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As far as is known, in order for recombination to occur naturally, 
the two DNA molecules must be within the same cell. Plasmids and 
viruses can be transferred from cell to cell by natural mechanisms 
that have been studied in some detail. Transfer of plasmids from 
one cell to another requires the physical contact of the two cells. 
Viruses, on the other hand, may be transferred without cell contact; 
their genetic material is protected from the effects of the environment 
by a coat of proteins. In addition, it is possible under some circum- 
stances for DNA segments, free in a solution, to enter a cell and 
undergo recombination therein. This process has been called 
transformation (6). Thus far, transformation of this type has only 
been detected in a small number of species of bacteria; it has not 
been observed with more complex organisms. 
In complex organisms that reproduce sexually, each offspring 
receives DNA from its parents. For example, every human being 
receives half of his complement of DNA from a sperm cell and half 
from an egg cell. In each generation the human genetic material 
is recombined. 
Recombination in the laboratory . In the recombinant DNA exper- 
iments that are the subject of this Statement and the accompanying 
Guidelines, DNA is first isolated from two different cell types. Each 
DNA is then broken into segments. Each segment may contain one or 
more genes, or it may contain a portion of the DNA that lacks functional 
genes. The breaking can be accomplished by several means, including 
bacterial enzymes (e.g., restriction endonucleases), which cut the 
DNA in such a way that the chemical structure at the ends of the 
segments permits interchangeable rejoining when the two different 
DNAs are mixed. Alternatively, the necessary structures at the ends 
of the DNA segments may be introduced after other types of breakage. 
In this way single DNA molecules containing portions of the two different 
DNAs are constructed. 
The DNA recombined in these experiments can be derived from 
widely divergent sources. DNA from one of the sources may serve 
as a carrier, or vector, for the insertion of the recombined DNA into 
a cell, or host. The vector may be a plasmid, usually derived from 
the same species as will serve as the host, or a virus. The DNA 
to be inserted is called the "foreign" DNA. When a large mixture of 
DNA fragments from the foreign source is used in the joining, the 
experiment is referred to as a "shotgun" experiment. In other instances, 
a particular DNA fragment of interest will be purified and then joined 
to the vector. 
