42 • Impacts of Applied Genetics— Micro-Organisms, Plants, and Animals 
Fiqure 15.— An Example of How the Recombinant DNA Technique May Be Used 
^ To Insert New Genes Into Bacterial Cells 
The first part of the technique invoives the manipuia- 
tions necessary to isoiate and reconstruct the desired 
gene from the donor: 
a) The RNA that carries the message (mRNA) for the 
desired protein product is isoiated. 
b) The doubie-stranded DNA is reconstructed from the 
mRNA. 
c) In the finai step of this sequence, the enzyme ter- 
minai transferase acts to extend the ends of the 
DNA strands with short sequences of identical 
bases (in this case four guanines). 
a) 
b) 
I 
Messenger RNA 
from animal cell 
Double-strand DNA 
Enzymatic 
reconstruction 
c) 
Terminai 
transferase 
GGG(5i 
III. The final product, a bacterial plasmid containing the 
new gone, is obtained. This piasmid can then be in- 
serted into a bacterium where it can be repiicated and 
produce the desired protein product: 
a) The gene obtained in part I and the plasmid DNA 
from part II are mixed together and anneal 
because of the complementary base-pairing be- 
tween them. 
b) Bacterial enzymes fill in any gaps in the circle, 
sealing the connection between the plasmid DNA 
and the inserted DNA to generate an intact cir- 
cular plasmid now containing a new gene. 
II. A bacterial plasmid, which is a small piece of circular 
DNA, serves as the vehicle for introducing the new gene 
(obtained in part I above) into the bacterium: 
a) The circular plasmid is cleaved by the appropriate 
restriction enzyme. 
b) The enzyme terminal transferase extends the DNA 
strands of the broken circle with identical bases 
(four cytosines in this case, to allow complemen- 
tary base pairing with the guanines added to the 
gene obtained in part I). 
II 
Bacterial plasmid DNA 
Uptake by cell; 
repair by 
SOURCE: Office of Technology Assessment. 
