Expression vectors (or Constructs)
Cloning vectors provide a backbone for the DNA insert to be reproduced and propagated in bacteria; however, these vectors are only useful for storing a genetic sequence. By themselves, they are incapable of allowing for transcription and translation of the gene into a functional protein product (Li et al., 2017).
The expression vectors are vectors which act as vehicles for DNA insert and also allow the DNA insert to be expressed efficiently. They are special types of cloning vectors containing the regulatory sequences necessary to allow the transcription and translation of a cloned gene or genes. These constructs are often derivatives of the plasmid vectors used in the host: Origin of replication, which is a DNA segment recognized by the cellular DNA-replication enzymes; selectable markers, which constitute genes showing resistance to certain antibiotics like ampicillin and tetracycline; multiple cloning sites; a promoter, is a sequence recognized by sigma subunit of RNA polymerase which is required for initiation of transcription of gene of interest; terminator, which is short nucleotide sequences present at the end of a gene where transcription of gene ends; and translation initiation site, such as ribosome binding site and start codon. Ribosome binding siteis a short nucleotide sequence recognized by the ribosome as the point at which it should attach to the messenger molecule. The initiation codon of the gene is always a few nucleotides downstream of this [7] . For a gene to give rise to a protein product, an expression vector must be used that contains the necessary elements for a host cell to transcribe and translate the gene. The most common reason for this is that the promoter may not be recognized by the RNA polymerase of the new host. In the case of a mammalian cell, a standard mammalian expression vector will contain an origin of replication, MCS, and selectable marker. However, the expression vector will also need a promoter found in mammalian cells that can drive the expression of the gene. The coding DNA needs other features to be transcribed and translated, such as the polyadenylation tail that normally appears at the end of transcribed pre-mRNA and a sequence that attracts the ribosome for translation [19] . Once the expression constructs are inside the host cell, the protein encoded by the gene of interest is produced by the transcription and translation, which utilizes the translation machinery and ribosomal complexes of the host organism. They are extensively used as tools which help in the production of mRNAs and, in turn, stable proteins like insulin. However, if the cloned gene is to be expressed across the prokaryotic-eukaryotic boundary, then different mechanisms are used in prokaryotes and eukaryotes for the translation machinery to identify the start codon. For example, expression of a eukaryotic gene in E. coli requires addition of a Shine-Dalgarno sequence at a position upstream of the start codon. Vectors that can be propagated in two or more different hosts (both in prokaryotes and eukaryotes) are called shuttle vectors [6, 17] . After the protein product is expressed, it is to be then purified. The purification of a protein poses a challenge since the protein of interest, whose gene is carried on the expression vector, is to be purified independently of the proteins of the host organism. To make the process of purification simpler, the gene of interest carried on the expression vector should always have a ‘tag’. This tag can be any marker peptide or histidine (His tag) [6] . Expression vectors have numerous applications in producing peptides and proteins for the pharmaceutical industry such as producing insulin, growth hormone, antibiotics, vaccines, and antibodies. Moreover, expression vectors help in enzyme production in food and garment industries. Not only that, expression vectors are essential in producing transgenic plants such as golden rice, insect resistant plants, herbicideresistant plants, etc. [15] . The most commonly used expression vector is pET vector system. It is the most powerful system yet developed for the cloning and expression of recombinant proteins in E. coli. Target genes are cloned in pET plasmids under control of strong bacteriophage T7 transcription and (optionally) translation signals; expression is induced by providing a source of T7 RNA polymerase in the host cell. They are initially cloned using hosts that do not contain the T7 RNA polymerase gene, thus eliminating plasmid instability due to the production of proteins potentially toxic to the host cell. Once established in a non-expression host, plasmids are then transferred into expression hosts containing a chromosomal copy of the T7 RNA polymerase gene in the genome, and expression is induced by the addition of IPTG. Ampicillin and kannamycin resistance genes are available in pET vectors as selection marker. pET28 and pET32 are the most commonly used pET vectors [1, 15] .
Similarities and Differences between Cloning Vector and Expression Vector
Cloning vector and expression vector are two types of vectors used in recombinant DNA technology and genetic engineering. Both of them contain multiple cloning sites, origin of replication and selectable marker [4, 15] . However, cloning vector is a small DNA molecule that carries a foreign DNA fragment into the host cell while expression vector is a type of vector that facilitates the introduction, expression of genes and production of proteins. Furthermore, another significant difference between cloning vector and expression vector is that a cloning vector introduces a foreign DNA fragment into a host while expression vectors express the introduced gene by producing the relevant protein [14, 19, 21] . Besides, a cloning vector consists of an origin of replication, restriction sites, and a selectable marker while the expression vector contains enhancers, promoter region, termination codon, transcription initiation sequence, an origin of replication, restriction sites, and a selectable marker. Plasmids, bacteriophages, cosmids, bacterial artificial chromosomes, yeast artificial chromosome, mammalian artificial chromosomes are examples of cloning vectors. Meanwhile, expression vectors are mostly plasmids [6] .