34 • Impacts of Applied Genetics— Micro-Organisms, Plants, and Animals 
teria gave rise only to bare progeny, while those 
with capsules produced only encapsulated 
forms. Only the encapsulated forms of both 
types II and III could cause disease; bare bac- 
teria were benign. (See figure 7a.) But when 
Griffith took some encapsulated type III bacteria 
that had been killed and rendered harmless and 
mixed them with bare bacteria of type II, the 
presumably safe mixture became virulent: Mice 
injected with it died of a massive pneumonia in- 
fection. Bacteria recovered from these animals 
were found to be of type II— the only living bac- 
teria the mice had received— now wrapped in 
type III capsules. (See figure 7b.) 
Avery’s group recognized Griffith’s finding as 
a genetic phenomenon; the dead type III bacte- 
ria must have delivered the gene for making 
capsules into the genetic complement of the 
living type II recipients. By meticulous research, 
Avery’s group found that the substance which 
caused the genetic transformation was DNA. 
It had been in 1868, just 3 years after Mendel 
had published his findings, that DNA was dis- 
covered by Friedrich Miescher. It is an extreme- 
ly simple molecule composed of a small sugar 
molecule, a phosphate group (a phosphorous 
atom surrounded by four oxygen atoms), and 
four kinds of simple organic chemicals known 
as nitrogenous (nitrogen-containing) bases. To- 
gether, one sugar, one phosphate, and one base 
form a nucleotide— the basic structural unit of 
the large DNA molecule. Because it is so simple, 
DNA had appeared to be little more than a 
monotonous conglomeration of simple nucleo- 
tides to scientists in the early 20th century. It 
seemed unlikely that such a prosaic molecule 
could direct the appearance of genetic traits 
while faithfully reproducing itself so that in- 
formation could be transferred between gen- 
erations. Although Avery’s results seemed clear 
enough, many were reluctant to accept them. 
Those doubts were finally laid to rest in a 
brief report published in 1953 by James Watson 
and Francis Crick. By using X-ray crystallo- 
graphic techniques and building complex mod- 
els— and without ever having actually seen the 
molecule itself— Watson and Crick reported that 
they had discovered a consistent scientifically 
sound structure for DNA. 
Figure 7.— The Griffith Experiment 
7a. There are two types of pneumococcus, each of which 
can exist in two forms: 
Type II Type III 
y\ 
®ii *^iii ®ni 
where R represents the rough, nonencapsulated, benign 
form; and 
S represents the smooth, encapsulated, virulent 
form. 
7b. The experiment consists of four steps: 
Virulent strain (1) 
Mice injected with the virulent Sm die. 
Living 
Nonvirulent 
strain 
( 2 ) 
Mice injected with nonvirulent Rn do not become infected. 
S|ii 
Virulent 
strain, 
heat-killed 
Living 
The virulent Sm is heat-killed. Mice injected with it do not 
die. 
When mice are injected with the nonvirulent R, and thp 
heat-killed Sm, they die. Type II bacteria wrapped m type III 
capsules are recovered from these mice 
SOURCE: Office of Technology Assessment 
