Курсовая работа: Vectors for moleculars cloning

However, packaging of phage DNA is prevented by the lack of cos sites on the prophage DNA. On the other hand, exogenous DNA with cos sites is packaged efficiently to produce an infectious bacteriophage particle. The single-strain system is superior to two-strain system in having a lower background of parental phages. In addition, it uses E. coli C, which lacks the EcoK restriction system, as the host for the lysogen.

7.3 Biological Containment

The biological containment of recombinant phages is an important aspect from the point of view of ethics and eventual biohazards. It is desirable that cloning vectors and recombinants have poor survival in the natural environment and require special laboratory conditions for their replication and survival. According to Blattner, the lytic phages offer a natural advantage in this respect since the phage and the sensitive bacteria coexist only briefly. A newly inserted segment may not be compatible with E. coli metabolism for extended periods. To make the phage vectors more safe, three amber mutations were introduced in its genome. The new vector Xgt WES XC is safer because an amber suppressor host strain is a very rare occurrence in the natural environment. Many vectors carry one of the amber mutations on the genome so that they can be propagated only on an appropriate suppressor host.

8. Phage vectors

Many phage vectors have been constructed in the recent past, each with its own special features. There is no universal lambda vector which can fulfill all the desired objectives of the cloning experiments.

The choice of a vector depends mainly on

- the size of a DNA fragment to be inserted,

- the restriction enzymes to be used,

- the necessity for expression of the cloned fragment,

- the method of screening to be used to select the desired clones.

Bacteriophage lambda vectors can be broadly classified into two types:

1. replacement vectors ,

2. insertion vectors.

Figure 7. Lambda Phage genome

8.1 Replacement Vectors

Taking advantage of the maximum and minimum genome size essential for efficient packaging and the presence of the nonessential central fragment, it is possible to remove the stuffer fragment and replace it with a foreign DNA fragment in the desired size range. This forms the basis of lambdaderived replacement vectors.

Cloning of a foreign DNA in these vectors involves

· preparation of left and right arms by physical elimination of the nonessential region,

· ligation of the foreign DNA fragment between the arms,

· in vitro packaging and infection.

The replacement vectors contain a pair of restriction sites to excise the central stuffer fragments, which can be replaced by a desired DNA sequence with compatible ends. The presence of identical sites within the stuffer fragment but not in the arms facilitates the separation of the arms and the stuffer on density gradient centrifugation. In many vectors, sets of such sites are provided on attached polylinkers so that an insert can be easily excised. Two purified arms cannot be packaged despite their being ligated to each other, because they fall short of the minimum length required for packaging. This provides positive selection of recombinants. The replacement vectors are convenient for cloning of large (in some cases up to 24 kbp) DNA fragments and are useful in the construction of genomic libraries of higher eukaryotes. Charon and EMBL are among the popular replacement vectors.

8.2 Insertion Vectors

Because the maximum packagable size of lambda genome is 53 kb, small DNA fragments can be introduced without removal of the nonessential (stuffer) fragment. These vectors are therefore called insertion vectors. Cloning of foreign DNA in these vectors exploits the insertional inactivation of the biological function, which differentiates recombinants from nonrecombinants. Insertion vectors are particularly useful in cloning of small DNA fragments such as cDNA.

AgtlO and Agtll are examples of this type of vector. In recent years a multitude of lambda vectors have been constructed. Many innovative approaches have been used to introduce desired properties into the vectors. The following section deals with the strategies adopted for the construction of some of the commonly used vectors and their salient features, utilities, and limitations.