A vector is used to carry a gene into a host cell. The two main types are plasmids and viruses.Plasmids
Plasmids are small circles of DNA found in bacteria. Plasmids used as vectors contain:
• an origin of replication, so the plasmid is copied inside the bacterium
• one or more restriction sites where the gene can be inserted
• a marker gene, such as antibiotic resistance, to identify transformed cells
• a promoter so the inserted gene is transcribed.Bacteria do not normally take up plasmids easily. They are made more permeable by treatment with ice-cold calcium chloride solution followed by a brief heat shock, or by electroporation, where a short electric pulse makes temporary pores in the membrane.Viruses
Viruses naturally inject their genetic material into cells, so they make efficient vectors. They are modified first so they cannot reproduce or cause disease.
• Bacteriophages are used to carry genes into bacteria.
• Adenoviruses and adeno-associated viruses are used to carry genes into human cells in gene therapy.
• Retroviruses and lentiviruses insert their genes into the host cell's chromosomes, so the gene is passed on when the cell divides. This gives long-lasting expression but the insertion point is random.Agrobacterium tumefaciens
This soil bacterium naturally infects plants and transfers part of its Ti plasmid into the plant's chromosomes. Scientists replace the tumour-causing genes with the desired gene, making it a vector for many broad-leaved plants.
Plasmids are small circles of DNA found in bacteria. Plasmids used as vectors contain:
• an origin of replication, so the plasmid is copied inside the bacterium
• one or more restriction sites where the gene can be inserted
• a marker gene, such as antibiotic resistance, to identify transformed cells
• a promoter so the inserted gene is transcribed.Bacteria do not normally take up plasmids easily. They are made more permeable by treatment with ice-cold calcium chloride solution followed by a brief heat shock, or by electroporation, where a short electric pulse makes temporary pores in the membrane.Viruses
Viruses naturally inject their genetic material into cells, so they make efficient vectors. They are modified first so they cannot reproduce or cause disease.
• Bacteriophages are used to carry genes into bacteria.
• Adenoviruses and adeno-associated viruses are used to carry genes into human cells in gene therapy.
• Retroviruses and lentiviruses insert their genes into the host cell's chromosomes, so the gene is passed on when the cell divides. This gives long-lasting expression but the insertion point is random.Agrobacterium tumefaciens
This soil bacterium naturally infects plants and transfers part of its Ti plasmid into the plant's chromosomes. Scientists replace the tumour-causing genes with the desired gene, making it a vector for many broad-leaved plants.