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5. Nucleotides and Nucleic Acids — Biochemistry MCQs
KEMU · 158 MCQs with clinical pearls · Prof Jariya
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Sections in this kemu
- Nucleotide Derivatives — 23 MCQs
- Genetic Terminology — 20 MCQs
- Inheritance Patterns and Pedigrees — 17 MCQs
- Nucleotide Structure and Nomenclature — 11 MCQs
- Gene Mutations — 9 MCQs
- Chromatin Structure — 7 MCQs
- Anticancer Drug Mechanisms — 7 MCQs
- Tumour Lysis Syndrome — 7 MCQs
- Chromosome Structure and Karyotype — 7 MCQs
- Mendelian Principles — 6 MCQs
- Polymerase Chain Reaction — 5 MCQs
- Structural Chromosomal Abnormalities — 5 MCQs
- DNA Damage and Repair — 4 MCQs
- Purine Degradation and Gout — 4 MCQs
Sample MCQs with clinical pearls
A 58-year-old male diagnosed with adenocarcinoma of the colon is treated with 5-fluorouracil. This drug inhibits which of the following enzymes?
A) Adenylosuccinate synthetase
B) Carbamoyl phosphate synthetase
C) CTP synthetase
D) Thymidylate synthase ✔
E) UMP synthase
5-FLUOROURACIL is converted to FdUMP, which forms a covalent ternary complex with THYMIDYLATE SYNTHASE and its folate cofactor - a SUICIDE inhibition. Without thymidylate synthase, dUMP cannot be methylated to dTMP, so DNA synthesis halts while RNA synthesis continues. Toxicity falls on rapidly dividing tissue: bone marrow and gut mucosa. Leucovorin paradoxically ENHANCES its action by stabilising that complex. MEMORY: 'No thymidylate synthase, no thymine, no DNA.'
A 60-year-old male receiving 5-fluorouracil as part of chemotherapy. This drug blocks synthesis of which of the following nitrogenous bases?
A) Adenine
B) Cytosine
C) Guanine
D) Thymine ✔
E) Uracil
Thymidylate synthase makes dTMP, so blocking it depletes THYMINE nucleotides specifically. Thymine is the ONLY base unique to DNA - RNA uses uracil instead - which is why 5-FU halts DNA synthesis while leaving RNA largely intact, and why the effect is called THYMINELESS DEATH. Note the methyl group of thymine comes from N5,N10-methylene-THF. MEMORY: 'Thymine is DNA's own base - block it and only DNA suffers.'
A 3-month-old infant presents with repeated infections and failure to thrive diagnosed with SCID. Accumulation of which compound leads to development of symptoms?
A) Adenosine ✔
B) Cytosine
C) Guanosine
D) Thymidine
E) Uridine
In ADA deficiency, ADENOSINE and its precursor deoxyadenosine cannot be deaminated, so they accumulate. Deoxyadenosine is phosphorylated to dATP, which INHIBITS RIBONUCLEOTIDE REDUCTASE, starving the cell of all four deoxyribonucleotides. LYMPHOCYTES are hit hardest because they have the highest deoxynucleoside kinase activity and divide rapidly. MEMORY: 'Adenosine piles up and poisons the enzyme that makes DNA precursors.'
158 MCQs are arranged to the learning outcomes of this kemu.
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