In medicinal studies, a vector is an agent which acts to transmit pathogens from one host to another; in genetics, the term vector refers to a vehicle used to transfer DNA sequences from one organism to another organism. We are focused on the genetics-related definition. Two biological agents which act as vectors include:

• Plasmid vectors

• Viral vectors

How do plasmid vectors work? Agrobacterium tumefaciens is a bacterium which acts like a vector in nature by transferring genes found on its plasmids to other organisms, and it is commonly used in recombinant DNA technology. Plasmids can be copied numerous times, regardless of whether the bacterial host is replicating its own DNA; every time a plasmid vector is replication, the introduced DNA is also replicated. Purified recombinant plasmids can be inserted into a new organism directly.

How do viral vectors work? Viruses act to infect target cells by injecting their nucleic acid into the host cell. By using recombinant DNA techniques, it is possible to insert genes desired for replicating into viral DNA or RNA, and to use the virus to insert the new gene into the target cells. Viruses can accept large DNA inserts, making it relatively easy to accept foreign genes; and as viruses are a type of pathogen, it is also necessary to either disable or entirely remove the genes which cause symptoms of disease. The two types of viruses currently being used in this way include adenoviruses and retroviruses. There are issues with using viral vectors however;

• The human immune system naturally attacks viruses, and this can decrease their chance of survival within their new host

• Viral DNA insertion into the hst genome can sometimes disrupt normal gene regulation and result in the development of cancer