Page 25 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

184 Chapter 6 DNA Structure, Replication, and Recombination

Figure 6.4 Transformation. (a) S bacteria are virulent and can cause lethal infections when injected into mice. (b) Injections of R
mutants by themselves do not cause infections that kill mice. (c) Similarly, injections of heat-killed S bacteria do not cause lethal infections.
(d) Lethal infection does result, however, from injections of live R bacteria mixed with heat-killed S strains; the blood of the dead host
mouse contains living S-type bacteria.

(a)

S Inject Dead

(b)

Mutates to

S R Inject Alive
(c)

Cell
S Heat-killed components Inject Alive

(d)

Cell
S Heat-killed components Combined Tissue analyzed

Inject Dead

Living S
R recovered

medium in the presence of components from dead S forms Once purified, the transforming principle had to be
(Fig. 6.5a). Avery then embarked on a quest that would characterized. In 1944, Avery and two coworkers, Colin
remain the focus of his work for almost 15 years: “Try to MacLeod and Maclyn McCarty, published the cumulative
find in that complex mixture, the active principle!” In other findings of experiments designed to determine the transform-
words, try to identify the heritable substance in the bacte- ing principle’s chemical composition (Fig. 6.5c). In these
rial extract that induces the transformation of harmless experiments, the purified transforming principle was active
R bacteria into pathogenic S bacteria. Avery dubbed the at the extraordinarily high dilution of 1 part in 600 million.
substance he was searching for the transforming principle Although the preparation was almost pure DNA, the inves-
and spent many years trying to purify it sufficiently to be tigators nevertheless exposed it to various enzymes to see
able to identify it unambiguously. He and his coworkers if some molecule other than DNA might have caused the
eventually prepared a tangible, active transforming principle. transformation. Enzymes that degraded RNA, protein, or
In the final part of their procedure, a long, whitish wisp polysaccharide had no effect on the transforming principle,
materialized from ice-cold alcohol solution and wound but an enzyme that degrades DNA completely destroyed its
around the glass stirring rod to form a fibrous wad of nearly activity. The tentative published conclusion was that the
pure transforming principle (Fig. 6.5b). transforming principle appeared to be DNA. In a personal