Biochemistry Nucleotide Chemistry and Metabolism 1 — Questions and Answers
Question 1: Which of the following nitrogenous bases is classified as a purine?
- Cytosine
- Adenine (Correct answer)
- Uracil
- Thymine
Correct answer: Adenine
Adenine and guanine are purines, characterized by a bicyclic structure of a pyrimidine ring fused to an imidazole ring, while cytosine, uracil, and thymine are monocyclic pyrimidines.
Question 2: What distinguishes a nucleotide from a nucleoside?
- Nucleosides contain a phosphate group; nucleotides do not
- Nucleotides contain one or more phosphate groups; nucleosides do not (Correct answer)
- Nucleosides contain deoxyribose; nucleotides contain ribose
- Nucleotides are found only in DNA; nucleosides are found only in RNA
Correct answer: Nucleotides contain one or more phosphate groups; nucleosides do not
A nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups, whereas a nucleoside contains only the base and the sugar without any phosphate.
Question 3: In the DNA double helix, adenine forms hydrogen bonds with which complementary base?
- Guanine
- Cytosine
- Thymine (Correct answer)
- Uracil
Correct answer: Thymine
Adenine pairs with thymine in DNA via two hydrogen bonds (A=T), following Chargaff's rules, while guanine pairs with cytosine via three hydrogen bonds.
Question 4: Which pyrimidine base is present in RNA but absent from DNA?
- Thymine
- Cytosine
- Uracil (Correct answer)
- Hypoxanthine
Correct answer: Uracil
Uracil replaces thymine in RNA; thymine contains a methyl group at C-5 that uracil lacks, and uracil pairs with adenine in RNA just as thymine does in DNA.
Question 5: De novo purine nucleotide synthesis begins with which metabolite as the core scaffold?
- Hypoxanthine
- Ribose-5-phosphate (Correct answer)
- Xanthine
- Inosine monophosphate (IMP)
Correct answer: Ribose-5-phosphate
De novo purine synthesis starts with ribose-5-phosphate (from the pentose phosphate pathway), which is converted to PRPP (5-phosphoribosyl-1-pyrophosphate), the activated ribose donor for ring assembly.
Question 6: Lesch-Nyhan syndrome results from a deficiency of which enzyme in purine metabolism?
- Adenosine deaminase
- Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) (Correct answer)
- Purine nucleoside phosphorylase
- Xanthine oxidase
Correct answer: Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
HGPRT deficiency impairs the purine salvage pathway, leading to massive uric acid overproduction, causing gout, intellectual disability, and self-mutilating behavior characteristic of Lesch-Nyhan syndrome.
Question 7: What is the final end product of purine catabolism in humans?
- Allantoin
- Uric acid (Correct answer)
- Xanthine
- Hypoxanthine
Correct answer: Uric acid
Humans lack uricase and therefore cannot convert uric acid to allantoin as most other mammals do; uric acid is thus the terminal product of purine breakdown, and its accumulation causes gout.
Biochemistry Nucleotide Chemistry and Metabolism 1
Which of the following nitrogenous bases is classified as a purine?