Biology 12

Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 221

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 7.6Erwin 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 7.7Looking somewhat like space capsules, these

T2 phages use leg-like structures to bind to the cell wall of

a bacterium.