Basics of Genetics - Nucleotide Metabolism and its Disorders MCQs

Explanation: Cytosine is a pyrimidine base, and during its catabolism, it is first deaminated to uracil, which is then further broken down via the pyrimidine degradation pathway. One of the final products of uracil (and thus cytosine) metabolism is β-alanine. This distinguishes pyrimidine breakdown from purine metabolism, where the final product is uric acid, and from muscle metabolism, where creatinine is formed

Option A: Xanthine oxidase enzyme convert Hypoxanthine into Xanthine and Xanthine into Uric acid. This enzyme presents in mammals and humans both, so both breakdown purines up to uric acid. Option B: HGPRTase is an enzyme involved in the salvage pathway. It converts Guanine & Hypoxanthine into GMP & IMP. Its deficiency can lead to Lesch Nyhan syndrome. Option C: Urease catalyzes the hydrolysis of Urea into ammonia and carbon dioxide. It is also not present in humans, so human urea is excreted in urine. Option D: Uricase enzyme convert uric acid into allantoin. Allantoin is more water soluble than uric acid. But in human uricase enzyme is absent, so the end product of purine catabolism is uric acid. Uricase is present in mammals, so they convert uric acid into allantoin. Reference: Harper's illustrated biochemistry 32 nd edition CH 33 PN 334

Explanation: Ribonucleotide reductase is the key enzyme that converts ribonucleotides (RNA precursors) into deoxyribonucleotides (DNA precursors), making it essential for DNA synthesis and repair. Ribonucleotide phosphorylase is involved in the breakdown of ribonucleosides during nucleotide metabolism. Nucleoside phosphorylase functions mainly in the salvage pathway by converting nucleosides into nitrogenous bases and sugar phosphates, helping recycle nucleotides. Nucleotidase simply removes phosphate groups from nucleotides to convert them into nucleosides and is mainly involved in nucleotide degradation and regulation.

The chemical nature of the monomeric deoxynucleotide units of DNA are deoxyadenylate, deoxyguanylate (Option C), deoxycytidylate (Option D), and thymidylate (Option B). But it does not contain deoxyuridylate (Option A). These monomeric units of DNA are held in polymeric form by 3′,5′-phosphodiester bonds constituting a single strand of DNA. So Option A is the correct answer, rest Option B, Option C and Option D are not correct. Reference: Harper's illustrated biochemistry 32 nd edition CH 34 PN 338

Ans. is 'd' i.e., Glycine [Ref Harper 29th/e p. 336-337] Amino acids involved in purine synthesis → Glycine, aspartate, glutamine. Amino acids involved in pyrimidine synthesis → Glutamine, aspartic acid (aspartate)

Explanation: HGPRT deficiency causes Lesch Nyhan syndrome , an X-linked recessive disorder in which failure of the purine salvage pathway leads to excess uric acid production and neurological symptoms like developmental delay and self-mutilation. Gout involves hyperuricemia and is primarily linked to purine metabolism and the overproduction or underexcretion of uric acid. Ornithine transcarbamylase deficiency is a urea cycle disorder leading to hyperammonemia and Adenosine deaminase deficiency is purine metabolism disorder but results in severe combined immunodeficiency (SCID).

Ans. is 'c' i.e., Fatty acid [Ref Harper's 28"/e p. 286] Nucleotides are monomeric units of nucleic acids. They are required for synthesis of nucleic acid. Each nucleotide is made up of : (i) A nitrogenous base, (ii) A sugar (pentose sugar) and (iii) A phosphate group (phosphoric acid). Nitrogenous base combines with a sugar to form nucleosides. The nucleoside combines with phosphoric acid to form a nucleotide.

Explanation: Option wise explanation Methotrexate inhibits dihydrofolate reductase, an enzyme required to convert dihydrofolate(DHF) into tetrahydrofolate(THF). THF is essential for one-carbon transfer reactions involved in the synthesis of purines and thymidine. By blocking DHFR, methotrexate depletes THF, leading to reduced availability of both purines and thymidine, which impairs DNA synthesis and cell division. This is why it's effective in rapidly dividing cells like cancer and in autoimmune disorders.