Vectors part 2 | molecular biology | biotechnology

In molecular cloning, a vector is a DNA molecule used as a vehicle to artificially carry foreign genetic material into another cell, where it can be replicated and/or expressed (e.g.- plasmid, cosmid, Lambda phages). A vector containing foreign DNA is termed recombinant DNA. The four major types of vectors are plasmids, viral vectors, cosmids, and artificial chromosomes. Of these, the most commonly used vectors are plasmids.[1] Common to all engineered vectors are an origin of replication, a multicloning site, and a selectable marker.

The vector itself is generally a DNA sequence that consists of an insert (transgene) and a larger sequence that serves as the "backbone" of the vector. The purpose of a vector which transfers genetic information to another cell is typically to isolate, multiply, or express the insert in the target cell. All vectors may be used for cloning and are therefore cloning vectors, but there are also vectors designed specially for cloning, while others may be designed specifically for other purposes, such as transcription and protein expression. Vectors designed specifically for the expression of the transgene in the target cell are called expression vectors, and generally have a promoter sequence that drives expression of the transgene. Simpler vectors called transcription vectors are only capable of being transcribed but not translated: they can be replicated in a target cell but not expressed, unlike expression vectors. Transcription vectors are used to amplify their insert.

The manipulation of DNA is normally conducted on E. coli vectors, which contain elements necessary for their maintenance in E. coli. However, vectors may also have elements that allow them to be maintained in another organism such as yeast, plant or mammalian cells, and these vectors are called shuttle vectors. Such vectors have bacterial or viral elements which may be transferred to the non-bacterial host organism, however other vectors termed intragenic vectors have also been developed to avoid the transfer of any genetic material from an alien species.[2]

Insertion of a vector into the target cell is usually called transformation for bacterial cells,[3] transfection for eukaryotic cells,[4] although insertion of a viral vector is often called transduction.[5]

2. Thecosmidcanbedefinedasthehybridderivedfrom
plasmidswhichcontaincossiteofphageλ(cosmid=
cossite+plasmid).
Acosmid,firstdescribedbyCollinsandHohnin1978,
isatypeofhybridplasmid(oftenusedasacloning
vector)thatcontainscossequences,DNAsequences
originallyfromtheLambdaphage.

3. Characteristics of Cosmid
Cosmidsareabletocontain37to52kbpofDNA,
whilenormalplasmidsareabletocarryonly1-20kbp
Theycanreplicateasplasmidsiftheyhaveasuitable
originofreplication.
Theyfrequentlyalsocontainageneforselection
suchasantibioticresistance.
Thosecellswhichdidnottakeupthecosmidwould
beunabletogrow.

4. Construction of Cosmid Vectors:
A cosmid is basically a plasmid that carries a cos site.
It also needs a selectable marker, such as ampicillin resist
ant gene, and a plasmid origin of replication.
This is important to note that as cosmid lacks all the lamb
da genes, so at does not produce plaques.
Instead colonies are formed on the selective media just as
with plasmid vectors.

6. Cloning Experiment with Cosmid Vectors:
This is carried out as follows.
The cosmid is opened and its unique restriction site and our gene
of interest is inserted.
These fragments are usually produced by partial digestion with a
restriction endonuclease, as total digestion almost invariably resu
lts in fragments that are too small to be cloned with a cosmid.
Ligation is carried out so that catenanes are formed.
These lambda phages are then used to infect an E. coli culture.
All colonies are recombinant colonies as non-recombinant lambd
a phages cannot be packaged into the head of the lambda bacter
iophage.

8. Uses of Cosmid Vectors:
Cosmid are used for construction of genomic libraries of e
ukaryotes since these can be used for cloning large fragm
ents of DNA.
Advantages of Cosmids:Followings are advantages of co
smid vectors:
1. These can be used to clone gene of interest up to 40 kb.
2. As the lambda phage will insert the recombinant DNA i
nto the host cell, an extra step of inserting the recombina
nt DNA into the host cell is not performed.
3. Easy screening method is found.