*2.2.2 Plasmids*

These are extrachromosomal DNA that possess the ability to replicate independently within the host cell. Plasmids are circular in shape and impart special properties to the organism possessing it as it may code for antibiotic resistance, bactericide production, etc. [13].

The plasmids can be classified majorly into two categories:

The first type is self-transmissible, i.e., the ones that possess the ability to promote conjugation and are transferred quickly amid the bacterial population. These kinds are not generally employed as vectors.

The second type is the non-self-transmissible, i.e., the ones that cannot regulate their own transfer among bacterial population. These categories of plasmids are extensively used in genetic engineering as vectors [14].

The gene of interest or the foreign gene is inserted within the plasmid DNA vector; for that the closed circular plasmid is cleaved using restriction enzyme to make it linear. A plasmid contains a single cleavage site for a restriction enzyme to avoid multiple digestions. The gene of interest is inserted in the linear plasmid, and then the recombinant is converted to its original circular form [15] (**Figure 1**).

#### *2.2.3 Phage vectors*

The viruses that infect the bacteria are termed as bacteriophages. They possess a very simple structure consisting of a genetic material which might be a DNA or RNA surrounded by a protein coat termed as capsid. These phages can also be employed as a vector as large piece of DNA of interest can be incorporated in the genetic material of the phage. Different phages have been developed for the

**5**

**Figure 1.**

*Genetic Engineering: Altering the Threads of Life DOI: http://dx.doi.org/10.5772/intechopen.92618*

blue colonies.

*2.2.4 Cosmids*

the case of plasmids (**Table 1**).

**2.3 Introducing vector into host cells**

foreign DNA. This goal can be achieved by different ways.

*Plasmid pBR322 exhibiting ori site, ampicillin- and tetracycline-resistant selectable markers.*

purpose of genetic engineering containing a single site for the restriction enzymes [16]. These commonly used phage vectors are M13 phage, T4 phage, T7 phage, etc. The DNA of these phage vectors are altered by ligating the alien DNA fragment, and further the phage vector is inserted into the host cell [17]. These cells are later grown in a culture medium containing X-gal. The cells transformed with the recombinants will produce white colonies, and the non-transformants will develop

A cosmid is basically a plasmid with cos site, which contains cos sequences necessary for packaging. Cos sequences are ~200 base pair in length. Cosmids are hybrids comprising phage DNA molecule and plasmid of bacteria. Just like any other vector system, cosmids also require a selectable marker and an ori site. The cosmids lack λ-genes so unlike phages they do not produce plaques. Instead plaques colonies are formed by using selective media according to the marker just as done in

The membrane of the cell prevents the DNA from diffusing in or out. This barrier must be manipulated or altered so that the host cells are able to take up the *Genetic Engineering: Altering the Threads of Life DOI: http://dx.doi.org/10.5772/intechopen.92618*

purpose of genetic engineering containing a single site for the restriction enzymes [16]. These commonly used phage vectors are M13 phage, T4 phage, T7 phage, etc. The DNA of these phage vectors are altered by ligating the alien DNA fragment, and further the phage vector is inserted into the host cell [17]. These cells are later grown in a culture medium containing X-gal. The cells transformed with the recombinants will produce white colonies, and the non-transformants will develop blue colonies.
