Vectors are carriers, transporters and may also be termed as vehicles for genetic material. DNA bodies used to transfer alien genetic material to the cells are known as vectors. Vectors are generally classified in to four groups on the basis of their transmitting form e.g. (1) Synthetic chromosomes (2) Plasmids (3) Viruses (4) Cosmids. Most common of these vectors are viruses and plasmids.

Basically vectors are transgenic DNA bodies containing two units, larger unit is the foundation and smaller unit is the foreign DNA. This foreign DNA is expressed in the cell where it's to be transmitted. But before its expression it undergoes severance, burgeoning and then it finally expresses itself. Vectors can be transcribed, translated, inserted and expressed. Expression vectors are both transcribed and translated unlike transcription vectors are only transcribed. Transcription vectors enlarge the alien gene in them. When a vector is incorporated in a bacterial cell this phenomenon is known as transformation, when incorporated into eukaryotic cells known as transfection and in general incorporation of viral vector is known as transduction.

Plasmids
They are double stranded mostly spherical DNA and self replicating in the host. They don't need the spherical DNA to replicate, they are independent. Plasmids are characteristic of bacteria but rarely found in eukaryotes too. Because of their automated replication they are known as replicons. Plasmids are neither parasites nor symbionts. Plasmids carry genes for antibiotic resistance. There are two types of plasmids, integrating plasmids and non-integrating plasmids. Non-integrating plasmids don't merge with the bacterial DNA and replicate on their own when the bacterial cell divides. Whereas integrating plasmids merge with the bacterial DNA and then are replicated.

Applications of plasmids
• They are used to manufacture large number of proteins like insulin, antibiotics etc Because of their automated spontaneous replication they are very vastly used in genetic engineering. They are used to produced resistance against antibiotics like streptomycin, penicillin etc.
• They are used to make human disease specimen. Earlier rat disease specimen was made. Now utilizing adeno virus and zinc finger nucleases scientists are trying to make Isogenic human disease specimens.
• Gene therapies are only successful if there is proper transfer of foreign gene and this task is marvellously achieved by using plasmids. They also inhibit uncontrolled division of cells in cell i.e. cancer, mutations, cell damage or an auto-immune response.
• Plasmids are also used for research process for biotechnology purposes. Plasmids are non infectious, they have no effect on cell growth even if detached from cell hence are very useful.

Viral Vectors
Viruses that play the role of carriers are known as viral vectors. They perform this function in two ways IN VIVO and IN VITRO. They transfer genes by transduction. First discovered in 1970 by Paul Berg. As they are viral vectors they are pathogenic in nature but their infectious properties are reduced to minimum by modifying them by removing the gene for viral replication, these viruses can infect but they can't replicate due to which they are not that alarming. They are not wobbly and unstable. They are usually not cell specific and can infect many types of cells but sometimes they counterfeit this situation. They are given certain genes by which they can be easily identified.

Applications
• They are used in research work. Retro viruses are used to demonstrate GFP they mark them and use them. They are cheaper, easy to use and are used in xenotransplantation, here cells are infected by in vitro method and incorporated in to host cell. Transfection is the opted process and is arduous.
• These viruses carry infectious proteins which on being attenuated can be used as vaccines. Infected cells are identified by T-lymphocytes. These are then used as vaccines. Adeno viruses are now days being used for this purpose.
• Genetic ailments can be controlled and treated well by using viral vectors. Cystic fibrosis, SCID, haemophilia A, SCID, rickets etc. These disorders are generally caused by insertion, deletion or changing of sequence of nucleotides in DNA i.e. mutation. Viruses carry the genes required for proper functioning they are replaced and these disorders are treated. Adino virus vectors are safer, cheaper and easily available.

Summary
Vectors are used as carriers of different genes. These are used in different purposes like treatment or multiple disorders, research work, gene therapy and many others. They are the most vitally used belongings used in biotechnology. They have revolutionised biotechnology and has helped mankind in achieving number of targets.

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