Ch. 2— Introduction • 41 
Figure 14.— Recombinant DNA: The Technique of 
Recombining Genes From One Species 
With Those From Another 
amount of DNA protein 
Restriction enzymes recognize certain sites along the DNA 
and can chemically cut the DNA at those sites. This makes 
it possible to remove selected genes from donor DNA mole- 
cules and insert them into plasmid DNA molecules to form 
the recombinant DNA. This recombinant DNA can then be 
cloned in its bacterial host and large amounts of a desired 
protein can be produced. 
SOURCE; Office of Technology Assessment. 
a bacterial host as a suitable en\ ironnient for 
replication. The desired piece of D\,A could be 
recombined \\ ith a plasmid \ ector, a procedure 
that ga\ e rise to recombinant D.\.A (rDX.A), also 
known as gene splicing. Since bacteria can be 
grown in \ast quantities, this process could 
result in large-scale production of otherwise 
scarce and e.\pensi\ e proteins. 
.Although placing genes inside of bacteria is 
now a relati\ ely straightforward procedure, ob- 
taining precisely the right gene can be difficult. 
Three techniques are currently ax ailable: 
• Ribonucleic acid— R\A— is the \ehicle 
through which the message of D\A is read 
and transcribed to form proteins. The Ri\A 
that carries the message for the desired 
protein is first isolated. An enzyme, called 
‘reverse transcriptase/ is then added to the 
RNA. The enzyme triggers the formation of 
D\.A— rex ersing the normal process of pro- 
tein production. The DNA is then inserted 
into an ap|)ro[)riate \eclor. This was the 
procedure used to obtain the gene for hu- 
man insulin in 1979. (See figure 15.) 
• The gene can also he synthesized, or 
created, directly, since the nucleotide se- 
(|uence of the gene can he deduced from 
the amino acid seciuence of its protein 
product. This procedure has worketl well 
foi' small protein.s— like the growth regu- 
latory hormone somatostatin— which ha\e 
relatixely short stretches of DNA coding. 
Rut somatostatin is a tiny protein, only 14 
amino acids long. With three nucleotides 
coding for each amino acid, scientists had 
to synthesize a DNA chain 42 nucleotides 
long to [)i'otluce the coni|)lete hormone. For 
larger proteins, the gene-synthesis ap- 
|)i'oach rapidly becomes highly impractical. 
• The third method is also the most con- 
troversial. In this "shotgun” approach, the 
entire genetic complement of a cell is 
chopped up by restriction enzymes. Each 
of the DNA fragments is attached next to 
vectors and transferred into a bacterium; 
the bacteria are then screened to find those 
making the desired product. Screening 
thousands of bacterial cultures was part of 
the technique that enabled the isolation of 
the human interferon gene.* 
At present, these techniques of recombina- 
tion work mainly with simple micro-organisms. 
Scientists have only recently learned how to in- 
troduce novel genetic material into cells of 
higher plants and animals. These higher cells 
are being ‘engineered’ in totally different ways, 
by grow ing plant or animal cells in ‘tissue cul- 
ture’ systems, in vitro. 
Tissue culture systems work with isolated 
cells, with entire pieces of tissue, and to a far 
more limited extent, with whole organs or ev en 
early embryos. The techniques make it possible 
to manipulate cells experimentally and under 
controlled conditions. Several techniques are 
available. For example, in one set of experi- 
ments, complete plants have been grown from 
single cells— a breakthrough that may permit 
'Strictly speaking, R\A was transcribed using the shotgun 
approach into DNA, which was then cloned into bacteria and 
screened. 
