Biochemistry Nucleotide Chemistry and Metabolism

Question 1

Which of the following nitrogenous bases is classified as a purine?

Accepted Answer

Adenine

Answer

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?

Accepted Answer

Nucleotides contain one or more phosphate groups; nucleosides do not

Answer

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?

Accepted Answer

Thymine

Answer

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?

Accepted Answer

Uracil

Answer

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?

Accepted Answer

Ribose-5-phosphate

Answer

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.

Biochemistry Practice Test

Biochemistry Practice Test

Biochemistry Nucleotide Chemistry and Metabolism