the structure of DNA. The race that
ensued crossed the boundaries of scientific
disciplines and became swept up in politics, social
values, and debates on ethics, as individuals and
teams from different nations competed with one
another. The race finally ended in 1953 with the
publication of a landmark paper describing the
molecular structure of DNA. You will study this
structure in more detail in the next section.
Define Chargaff’s rule and explain its
significance.
8.
I You are given an enzyme that replaces the 2′
hydroxyl group of a sugar molecule with a methyl
(−CH3) group. The enzyme has no other effect. If you
treat a suspension of heat-killed, pathogenic bacteria
with this enzyme and then add a culture of live,
non-pathogenic bacteria, will transformation occur?
Explain your reasoning.
9.
Several researchers besides Mendel made
outstanding contributions that eventually helped to
pinpoint DNA as the molecule of heredity. Develop a
flowchart that summarizes the work of the following
scientists and shows how their discoveries
contributed to the discoveries that followed.
(a) Miescher
(b) Levene
(c) Griffith
(d) Avery, MacLeod, and McCarty
K/U
6.
K/U Explain why researchers believed for many
years that DNA was too simple a molecule to serve
as the material of heredity. Whose research
conclusion lent support to this belief?
7.
Hershey and Chase used radioactive isotopes of
phosphorous and sulfur in their experiments to isolate
the factor responsible for the transmission of genetic
information. Design an experiment that would show
the results they could have expected if they had used
radioactive carbon and nitrogen, respectively, in the
place of the radioactive phosphorus and sulfur. Use
diagrams to illustrate the results of tests on both the
virus ghosts and the infected bacterial cells.
I
C
10.
MC You have learned about several milestones in the
development of the science of genetics. In your
opinion, what technologies or cultural issues might
have influenced the timing of these milestones and
other discoveries in genetics?
11.
Historians debate the degree to which key
people actually change the course of history. Do you
think the individual scientists discussed in this section
influenced the actual progress of knowledge? Why
or why not?
MC
Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR
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