Explanation

(i) $\rightarrow$ (ii) $\rightarrow$ (iii) $\rightarrow$ (iv). Adenosine deaminase enzyme is very important for the immune system to function. In the absence of adenosine deaminase enzyme, purine metabolism is disturbed and T-lymphocytes fails to function. ADA deficiency can lead to Severe combiuned Immune Deficiency (SCID) SCID is caused due to defect in the genes for the enzyme adenosine deaminase. The genetic diseases that are being investigated for gene therapy ranges from sickle-cell anaemia to Severe Combined Immuno Deficiency (SCID). In some children, ADA deficiency can be cured by bone marrow transplantation However, in others it can be treated by the enzyme replacement therapy, in which functional ADA is given to the patient by injection. But in both approaches, the patients are not completely cured. For permanent cure, gene isolated from the bone marrow cells producing ADA at early embryonic stage can be a possible cure