DNA Cloning and Its Applications: A Preview
Most methods for
cloning pieces of DNA in the laboratory share certain general features. One
common approach uses bacteria, most often Escherichia coli. Recall from Figure 16.12 that the E. coli chromosome is a large circular molecule of DNA. In
addition, E.
coli and many other bacteria have plasmids, small circular DNA molecules that replicate separately from
the bacterial chromosome. A plasmid has only a small number of genes; these
genes may be useful when the bacterium is in a particular environment but may
not be required for survival or reproduction under most conditions.
To clone pieces of
DNA in the laboratory, researchers first obtain a plasmid (originally isolated
from a bacterial cell and genetically engineered for efficient cloning) and
insert DNA from another source (“foreign” DNA) into it (Figure 20.2). The resulting plasmid is now a recombinant DNA molecule. The
plasmid is then returned to a bacterial cell, producing a recombinant bacterium. This single cell reproduces through repeated cell
divisions to form a clone of cells, a population of genetically identical
cells. Because the dividing bacteria replicate the recombinant plasmid and pass
it on to their descendants, the foreign DNA and any genes it carries are cloned
at the same time. The production of multiple copies of a single gene is called gene cloning.
Gene cloning is
useful for two basic purposes: to make many copies of, or amplify, a particular gene and to produce a protein
product. Researchers can isolate copies of a cloned gene from bacteria for use
in basic research or to endow an organism with a new metabolic capability, such
as pest resistance. For example, a resistance gene present in one crop species
might be cloned and transferred into plants of another species. Alternatively,
a protein with medical uses, such as human growth hormone, can be harvested in
large quantities from cultures of bacteria carrying the cloned gene for the
protein. A single gene is usually a very small part of the total DNA in a cell.
For example, a typical gene makes up only about one-millionth of the DNA in a
human cell. The ability to amplify such rare DNA fragments is therefore crucial
for any application involving a single gene.
_ Figure 20.2 A preview of gene cloning and some uses of cloned genes. In this simplified diagram of gene cloning, we
start with a plasmid (originally isolated from a bacterial cell) and a gene of
interest from another organism. Only one plasmid and one copy of the gene of
interest are shown at the top of the figure, but the starting materials would
include many of each.
