Essential Cell Biology 5th edition

HOW WE KNOW
GENES ARE MADE OF DNA
193
By the 1920s, scientists generally agreed that genes
reside on chromosomes. And studies in the late nineteenth
century had demonstrated that chromosomes are
composed of both DNA and proteins. But because DNA
is so chemically simple, biologists naturally assumed
that genes had to be made of proteins, which are much
more chemically diverse than DNA molecules. Even
when the experimental evidence suggested otherwise,
this assumption proved hard to shake.
Messages from the dead
The case for DNA began to emerge in the late 1920s,
when a British medical officer named Fred Griffith made
an astonishing discovery. He was studying Streptococcus
pneumoniae (pneumococcus), a bacterium that causes
pneumonia. As antibiotics had not yet been discovered,
infection with this organism was usually fatal. When
grown in the laboratory, pneumococci come in two
living S strain of
S. pneumoniae
mouse dies
of infection
living R strain of
S. pneumoniae
mouse lives
S strain of
S. pneumoniae
living R strain
S strain of
S. pneumoniae
heat-killed
heat-killed
mouse dies
of infection
mouse lives
living, pathogenic
S strain recovered
Figure 5–30 Griffith showed that
heat-killed infectious bacteria can
transform harmless live bacteria
into pathogens. The bacterium
Streptococcus pneumoniae comes in
two forms that differ in their microscopic
appearance and in their ability to cause
disease. Cells of the pathogenic strain,
which are lethal when injected into
mice, are encased in a slimy, glistening
polysaccharide capsule. When grown
on a plate of nutrients in the laboratory,
this disease-causing bacterium forms
colonies that look dome-shaped and
smooth; hence it is designated the
S form. The harmless strain of the
pneumococcus, on the other hand, lacks
this protective coat; it forms colonies
that appear flat and rough—hence, it is
referred to as the R form. As illustrated
in this diagram, Griffith found that a
substance present in the pathogenic
S strain could permanently change, or
transform, the nonlethal R strain into the
deadly S strain.