Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 223found 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
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.SECTION REVIEW- What is the relationship between nuclein and a
chromosome? - Identify the five different nucleotides. Which one
is only found in RNA? - Draw the general structure of a DNA nucleotide
and label each of its components. - 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? - Define Chargaff’s rule and explain its
significance. - 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? - 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.
8. You are given an enzyme that replaces the 2 ′
hydroxyl group of a sugar molecule with a methyl
(−CH 3 )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 - 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? - 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?
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