Cloning Vectors, pBR322 and Recombinant Selection

A vector carries and maintains a chosen DNA fragment

A cloning vector is a DNA molecule designed to carry another DNA sequence into a compatible host and allow its maintenance or replication. Plasmids and modified bacteriophage DNA can perform this role. Typical vector features include an origin of replication, a way to select host cells carrying the vector and one or more defined locations for inserting foreign DNA. A vector need not automatically contain all the signals required for protein expression; cloning and expression vectors can have different designs.

Copy number is the approximate number of vector molecules maintained in one cell. Some plasmids are low-copy, whereas others are high-copy under suitable conditions. Higher copy number may increase the amount of insert DNA recoverable from a culture but can also affect cell burden and stability. Vector design must therefore account for the host species, desired purpose and size of the DNA insert.

Origin, marker and cloning site perform different jobs

The origin of replication (ori) permits the plasmid to replicate when recognised by the host system. A selectable marker allows only certain cells to survive or grow under a chosen condition—for example, the presence of a particular antibiotic. A cloning site provides a defined location where a DNA fragment can be inserted. These features answer different questions: Can the DNA be copied? Can cells containing the vector be found? Can the new sequence be placed into the vector reliably?

A single or well-characterised restriction site simplifies cloning because cutting at numerous equivalent sites could fragment the vector and make reconstruction unpredictable. Some modern vectors instead contain a multiple cloning site, a short designed region with different unique restriction sites. Having several different unique sites within a compact region does not mean the same enzyme cuts many times; that distinction is important when reading a plasmid map.

Reading a map of pBR322

pBR322 is a classic circular E. coli plasmid vector. It contains an origin of replication, ampicillin- and tetracycline-resistance determinants commonly represented as ampR and tetR, and an associated replication-control region that includes rop. The map also shows particular restriction sites. A map is a symbolic representation of DNA positions, not a picture of proteins physically located on a ring.

If foreign DNA is inserted into a suitable BamHI site within the tetracycline-resistance region, it can disrupt the functional tetR determinant while leaving ampR intact. Cells receiving this recombinant plasmid may grow on ampicillin but fail to grow when tetracycline is the only relevant selective pressure. Cells carrying an unmodified pBR322 molecule can retain resistance to both antibiotics. The observation depends on the correct host and selection conditions.

Selecting transformants versus identifying recombinants

After transformation, a selectable marker such as ampicillin resistance can distinguish bacteria carrying a suitable plasmid from cells lacking it. Growth on ampicillin indicates survival under those conditions; it does not tell whether the plasmid contains the desired insert. Screening asks a different question: among transformed cells, which received the recombinant plasmid rather than a reclosed or empty vector?

With pBR322 insertional inactivation, colonies growing on an ampicillin plate can be replica-plated onto tetracycline so the same colony positions are compared. Colonies that survive ampicillin but not tetracycline are candidates for an insert in the tetR site; colonies that survive both may contain intact tetR. A candidate must still be checked for the correct insert sequence and orientation. Selection and confirmation are not interchangeable.

Blue–white screening and β-galactosidase

Some cloning vectors carry a multiple cloning site within a lacZ α-complementation region. Under suitable host and plate conditions, an intact plasmid-borne α fragment complements the host's deficient β-galactosidase system. In the presence of the chromogenic substrate X-gal and appropriate induction, functional β-galactosidase generates blue pigment. Insertion of DNA into the α region often prevents complementation, producing a white or pale colony.

Blue colonies often indicate non-recombinant vector and white colonies often indicate insertional disruption, provided the proper strain and vector are used. White colour does not prove that the intended DNA is present, nor does the screening method alone select for plasmid-bearing cells. An independent antibiotic marker is commonly used first. Colour results require confirmation because unusual inserts, mutations or background effects can produce misleading colonies.

Vectors for plant and animal cells

Some bacteria and viruses naturally transfer nucleic acid to other cells. Agrobacterium tumefaciens can deliver a DNA segment, T-DNA, to susceptible plant cells through its Ti-plasmid-associated machinery. Researchers have engineered non-tumour-inducing versions that retain useful transfer functions while replacing pathogenic functions with sequences of interest. The organism's natural delivery system becomes a tool when the vector and host are chosen appropriately.

Engineered viral vectors can transfer genetic material into certain animal cells; retroviral vector designs are one example. The term disarmed means disease-causing or replication-associated functions have been modified or removed as appropriate, not that all biological risks have disappeared. Plant and animal vectors differ from common E. coli cloning plasmids in the host functions they must use and the way incoming DNA is maintained.

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