Q: Which molecule acts as a one-carbon donor in purine nucleotide synthesis?

N\textsuperscript{10}-formyl-tetrahydrofolate ($N^{10}$-formyl-THF)

Question 1

Which of the following statements best describes the difference between purine and pyrimidine bases in DNA and RNA?

Accepted Answer

DNA contains thymine (T) and RNA contains uracil (U) as their respective pyrimidine bases.

Question 2

Which enzyme catalyzes the committed step in de novo purine nucleotide synthesis, and what is its key regulatory mechanism?

Accepted Answer

Glutamine:phosphoribosylpyrophosphate amidotransferase (GPAT); inhibited by AMP, GMP, and activated by PRPP.

Question 3

A patient presents with hyperuricemia and gouty arthritis. Genetic analysis reveals a deficiency in hypoxanthine-guanine phosphoribosyltransferase (HGPRT). Which syndrome is most likely?

Accepted Answer

Lesch-Nyhan syndrome

Question 4

Which of the following is the final product of purine degradation in humans?

Accepted Answer

Uric acid

Question 5

Which of the following equations correctly represents the reaction catalyzed by nucleoside monophosphate kinase for AMP?

Accepted Answer

$AMP + ATP \rightarrow 2 ADP$

Question 6

Which molecule acts as a one-carbon donor in purine nucleotide synthesis?

Accepted Answer

N	extsuperscript{10}-formyl-tetrahydrofolate ($N^{10}$-formyl-THF)

Question 7

Which enzyme is responsible for the reduction of ribonucleoside diphosphates to deoxyribonucleotides during DNA synthesis?

Accepted Answer

Ribonucleotide reductase

Question 8

A patient with adenosine deaminase (ADA) deficiency is unable to produce deoxyribonucleotides. What is the most likely clinical consequence?

Accepted Answer

Severe combined immunodeficiency (SCID)

Question 9

Which of the following drugs inhibits dihydrofolate reductase, thereby blocking the regeneration of tetrahydrofolate and ultimately DNA synthesis?

Accepted Answer

Methotrexate

Question 10

Which of the following is a key regulatory step in pyrimidine nucleotide synthesis, and what molecule inhibits this step?

Accepted Answer

Carbamoyl phosphate synthetase II; inhibited by UTP

Question 11

Which of the following best explains why uric acid crystals precipitate in joints during gout?

Accepted Answer

Uric acid has low solubility, and when its concentration exceeds the saturation point ($6.5\,	ext{mg/dL}$), crystals form.

Question 12

Which of the following is a direct substrate for thymidylate synthase in the synthesis of dTMP?

Accepted Answer

dUMP

Question 13

Which of the following is a function of nucleotides in cellular metabolism?

Accepted Answer

They act as carriers of activated intermediates in the synthesis of carbohydrates, lipids, and conjugated proteins.

Question 14

Which of the following statements about the salvage pathway for purines is correct?

Accepted Answer

It uses PRPP and preformed bases to synthesize nucleotides.

Question 15

Which of the following reactions is catalyzed by xanthine oxidase in purine degradation?

Accepted Answer

$Xanthine \rightarrow Uric\ acid$

Question 16

Which of the following is a consequence of increased PRPP synthetase activity due to a genetic mutation?

Accepted Answer

Increased de novo purine synthesis and hyperuricemia

Question 17

Which of the following is the correct sequence of events in the degradation of dietary nucleic acids in the small intestine?

Accepted Answer

Pancreatic nucleases hydrolyze nucleic acids to oligonucleotides, which are then degraded to nucleosides and finally to free bases.

Question 18

Which of the following drugs is a structural analog of hypoxanthine and inhibits uric acid synthesis by blocking xanthine oxidase?

Accepted Answer

Allopurinol

Question 19

Which of the following statements about pyrimidine degradation is correct?

Accepted Answer

It produces highly soluble products such as $\beta$-alanine and $\beta$-aminoisobutyrate.

Nucleotide Metabolism: Structure, Synthesis, and Regulation

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