Bio-Rad Ligation and Transformation Module - Manual (Page 14)

Background
Cloning
Cloning is the production of multiple exact copies of a piece of DNA,
usually a gene, using molecular biology techniques. Cloning is frequently
the first step used in a research project, producing enough DNA for further
study. Once a gene or part of a gene has been amplified using PCR, the
next step is to insert the DNA into a plasmid or cloning vector so that the
DNA fragment can be propagated.
Plasmids as Cloning Vectors
Many cloning vectors are derived from bacterial plasmids. Plasmids are
circular extrachromosomal DNA molecules, usually around 2,000–100,000
base pairs (bp) long, although most plasmids used in cloning are
2,000–10,000 bp. Bacteria may naturally contain many copies of a single
plasmid, or single copies of others. Plasmids are able to replicate
independently of the host DNA and most plasmids carry at least one gene.
Frequently these genes code for a factor or function that helps the bacteria
survive. For example, resistance to the antibiotic ampicillin is conveyed by
a plasmid carrying an ampicillin-resistance gene. Plasmids are capable of
being transferred from one bacterium to another. These characteristics
have resulted both in wonderful new uses for plasmids (such as their use in
cloning, making many of the techniques of molecular biology possible) and
in the emergence of dangerous pathogenic organisms (namely bacteria
resistant to multiple antibiotics).
Plasmids thus already have many of the characteristics needed for use as
cloning vectors, and other useful features have been added through genetic
engineering. A wide variety of vectors are available commercially for various
applications. A plasmid designed to clone a gene is different from a plasmid
designed to express a cDNA (complementary DNA) in a mammalian cell
line, which is different again from one designed to add a tag to a protein for
easy purification. The primary characteristics of any good vector include:
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