40 • Impacts of Applied Genetics — Micro-Organisms, Plants, and Animals 
projections. The ability to form projections and 
donate genes to neighbors is a genetically con- 
trolled trait. The genes controlling this trait, 
however, are not located on the bacterial chro- 
mosomes. Instead, they are located on separate 
genetic elements called plasmids— relatively 
small molecules of double-stranded DNA, ar- 
ranged as closed circles and existing autono- 
mously within the bacterial cytoplasm. (See 
figure 13.) 
Plasmids and phages are two vehicles— or 
vectors— for carrying genes into bacteria. As 
such, they became tools of genetic engineering; 
for if a specifically selected DNA could he intro- 
duced into these vectors, it would then he pos- 
sible to transfer into bacteria the hluepi'ints for 
proteins— the building blocks of genetic charac- 
teristics. 
But bacteria had been confronting the inva- 
sion of foreign DNA for millennia, and they had 
evolved protective mechanisms that preserved 
their own DNA while destroying the DNA that 
did not belong. Bacteria survive by producing 
restriction enzymes. These cut DNA molecules 
in places where specific sequences of nucleo- 
tides occur— snipping the foreign DNA, yet leav- 
ing the bacteria’s own genetic complement 
alone. The first restriction enzyme that was iso- 
lated, for instance, would cut DNA only when it 
located the sequence: 
G-A-A-r-r-c 
C-T-T-A-A-G 
If the sequence occurred once in a circular plas- 
mid, the effect would simply he to open the 
circle. If the sequence were repeated se\(M’al 
times along a length of DNA, the DN.\ would he 
chopped into se\ eral small pieces. 
By the late 1970's, scores of different i’(\stric- 
tion enzymes had been isolated fi’om a \ ai'iety 
of bacteria, with each enzyme ha\ ing a uni(|ue 
specificity for one specific nucleotide se(|uence. 
These enzymes were another key to g(‘netic en- 
gineering: they not only allow cul plasmids to he 
opened up so that new DNA could he* in.serti'd, 
hut offered a way of obtaining manageahU* 
pieces of new' DNA as w(dl, (See figui'e 14.) 
Using restriction enzynu\s, almost any DNA 
molecule could he snipped, shapiul, and 
trimmed with |)recision. 
Cloning DNA— that is obtaining a large (|uanti- 
ty of exact copies of any chosen DNA molecule 
by inserting it into a host bacterium- became 
technically almost simpU;. The |)iece in {|ue.stion 
was merely snipped from th(' oi’iginal molecule, 
inserted into the \ ector DN,\, and pro\ ided w ilh 
Figure 13.— Conjugation: The Transfer of Genetic Material in Bacteria by Mating 
In conjugation, a plasmid inhabiting a bacterium can transfer the bacterial chromosome to a second cell where homologous 
segments of DNA can recombine, thus exchanging gene B from the first bacterium for gene b from the second. 
SOURCE: Office of Technology Assessment. 
