21 
then take whatever gene you want to purify, mix it with that plasmid, 
and cause the re-association of these two structures to reform circular 
molecules. It is only necessary then to re-introduce these into appropri- 
ate bacterial cells in order to create or propagate this new chromosome, 
and specifically the segment which was derived from the complex chromosome 
which is now in a simple structure which we can isolate and prepare in 
large quantities. 
Another attractive and potentially very useful approach to the iso- 
lation and amplification of genes has been pioneered by Noreen and Kenneth 
Murray in Edinburgh, and by Ron Davis at Stanford. Using the same method- 
ology that I have just described, they can incorporate DNA segments into 
the chromosome of a small bacterial virus. In this way the new genes be- 
have and multiply as if they have an integral part of the viral chromosome. 
Cloning or obtaining a pure preparation of a single chromosomal DNA 
segment is not restricted to bacterial vectors and cells. Some animal vi- 
ruses are analogous to bacterial plasmids in their structure, and because 
they can multiply in cultured animal cells, they can in fact be used. 
They can be joined to foreign DNA segments, and the hybrids can be propa- 
gated in animal or human cells. 
Now, why do biochemists want to do these kinds of experiments? There 
are many important, fundamental, and practical questions that could be an- 
swered or approached by this type of research. We need to understand the 
structure of genes and how they work, and this methodology provides us with 
a simple, inexpensive, and elegant way to prepare large quantities of spe- 
cific genetic bits of information in pure form. There is little doubt in 
my mind that our inability to achieve this earlier is what slowed down pro- 
gress in understanding the genetic chemistry of cells of higher organisms. 
In the short space of only a year, Hogness and his colleagues at Stan- 
ford, as well as Don Brown here in Baltimore, have shown how powerful a 
tool the recombinant DNA methodology can be for the isolation of genes and 
the analysis of chromosome structure. The precise chromosomal locations of 
discrete segments of DNA from the fruit fly Drosophila have been mapped, 
and that information promises new and important insight into the perplexing 
problem of how genes are organized into complex chromosomes. 
Understanding how the genes of higher organisms are expressed and reg- 
ulated, which is a pious dream we had a few years ago, is literally now 
within our grasp. 
So far I have emphasized only the ability to clone and amplify foreign 
genes in bacteria. But there is another and even more promising dimension. 
If the genes, for example, of toads, flies, or humans function in a simple 
bacterium, then we can certainly begin to study the expression and function 
f 
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