CBSE Biochem Exam Questions with 100% Correct Answers

Lesch-Nyhan Syndrome: what causes it and what are manifestations? Correct Answer HGPRT deficiency in purine salvage pathway (this enzyme normally converts hypoxanthine to IMP and guanine to GMP). This deficiency leads to excessive Uric acid production and de novo purine synthesis. It is X linked recessive. Symptoms: hyperuricemia, gout, aggression / self mutilation, mental retardation, dystonia. Treatment for Lesch-Nyhan? Correct Answer Allopurinol / febuxostat second line. Both of these drugs inhibit xanthine oxidase, which converts hypoxanthine to xanthine and xanthine to uric acid. Adenosine Deaminase Deficiency: what is it, what effects does it have? Correct Answer ADA is required for degradation of adenosine and deoxyadenosine--dATP builds up and is toxic to lymphocytes, which is why this disease is a major cause of SCID--severe combined immunodeficiency (Autosomal recessive). Why is purine salvage important? Correct Answer Because in some tissues, de novo purine synthesis is not possible. How are nucleotide monophosphates created? Correct Answer Activated ribose-5-phosphate (PRPP) is added to bases by phosphoribosyltransferases (HGPRT and APRT are the two types of this enzyme). tRNA Wobble: what is it and it causes which type of mutation? Correct Answer An unusual pairing of bases in the third position of the codon--causes silent mutation. Sickle Cell Anemia is caused by which type of mutation? Correct Answer Missense mutation--valine replaces glutamic acid. Tay-Sachs and Duchenne Muscular Dystrophy are caused by which mutation type? Correct Answer Frameshift Xeroderma Pigmentosum: is caused by a defect in what specifically? Correct Answer Nucleotide excision repair--pyrimidine dimers are not repaired (caused by UV exposure). In nucleotide excision repair, bulky helix-disturbing lesions are repaired by endonucleases. This occurs during G1 of cell cycle. How is spontaneous / toxic deamination repaired? Correct Answer Base excision repair--involves AP endonucleases throughout the cell cycle