A vector is a DNA molecule which is used for transporting exogenous DNA into the host cell. A vector is capable of self-replication and stable integration inside the host cell.
The molecular analysis of DNA has been made possible only after the discovery of vectors. The whole process of molecular cloning involves the following steps:
- Digestion of DNA fragments of the target segment and the vector DNA with the help of restriction enzymes,
- Ligation of the target segment with the vector DNA with the help of DNA ligases, and
- Introduction of the ligated segment into the host cell for propagation.
General characteristics of a vector:
- It should have an Origin of Replication, known as ori, so that the vector is capable of autonomous replication inside the host organism.
- It must possess a compatible restriction site for insertion of DNA molecule.
- A vector should always harbour a selectable marker to screen the recombinant organism. This selectable marker can be an antibiotic resistance gene.
- For easy incorporation into the host machinery, a vector should itself be small in size and be able to integrate large size of the insert.
A cloning vector is also a fragment of DNA which is capable of self-replication and stable maintenance inside the host organism. It can be extracted from a virus, plasmid or cells of a higher organism. Most of the cloning vectors are genetically engineered. It is selected based upon the size and the kind of DNA segment to be cloned.
The cloning vectors must possess the following general characteristics:
- It should small in size.
- It must have an origin of replication.
- It must also be compatible with the host organism.
- It must possess a restriction site.
- The introduction of donor fragment must not intervene with the self-replicating property of the cloning vector.
- A selectable marker, possibly an antibiotic resistance gene, must be present to screen the recombinant cells.
- It should be capable of working under the prokaryotic as well as the eukaryotic system.
- Multiple cloning sites should be present.
Importance of Cloning Vectors
Cloning Vectors are used as the vehicle for transporting foreign genetic material into another cell. This foreign segment of DNA is replicated and expressed using the machinery of the host organism.
A cloning vector facilitates amplification of a single copy DNA molecule into many copies. Molecular gene cloning is difficult without the use of the cloning vectors.
History of Cloning Vectors
Herbert Boyer, Keiichi Itakura, and Arthur Riggs were three scientists working in the Boyer’s lab, University of California, where they recognized a general cloning vector. This cloning vector had restriction sites for cloning foreign DNA and also, the expression of antibiotic resistance genes for the screening of recombinant/ transformed cells. The first vector used for cloning purposes was pBR322, a plasmid. It was small in size, nearly 4kB, and had two selectable markers.
Features of Cloning Vectors
1. Origin of Replication (ori)
- A specific set/ sequence of nucleotides where replication initiates.
- For autonomous replication inside the host cell.
- Foreign DNA attached to ori also begins to replicate.
2. Cloning Site
- Point of entry or analysis for genetic engineering.
- Vector DNA at this site is digested and foreign DNA is inserted with the aid of restriction enzymes.
- Recent works have discovered plasmids with multiple cloning sites (MCS) which harbour up to 20 restriction sites.
3. Selectable Marker
- Gene that confers resistance to particular antibiotics or selective agent which, under normal conditions, is fatal for the host organism.
- Confers the host cell the property to survive and propagate in culture medium containing the particular antibiotics.
4. Marker or Reporter Gene
- Permits the screening of successful clones or recombinant cells.
- Utilised extensively in blue-white selection.
5. Inability to Transfer via Conjugation
- Vectors must not enable recombinant DNA to escape to the natural population of bacterial cells.
Types of Cloning Vectors
E. Bacterial Artificial Chromosome (BAC)
F. Yeast Artificial Chromosome (YAC)
G. Human Artificial Chromosome (HAC)
H. Retroviral Vectors
- Plasmids were the first vectors to be used in gene cloning.
- They are naturally occurring and autonomously replicating extra-chromosomal double-stranded circular DNA molecules. However, not all plasmids are circular in origin.
- They are present in bacteria, archaea, and eukaryotes.
- The size of plasmids ranges from 1.0 kb to 250 kb.
- DNA insert of up to 10 kb can be cloned in the plasmids.
- The plasmids have high copy number which is useful for production of greater yield of recombinant plasmid for subsequent experiments.
- The low copy number plasmids are exploited under certain conditions like the cloned gene produces the protein which is toxic to the cells.
- Plasmids only encode those proteins which are essential for their own replication. These protein-encoding genes are located near the ori.
Examples: pBR322, pUC18, F plasmid, Col plasmid.
Nomenclature of plasmid cloning vector: pBR322 cloning vector has the following elements:
- p= plasmid
- B= Bolivar (name of the scientist)
- R= Rodriguez (name of the scientist)
- 322= number of plasmid discovered in the same lab
Advantages of using Plasmids as vectors:
- Easy to manipulate and isolate because of small size.
- More stable because of circular configuration.
- Replicate independent of the host.
- High copy number.
- Detection easy because of antibiotic-resistant genes.
Disadvantages of using Plasmids as vectors:
- Large fragments cannot be cloned.
- Size range is only 0 to 10kb.
- Standard methods of transformation are inefficient.
- Bacteriophages or phages are viruses which infect bacterial cells.
- The most common bacteriophages utilized in gene cloning are Phage λ and M13 Phage.
- A maximum of 53 kb DNA can be packaged into the phage.
- If the vector DNA is too small, it cannot be packaged properly into the phage.
Examples: Phage Lambda, M13 Phage, etc.
Phage Lambda λ
- It has head, tail, and tail fibers.
- Its genome consists of 48.5 kb of DNA and 12 bp ss DNA which comprise of sticky ends at both the terminals. Since these ends are complementary, they are cohesive and also referred to as cos sites.
- Infection by λ phage requires adsorption of tail fibers on the cell surface, contraction of the tail, and injection of the DNA inside the cell.
- These vectors are used for obtaining single-stranded copies of the cloned DNA.
- They are utilized in DNA sequencing and in vitro mutagenesis.
- M13 phages are derived from filamentous bacteriophage M13. The genome of M13 is 6.4 kb.
- DNA inserts of large sizes can be cloned.
- From the double-stranded inserts, pure single-stranded DNA copies are obtained.
Types of Phage Vectors
There are 2 types of phage vectors:
- Insertion vectors- these contain a particular cleavage site where the foreign DNA of up to 5-11 kb can be inserted.
- Replacement vectors- the cleavage sites flank a region which contains genes not necessarily important for the host, and these genes can be deleted and replaced by the DNA insert.
Advantages of using Phage Vectors
- They are way more efficient than plasmids for cloning large inserts.
- Screening of phage plaques is much easier than identification of recombinant bacterial colonies.
C. Phagemids or Phasmid
- They are prepared artificially.
- Phasmid contains the F1 origin of replication from F1 phage.
- They are generally used as a cloning vector in combination with M13 phage.
- It replicates as a plasmid and gets packaged in the form of single-stranded DNA in viral particles.
Advantages of using Phagemids:
- They contain multiple cloning sites.
- An inducible lac gene promoter is present.
- Blue-white colony selection is observed.
- Cosmids are plasmids.
- They are capable of incorporating the bacteriophage λ DNA segment. This DNA segment contains cohesive terminal sites (cos sites).
- Cos sites are necessary for efficient packaging of DNA into λ phage particles.
- Large DNA fragments of size varying from 25 to 45 kb can be cloned.
- They are also packaged into λ This permits the foreign DNA fragment or genes to be introduced into the host organism by the mechanism of transduction.
Advantages of using cosmids as vectors:
- They have high transformation efficiency and are capable of producing a large number of clones from a small quantity of DNA.
- Also, they can carry up to 45 kb of insert compared to 25 kb carried by plasmids and λ.
Disadvantages of using cosmids as vectors:
- Cosmids cannot accept more than 50 kb of the insert.
E. Bacterial Artificial Chromosomes (BACs)
- Bacterial artificial chromosomes are similar to E. coli plasmid vectors.
- They contain ori and genes which encode ori binding proteins. These proteins are critical for BAC replication.
- It is derived from naturally occurring F’ plasmid.
- The DNA insert size varies between 150 to 350 kb.
Advantages of BACs:
- They are capable of accommodating large sequences without any risk of rearrangement.
- BACs are frequently used for studies of genetic or infectious disorders.
- High yield of DNA clones is obtained.
Disadvantages of BACs:
- They are present in low copy number.
- The eukaryotic DNA inserts with repetitive sequences are structurally unstable in BACs often resulting in deletion or rearrangement.
F. Yeast Artificial Chromosomes (YACs)
- A large DNA insert of up to 200 kb can be cloned.
- They are used for cloning inside eukaryotic cells. These act as eukaryotic chromosomes inside the host eukaryotic cell.
- It possesses the yeast telomere at each end.
- A yeast centromere sequence (CEN) is present which allows proper segregation during meiosis.
- The ori is bacterial in origin.
- Both yeast and bacterial cells can be used as hosts.
Advantages of using YACs:
- A large amount of DNA can be cloned.
- Physical maps of large genomes like the human genome can be constructed.
Disadvantages of using YACs:
- Overall transformation efficiency is low.
- The yield of cloned DNA is also low.
Advantages of BACs over YACs
- Comparatively stable.
- Easy to transform.
- Simple purification required.
- User- friendly.
- Aid in the development of vaccines.
G. Human Artificial Chromosome (HACs)
- Human artificial chromosomes are artificially synthesized.
- They are utilized for gene transfer or gene delivery into human cells.
- It can carry large amounts of DNA inserts.
- They are used extensively in expression studies and determining the function of the human chromosomes.
Advantages of using HACs:
- No upper limit on DNA that can be cloned.
- it avoids the possibility of insertional mutagenesis.
H. Retroviral Vectors
- Retroviruses are the virus with RNA as the genetic material.
- Retroviral vectors are used for introduction of novel or manipulated genes into the animal or human cells.
- The viral RNA is converted into DNA with the help of reverse transcriptase and henceforth, efficiently integrated into the host cell.
- Any gene of interest can be introduced into the retroviral genome. This gene of interest can then integrate into host cell chromosome and reside there.
Advantages of using retroviral vectors:
- They are widely used as a tool to study and analyze oncogenes and other human genes.
The things which matter while choosing a cloning vector are:
- DNA insert size
- Size of the vector
- Restriction Size
- Efficiency of cloning