Biology Reference
In-Depth Information
Figure 5.2 The structure of the cloning plasmid pBR322 showing the unique sites where restriction
endonucleases can cleave the DNA. The thin arrows inside the circle indicate the direction of tran-
scription of the ampicillin and tetracycline resistance genes, which serve as selectable markers. The
thick arrow shows the direction of DNA replication.
This gives the genetic engineer options as to which restriction enzyme to use. If
the polylinker is placed within a selectable marker gene such as lacZ , gene func-
tion is disrupted when exogenous DNA is cloned into the polylinker site at any
of the restriction sites and the recombinant colonies can be identified by their
color.
Plasmids have been engineered by sophisticated techniques to perform a vari-
ety of defined tasks. Expression vectors facilitate expression of proteins; for
example, baculovirus vectors are used to produce large amounts of foreign pro-
teins in insect cells (see Chapter 6). Some vectors help identify regulatory signals
that turn genes on or off, some are used for direct selection of recombinants,
some have increased stability so that they are not eliminated from their host
cells, and others are genetically altered so that high copy numbers/host cell can
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