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It should be pointed out that the 
likelihood of such a mechanism caus- 
ing Inheritable changes in the offspr- 
ing of complex organisms is extremely 
low because of the protection afforded 
germ-line cells (eggs and sperm) by 
their location. Thus, it is highly Im- 
probable or perhaps Impossible for re- 
combined foreign DNA to reach germ- 
line cells at a time in their life when 
secondary recombination can occur. 
With one-celled organisms, plants, or 
simple multicellular organisms, the 
probability of heritable change result- 
ing from secondary recombination is 
higher. 
What Is the probability of secondary 
recombination between prokaryotes 
and eukaryotes In nature? It is gener- 
ally held that the recombination In 
nature is more likely if similar or iden- 
tical sequences of bases (rungs In the 
DNA ladder) occur In the two recom- 
bining DNA's.<7) The greater the 
degree of similar sequences, the more 
likely Is recombination. In general, the 
more closely two species are related, 
the more likely It is that similar se- 
quences will be found In their DNA’s. 
Thus. DNA from primates has more 
DNA sequences in common with 
human DNA than does DNA from 
mice, or fish, or plants. Recombination 
may also occur between DNA's not 
sharing sequences, but at lower fre- 
quencies. 
It Is possible that the capacity for in- 
terspecles recombination between dis- 
tantly related species exists in nature. 
For example, bacteria in animal Intes- 
tines are constantly exposed to frag- 
ments of animal DNA released from 
dead intestinal cells. Significant re- 
combination. however, would require 
the uptake of Intact segments of 
animal DNA and their subsequent in- 
corporation Into the bacterial DNA. 
Such uptake and incorporation has 
been demonstrated experimentally. 
The frequency of such events in 
nature is unknown. 
Similarly, there are very few data 
permitting assessment of the reverse 
process: namely, the incorporation of 
bacterial DNA into the cells, or DNA. 
of more complex organisms. Although 
there are reports of experiments In 
which bacterial DNA was inserted into 
animal and plant species and produc- 
tion of the bacterial protein followed, 
the process Is very inefficient and 
many investigators have been unable 
to repeat these experiments.* 8-10) 
There are certain well-documented 
Instances in which the DNA's of dif- 
ferent living things become more or 
less permanently recombined In 
nature. These instances Involve recom- 
bination between the DNA's of nonch- 
romosomal genes, such as those of vir- 
uses or plasmids, or between the 
DNA's of viruses or plasmids and chro- 
mosomal genes. The former instance. 
for example, is the mechanism behind 
the rapid spread of resistance to anti- 
biotics among different bacterial spe- 
cies.^, 11) This spread accompanied 
the prevalent use of antibiotics in 
medicine and agriculture. Another ex- 
ample is the insertion of DNA from 
the bacterium Agrobacterium tume/a- 
ciens into plant cells.(JJ) 
Expected Benefits of Recombinant 
DNA Research 
Benefits may be divided into two 
broad categories: an increased under- 
standing of basic biological processes, 
and practical applications for medi- 
cine. agriculture, and Industry. 
At this time, most of the practical 
applications are speculative. It is im- 
portant to stress that the most signifi- 
cant results of this work, as with any 
truly innovative endeavor, are likely to 
arise in unexpected ways and will 
almost certainly not follow a predict- 
able path. 
Increased understanding of basic bio- 
logical processes 
There are many important funda- 
mental biomedical questions that can 
be answered or approached by DNA 
recombinant research. In order to ad- 
vance against inheritable diseases, we 
need to understand the structure of 
genes and how they work. The DNA 
recombinant methodology provides a 
simple and inexpensive way to prepare 
large quantities of specific genetic in- 
formation in pure form. This should 
permit elucidation of the organization 
and function of the genetic Informa- 
tion in higher organisms. For example, 
current estimates of the fraction of 
this information that codes for pro- 
teins are simply educated guesses. 
There are almost no clues about the 
function of the portions of DNA that 
do not code for proteins, although 
these DNA sequences are suspected of 
being Involved in the regulation of 
gene expression. 
The existing state of ignorance is 
largely attributable to our previous in- 
ability to Isolate discrete segments of 
the DNA in a form that permits de- 
tailed molecular analysis. Recombin- 
ant DNA methodology removes this 
barrier. Furthermore, ancillary tech- 
niques have been developed whereby 
pure DNA segments that contain par- 
ticular sequences of Interest can be 
identified and selected. Of particular 
interest Is the isolation of pure DNA 
segments that contain the genes for 
the variable and constant portions of 
the immunoglobin proteins. The anal- 
yses of such segments obtained from 
both germ-line and somatic cells 
should be of inestimable value In de- 
termining the mechanism of immuno- 
logic diversity. 
A major problem in understanding 
the mechanism by which certain vir- 
uses cause cancer is how and where 
the infecting or endogenous viral gen- 
omes are integrated into the cell's 
chromosome. (12) This bears on the 
question of how the integrated viral 
genes affect cellular regulation, thus 
leading to the abnormal g^ovth char- 
acteristics of cancer cells. With the re- 
combinant DNA techniques for isola- 
tion and purification of specific genes, 
this research problem is reduced to 
manageable proportions. It is possible 
to isolate the desired DNA segment in 
pure form. Large quantities can be ob- 
tained for detailed study by simply ex- 
tracting a culture of the bacteria car- 
rying the viral DNA segment in a plas- 
mid. 
Important recent achievements in re- 
combinant DNA research 
It was anticipated (see F.IS of Octo- 
ber 1977) that the ability to excise, iso- 
late. and amplify specific segments of 
DNA from higher organisms would 
provide an unprecedented opportunity 
to study the structure of eukaryotic 
genomes and to correlate the results 
with concepts of how they evolved and 
are regulated. Recent work has yielded 
much more. Indeed, some of the geno- 
mic structures discovered through use 
of recombinant DNA techniques have 
occasioned a substantial reassessment 
of several major paradigms of molecu- 
lar biology. 
Many of the initial studies using re- 
combinant DNA techniques focused on 
DNA sequences that are repeated in 
the genomes of eukaryotes. In some 
instances, these repeated sequences 
specify an RNA product, such as ribo- 
somal RNA. or fulfill a function as yet 
unknowm— for example, the sequences 
called "satellite'' DNA. The organiza- 
tion of such sequences is being exam- 
ined extensively with recombinant 
DNA techniques* 14-20). 
The genes that specify ribosomal 
RNA are repeated in the eukaryotic 
genome several hundredfold. It has 
been known for some years that in a 
variety of species, such as the frog 
Xenopus laevis, these genes are ar- 
ranged as a series of tandem repeats. 
Each set of ribosomal genes is separat- 
ed from its neighbors by regions of 
DNA. of varying length, that are not 
transcribed. Cloning of several of 
these nontranscribed, or "spacer." re- 
gions has allowed analysis of the 
manner in which they are related to 
one another* 21, 22) and proposals of 
evolutionary mechanisms by which 
they may have arisen. 
Moreover, the availability of these 
cloned DNA sequences has made it 
possible* 23) to localize the DNA site at 
which the transcription of the riboso- 
mal genes Is initiated. The exact DNA 
sequence at this site is being deter- 
mined. Such information will further 
the understanding of the mechanisms 
FEDERAL REGISTER, VOL 43, NO. 146— FRIDAY, JULY 2S, 197S 
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