Bio-Rad Ligation and Transformation Module - Manual (Page 15)

• Self-replication — Plasmids have an origin of replication so they can
reproduce independently within the host cell; since the origin of
replication engineered into most cloning vectors is bacterial, the plasmid
can be replicated by enzymes already present in the host bacteria
• Size — Most bacterial vectors are small, between 2,000–10,000 bp
long (2–10 kilobases or kb), making them easy to manipulate
• Copy number — Each plasmid is found at specific levels in its host
bacterial strain. A high copy number plasmid might have hundreds of
copies in each bacterium, while a low copy number plasmid might
have only one or two copies per cell. Cloning vectors derived from
specific plasmids have the same copy number range as the original
plasmid. Most commonly used cloning vectors are high copy number
• Multiple cloning site (MCS) — Vectors have been engineered to contain
an MCS, a series of restriction sites, to simplify insertion of foreign
DNA into the plasmid. An MCS may have 20 or more different enzyme
sites, each site unique both in the MCS and in the plasmid. This
means that for each restriction site included in the MCS, the
corresponding restriction enzyme will cut the plasmid only at its single
site in the MCS
• Selectable markers — Plasmids carry one or more resistance genes
for antibiotics, so if the transformation is successful (that is, if the
plasmid enters and replicates in the host cell), the host cell will grow in
the presence of the antibiotic. Commonly used selectable markers are
genes for resistance to ampicillin (
amp r ), tetracycline ( tet r ), kanamycin
( kan r ), streptomycin ( sm r ), and chloramphenicol ( cm r )
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