(a) Plasmids are extra-chromosomal, self-replicating, usually circular double-stranded DNA molecules found in bacteria and in some yeast.
(b) ADA is adenosine deaminase deficiency, this enzyme is crucial for the immune system to function. The patient lacks functional T-lymphocytes and fails to fight the infecting pathogens.
Children with ADA deficiency are cured by bone marrow transplantation or enzyme replacement therapy, where ADA is given by injection. By using gene therapy techniques, Iymphocytes are taken from the patient's bone marrow and the normal gene for ADA is introduced into the lymphocytes using retrovirus. These cells are re-introduced in the patient's immune system.
As these cells are not immortal, the patient requires periodic infusion of such genetically engineered lymphocytes. Hence, it is not a permanent cure. If the functional gene is introduced into the bone marrow cells at early embryonic stage, it would be a permanent cure.
