Difference Between Cloning Vector and Expression Vector

Main Difference – Cloning Vector vs Expression Vector

Cloning vector and expression vector are two types of vectors, used in recombinant DNA technology to carry foreign DNA segments into a target cell. Both cloning and expression vectors comprise of the origin of replication, unique restriction sites, and selectable marker gene in their vector sequences. Both cloning and expression vectors are self-replicative due to the presence of an origin of replication. Cloning vectors can be either plasmids, cosmids or bacteriophages. The main difference between cloning vector and expression vector is that cloning vector is used to carry foreign DNA segments into a host cell, whereas expression vector is a type of a cloning vector, which contains suitable expression signals with maximal gene expression.

Key Areas Covered

1. What is a Cloning Vector
      – Definition, Types, Uses
2. What is an Expression Vector
      – Definition, Types, Uses
3. What are the similarities between Cloning Vector and Expression Vector
      – Outline of Common Features
4. What are the differences between Cloning Vector and Expression Vector
      – Comparison of Key Differences

Key Terms: Bacteriophages, Cloning Vector, Cosmids,  DNA, DNA Technology, Expression Construct, Expression Vector, Origin of Replication, Promoter Region, Recombinant  RNA, Plasmids, Restriction sites, Selectable marker

What is a Cloning Vector

Cloning vectors serve as carrier DNA molecules. All cloning vectors bear four special features: 

• They are self-replicative along with the foreign DNA segment they carry
• They contain several restriction sites, which are present only once in the vector
• They carry a selectable marker, typically in the form of antibiotic resistance genes, which are absent in the host genome
• They are relatively easy to recover from the host cell.

There are many choices of classic cloning vectors like plasmids, phages, and cosmids depending on the purpose. The choice of a cloning vector depends on the size of the insert and application.

Plasmids

Plasmids are naturally occurring, extrachromosomal, double-stranded DNA molecules, which are capable of autonomously replicating inside bacterial cells. The size limit of the insert in plasmids is 10 kb. Plasmids are used as cloning vectors in subcloning and downstream manipulation, cDNA cloning and expression assays. The pBR322 is one of the first plasmids genetically engineered to be used in recombinant DNA technologies. The pBR322 plasmid is shown in figure 1.

Figure 1: pBR322

Phages

Phages are derived from the bacteriophage lambda in which the cos site of the bacteriophage lambda allows it to be packaged into a phage head. The replication of vector DNA inside the host cell will ultimately cause cell lysis. The size of the insert which can be inserted into a phage vector is 5-12 kb. Phage vectors are used in genomic DNA cloning, cDNA cloning, and expression libraries.

Cosmids

Cosmids are a kind of plasmids containing cos site of bacteriophage lambda. The cos site of the bacteriophage lambda allows it to be packaged into a phage head. Although it is a plasmid, the replication of cosmids inside the host cell might not lyse the cell as in phage vectors. The size of the insert which can be cloned into a cosmid vector is 35-45 kb. Cosmid vectors are used in genomic library constructions. 

Since mammalian genes are often greater than 100 kb in size, the complete gene sequence cannot be cloned with classical cloning vectors. This problem is circumvented by mimicking the properties of host cell chromosomes into vectors. This type of vectors is called artificial chromosome vectors. BACs (bacterial artificial chromosome vectors), YACs (yeast artificial chromosome vectors), and MACs (mammalian artificial chromosome vectors) are types of artificial chromosome vectors.

BACs

Bacterial artificial chromosome vectors are based on Escherichia coli F factor plasmid. The size of the insert which can be cloned into a BAC vector is 75-300 kb. BAC vectors are used in the analysis of large genomes.

YACs

Yeast artificial chromosome vectors are based on Saccharomyces cerevisiae centromere, telomere, and other autonomously replicating sequences. The size of the insert which can be cloned into a YAC vector is 100-1 Mb. YAC vectors are used in the analysis of large genomes.

MACs

Mammalian artificial chromosome vectors are based on the mammalian centromere, telomere and the origin of replication. The insert size in MACs is 100 kb to 1 Mb. MACs are used in animal biotechnology and human gene therapy.  

What is an Expression Vector

Expression vectors, also referred to as expression construct, are a type of plasmids. A special gene is introduced into a host cell by expression vectors where the expression of the transformed gene is facilitated by the expression vector with the use of cellular-transcriptional and translational machinery. An expression vector comprises regulatory sequences like enhancers and promoter regions, which lead to an efficient gene expression. After the expression of a particular protein like insulin inside a host cell, the product should be purified from the proteins of the host cell. On that account, introduced protein is either tagged with histidine (His tag) or any other protein. In order to obtain an efficient expression of the introduced gene inside a host cell, the following expression signals should be introduced into an expression vector.

• Insertion of a strong promoter.
• Insertion of a strong termination codon.
• Considerable distance between promoter region and the cloned gene.
• Insertion of a transcription initiation sequence.
• Insertion of a translation initiation sequence.

Figure 2: pGEX-3X

Similarities Between Cloning Vector and Expression Vector

• Both cloning and expression vectors are used in introducing foreign DNA segments into a target cell known as the host cell.

• Both cloning vectors and expression vectors share common features like the origin of replication, unique restriction sites, and selectable marker gene in their vector sequence.

• Both cloning vectors and expression vectors are capable of replicating independently inside the host cell.

Difference Between Cloning Vector and Expression Vector

Definition

Cloning Vector: Cloning vector is a small piece of DNA which can be stably maintained within a host cell. It is used to introduce genes into cells while obtaining numerous copies of the insert.

Expression Vector: Expression vector is a plasmid which is used to introduce a specific gene into a target cell and commandeer cell’s mechanisms to produce the relevant gene product.

Role

Cloning Vector: Cloning vectors are used to obtain numerous copies of the inserted DNA segment.

Expression Vector: Expression vectors are used to obtain gene product of the inserted DNA segment, either a protein or RNA.

Types

Cloning Vector: Cloning vectors can be plasmids, cosmids, phages, BACs, YACs, or MACs.

Expression Vector: Expression vector is a plasmid vector.

Features of the Vector

Cloning Vector: Cloning vectors comprise an origin of replication, unique restriction sites, and a selectable marker.

Expression Vector: Expression vector comprises enhancers, promoter region, termination codon, transcription initiation sequence, and translation initiation sequence in the vector in addition to the typical features of a cloning vector.

Conclusion

Cloning vectors and expression vectors are readily used in recombinant DNA technology in order to introduce foreign DNA segments into target cells. Both cloning vectors and expression vectors are capable of replicating by themselves inside the host cell. Cloning vectors are typically used for introducing foreign genes into target cells while achieving numerous copy of the introduced gene. Expression vectors are used to obtain the gene product, either a protein or RNA of the introduced gene inside the host cell. Most of the recombinant proteins like insulin is produced by the use of expression vectors. The main difference between cloning vector and expression vector is the application of each vector in recombinant DNA technology.

Reference:

1. “Cloning Vectors.” Cloning and Molecular Analysis of Genes. N.p., n.d. Web. Available here. 18 June 2017. 
2. “Shuttle Vectors and Expression Vectors.” Boundless. Boundless, 26 May 2016. Web. Available here. 18 June 2017. 

Image Courtesy:

1. “PBR322” By Ayacop (+ Yikrazuul) – Own work (Public Domain) via Commons Wikimedia
2. “PGEX-3X cloning vector”By Magnus Manske – Created by Magnus Manske (CC BY-SA 3.0) via Commons Wikimedia

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