50 
The guidelines define purified as meaning that more than 99 percent 
of the purified DNA by weight represents the desired fragment, and further 
they require evidence that no harmful genes are present. Under such cir- 
cumstances the investigator may lower the containment conditions from those 
recommended for shotgun experiments with DNA of the same source, either by 
one step in physical containment or by one step in biological containment. 
In these experiments where the foreign DNA is purified before making 
the recombinant, all of the offspring of the recipient cell will ideally 
contain only a single kind of recombinant molecule, one containing the 
original purified fragment of DNA. It is distinguished from the case of 
shotgun experiments where the foreign DNA was a mixture of many fragments, 
as shown on the next slide (12). 
Slide 12 
"Shotgun" Experiment 
a ,b,c.e.c=Q Q 0 
Clone 
During the recombination in the shotgun experiment, each of the frag- 
ments, listed here as A, B, C, et cetera, can be combined separately with 
molecules of a vector and a large number of different recombinants indicated 
here are generated. Each recombinant DNA molecule may then enter a differ- 
ent individual cell in the populaton of host cells used as recipients. When 
each of these divides, each of its offspring will, like itself, contain the 
recombinant that was originally received by that parent, so all of the off- 
spring of this cell will contain recombinant A, all of the offspring of 
this, recombinant B, and so forth. 
There are a variety of techniques available for isolating from such a 
mixture only those cells which contain a single desired recombinant, and 
these procedures are referred to as cloning, and the isolated population a 
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