14 
Figure IV-3 
Host Cell 
The cell represented at the upper left contains chromosomal DNA 
and several separately replicating DNA molecules. The nonchromosomal 
DNA molecules can be isolated from the cell and manipulated to serve 
as vectors (carriers) for DNA from a foreign cell. Most DNA molecules 
used as vectors are circular. They can be cleaved, as shown, by 
enzymes (restriction endonucleases) to yield linear molecules with 
rejoinable ends. 
At the upper right is another cell, represented here as a rectangle. 
It serves as the source of the foreign DNA to be inserted into the vector. 
This DNA can also be cleaved by enzymes. The rectangular cell could 
be derived from any living species, and the foreign DNA might contain 
chromosomal or nonchromosomal DNA, or both. 
In the next steps, the foreign DNA fragment is mixed and combined 
with the vector DNA, and the recombinant DNA is reinserted into a 
host cell. In most experiments this host cell will be of the same 
species as the source of the vector. The recipient cells are then placed 
under conditions where they grow and multiply by division. Each new 
cell will contain recombinant DNA. 
