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B Saccharomyces cerevisiae
The process by which resistance to a disease is acquired or induced in an organism is called immunisation.
Immunisation can occur naturally when a person is exposed to a pathogen or it may be brought about artificially through a vaccine. In both the cases, immunisation provides resistance to a particular pathogen by means of production of antibodies that are aimed to eliminate that pathogen from the body.
Vaccines are suspension of weakened, killed, or fragmented pathogens or toxins that are administered to prevent disease. Vaccines stimulate a person's immune system to produce antibodies thereby protecting the person from that disease.
Hepatitis B is caused due to hepatitis B virus. It is a serious infection of the liver. This disease can be prevented by taking vaccines.
Hepatitis B vaccine is prepared using the recombinant DNA technology.
Hepatitis B virus has surface antigens. These antigens trigger the immune response in the body. The gene encoding the surface antigen is identified in the virus, then isolated and cloned into a proper vector. After this, the vector is transferred into the host cell for multiplication. The host used here is yeast, Saccharomyces cerevisiae. The yeast is grown in large fermenters to produce these surface antigens. The yeast cells are then harvested and the surface antigen is obtained by rupturing the cells. These surface antigens are then purified using various purification techniques and used as vaccines.
Agrobacterium tumefaciens contains Ti plasmid (Tumour-inducing plasmid, an extrachromosomal circular, double- stranded DNA). When it infects a plant cell, it causes crown gall disease due to the introduction of the Ti plasmid. It is widely used to transfer foreign genes in plants.
Bacillus thuringiensis is a soil bacterium. It contains a gene called the cry gene, which codes for a crystalline protein. This protein is insecticidal in nature. The cry genes from the bacteria are used to create genetically modified crops. Example is Bt brinjal.