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With the recombinant DNA method, discrete DNA segments can be 
isolated in pure form and virtually unlimited quantities from the 
hopelessly complex mixtures obtained from cells. Molecular cloning 
provides the means to reconstruct extended regions of chromosomes, 
and eventually the entire genome, of any organism from the bits and 
pieces of its DNA. This opens the way for analysis of the detailed 
molecular structure, e.g. the subunit sequence, of individual genes 
and extended regions of mammalian chromosomes. 
Progress During the Last 3 Years. Recombinant DNA techniques 
have proved far superior to traditional means for exploring genetic 
systems. Through use of the new approaches, impressive analyses 
have already been made of chromosomal segments from simple higher 
organisms, such as a fruit fly ( Drosophila melanogaster ), a toad 
( Xenopus laevis ), and sea urchins. Cloned (pure) segments of virtually 
any chromosome can be obtained. 
DNA fragments can be joined to a vector DNA molecule. The vector 
DNA, a plasmid (or bacterial virus chromosome), and a segment of 
foreign DNA are joined in the test tube and then introduced into a 
special strain of E_. coli K-12. Conditions can be selected so that 
only those bacteria or viruses that acquire the recombinant DNA will 
grow or be detected. Because no more than a single recombinant DNA 
molecule is taken up by the bacterial cell, all the descendants of 
that transformed bacterium will contain only one particular foreign DNA 
segment. 
There are a number of different ways to identify cloned mammalian 
DNA segments and thereby select the clone of interest in a particular 
experiment. Whatever way they are detected, the size and chemical 
features of each DNA segment can be analyzed and cataloged. 
But can the cloned DNA segments be mapped to chromosomal 
locations ? This has already been accomplished with the DNA of one 
multicellular organism. Several cloned Drosophila DNA segments have 
been mapped to specific chromosomal sites. In one instance the data 
show that the isolated DNA segment occurs at many regions on all 
four chromosomes. Another cloned DNA segment is found, using the 
same technique, to occur at only one site on a particular chromosome. 
Structural analysis of the chromosomes of simple animals has 
already transformed our views about the organization of their genes. 
We know that some genes occur as tandemly repeated, multigene 
families. And we have learned that some genes contain an insertion 
of extraneous DNA. The significance of the extraneous insertion is 
still unclear, but implications for control of gene expression are 
tantalizing. 
