A clone is an identical copy. This term originally applied to cells of a
single type, isolated and allowed to reproduce to create a population of
identical cells. DNA cloning involves separating a specific gene or DNA segment from a larger
chromosome, attaching it to a small molecule of carrier DNA, and then
replicating this modified DNA thousands or millions of times through both an
increase in cell number and the creation of multiple copies of the cloned DNA
in each cell. The result is selective amplification of a particular gene or DNA
segment.
Cloning
of DNA from any organism entails five general procedures:
1. Cutting DNA at precise
locations.
Sequence-specific endonucleases (restriction
endonucleases) provide the necessary molecular scissors.
2.
Selecting
a small molecule of DNA capable of self-replication.
These DNAs are called cloning vectors (a vector is a delivery agent). They are typically plasmids or viral
DNAs.
3.
Joining
two DNA fragments covalently.
The enzyme DNA ligase links the cloning
vector and DNA to be cloned. Composite DNA molecules comprising covalently
linked segments from two or more sources are called recombinant DNAs.
4. Moving recombinant DNA from the
test tube to a host cell that
will provide the enzymatic machinery for DNA replication.
5. Selecting or identifying host
cells that contain recombinant DNA.
The
methods used to accomplish these and related tasks are collectively referred to
as recombinant DNA
technology or, more informally, genetic engineering.
Figure: Schematic illustration of DNA cloningA cloning
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