In molecular biology, a vector is a DNA molecule, used as a vehicle to transfer genetic materials to the host cells. It can either be a plasmid or virus that carries a piece/segment of DNA and introduces it into the target organisms through recombinant DNA or cloning techniques. The vector containing the DNA segment of interest is known as recombinant DNA.
Typically, the vector provides host cells with the machinery for replicating and expressing the inserted DNA sequence.
Four widely known vectors include cosmids, viral (such as retroviral and lentiviral), DNA plasmids, and artificial chromosomes. Among these, the most frequently used ones are plasmid vectors. Though all vectors have different features and are used for diverse applications, however, they all contain some common elements, which include:
How come viruses are used as a vector?
Viruses are an efficient model for inserting target genetic material into the host genome or for gene delivery. They have an effective system to enter host cells. However, to employ them as vectors, the viral disease-causing segments must be removed from their genome.
The insertion of a vector into the target cell (such as stem cells) is commonly referred to as transformation for bacterial cells, transfection for eukaryotic cells, and transduction for viral vectors.
The vectors have extensive uses in the screening of diseases, like CMV and HIV, and gene therapy for introducing nucleases and transgene.
In this article, we will cover how genetic vectors work, their types, and the industries that frequently employ them in their workflows.
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Genetic vectors are essential tools for transferring specific genes into host cells. These vectors undergo careful selection, DNA insertion (after digestion with restriction enzyme), cloning, and purification processes before being introduced to target cells. Once inside, they facilitate the isolation, multiplication, or expression of the desired gene that has been integrated into their nucleic acid or genetic material.
In gene therapy, vectors are used to modify the function of erroneous proteins by correcting their genetic code. This is particularly important when normal protein function is compromised by genetic mutations within the cell.
The goal is to restore the normal function of essential proteins in the body by targeting and compensating for genetic alterations. This aims to improve overall health and proper functioning of the body of organisms.
Figure: Gene delivery to the host cell using genetic vector.
The vectors are either classified based on the biological element being used to act as a vehicle, such as a plasmid and viruses, or based on their purpose or applications, such as cloning or expression vectors (for protein and gene expression or transcription). Vectors used for cloning vectors include bacteriophage and cosmid vectors, and expression includes M13 phage vectors.
Vectors derived from viruses are known as viral vectors. While using viruses as a vector, their viral genes are removed and modified to integrate and deliver only therapeutic genes. The capsid or the shell of the virus is used in creating vectors to transport genes of interest to the target cell.
Some extensively used viruses to manufacture the vectors include poxviruses, retroviruses, adenoviruses, and adeno-associated viruses (AAV). It’s mainly because they have low toxicity, low pathogenicity, and long-term transgene expression. However, for some of them, such as AAV, long-term administration triggers strong immune responses.
Gene therapy that utilizes viral vectors not only treats symptoms but also directly targets the underlying cause of disease and modifies cellular function.
Figure: Four types of viral vectors.
Artificial vectors are carriers designed to transfer large segments of DNA. Some widely known examples of these vectors include bacterial artificial chromosomes (BACs), yeast artificial chromosomes (YACs), and human artificial chromosomes (HACs).
YACs and BACs have the capacity to transport DNA fragments spanning up to 300,000 nucleotides in length.
An artificial chromosome must possess three essential components: a replication origin, a centromere, and telomeric end sequences. These structural elements are crucial for the functionality and stability of the vector.
Plasmids are circular, double-stranded DNA sequences capable of replicating using the host cell's machinery. The plasmid vectors contain an origin of replication for semi-independent replications inside the host cell. The vector has applications in protein expression, in vitro transcription and translation, and cloning. A few examples of plasmids used as a vector include col plasmid, F plasmid, and R plasmid.
Here are a few elements added to a vector to ensure the successful manufacture, selection, purification, and delivery of genes of interest:
Vectors have been extensively used in molecular biology as a tool for a variety of research associated with diseases and their treatment. Many of them are also a part of clinical trials, such as AAV vectors.
Vectors are used for cloning a specific gene for the production of a specific protein, such as insulin. The transcription of the cloned gene by the vector generates multiple mRNA copies, which act as templates for protein production in cell lines. The choice of promoter determines the expression type, whether constitutive or inducible, allowing for continuous or conditional protein synthesis.
Additionally, the vector is a crucial component of any research related to the understanding of nucleic acid composition, cell structure, and genetic engineering.
Gene therapy extensively involves vectors to deliver genes of interest for treating a range of diseases, such as cancer and HIV. The therapy involves introducing DNA into cells, which can be achieved through various techniques.
The primary methods include:
The adenovirus-associated vector is involved in a variety of clinical trials. When utilized in vivo gene therapy applications, AAV demonstrates both safety and effectiveness. In this approach, gene therapy is directly administered to specific areas of the body, enabling targeted delivery of new genetic instructions to the cells within.
Lentivirus vectors, in particular, have been utilized for gene therapy applications involving RNA silencing in the CNS.
Despite advancements in gene manipulation techniques like CRISPR and improved delivery methods for nonviral vectors, viral vectors continue to retain their appeal.
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Genetic vectors serve as carriers to transport foreign DNA into host cells. These vectors possess the ability to self-replicate and often contain specific elements for DNA manipulation, along with a genetic marker for identification. DNA plasmids, viruses, and artificial chromosomes are among the widely used types of vectors in genetic engineering.
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