Nucleic Acid Chemistry Questions and Answers

Answer: A

The nucleotide is adenosine monophosphate (AMP).

A nucleotide has three parts: a nitrogenous base, a pentose sugar, and one or more phosphate groups. AMP is adenine attached to ribose, with a phosphate on the 5' carbon.

Uridine is a nucleoside, uracil plus ribose with no phosphate. RNA and DNA are polynucleotides, long chains of nucleotides joined by phosphodiester bonds, so they are built from nucleotides rather than being single examples of one.

Answer: A

The bases in DNA are adenine, cytosine, guanine and thymine.

DNA uses two purines, adenine and guanine, and two pyrimidines, cytosine and thymine, paired A with T and G with C in the double helix. Thymine is 5-methyluracil.

Uracil replaces thymine in RNA, not DNA, so every option that lists uracil is wrong: one also drops cytosine, one drops guanine, and one drops adenine. Only the first option lists all four DNA bases and nothing else.

Answer: B

The compound is a nucleotide.

A nucleotide is a nitrogenous base joined to a pentose sugar (ribose or deoxyribose) that carries one or more phosphate groups on its 5′ carbon. A nucleoside triphosphate such as ATP or dGTP, one base, one five-carbon sugar and a triphosphate, is therefore a nucleotide.

A nucleoside is only the base and the sugar, with no phosphate. 'Base' names just the nitrogenous ring. A nucleic acid is a polymer of many nucleotides joined by phosphodiester bonds, so it contains many sugars, not a single one.

Answer: C

An adenine-thymine pair is held by 2 hydrogen bonds and a guanine-cytosine pair by 3, so the answer is 2 and 3.

In the Watson-Crick A-T pair, the N-H of thymine bonds to a ring nitrogen of adenine and the amino N-H of adenine bonds to a carbonyl oxygen of thymine: two bonds. The G-C pair adds a third, between the amino group of guanine and the carbonyl oxygen of cytosine. That extra bond is one reason DNA rich in G-C pairs melts at a higher temperature.

'3 and 2' reverses the pairs, while '3 and 3' and '2 and 2' ignore the difference between them, so all three are wrong.

Answer: A

The base not present in DNA is uracil.

DNA contains adenine, guanine, cytosine and thymine. RNA contains uracil in place of thymine.

Adenine and guanine (purines) and cytosine and thymine (pyrimidines) are all found in DNA, so they cannot be the answer.

Answer: E

The linkage is a phosphodiester linkage.

In a nucleic acid chain, a phosphate group joins the 5' carbon of one nucleotide's pentose sugar to the 3' carbon of the next. Because the phosphate is esterified to two sugar hydroxyl groups, the link is a phosphodiester, and these bonds form the sugar-phosphate backbone.

The glycosidic linkage joins the base to the 1' carbon of the sugar. A phosphomonoester is a phosphate attached to only one sugar, as at the end of a chain. Amide and peptide bonds join amino acids in proteins, not nucleotides.

Answer: D

The base not present in DNA is Uracil (U).

DNA is built from four nitrogenous bases: adenine, guanine, cytosine and thymine. RNA uses the same first three but has uracil in place of thymine.

Adenine and guanine are the purines found in both nucleic acids, and cytosine is a pyrimidine found in both. So uracil is the only option that is not part of DNA structure.

Answer: A

The atoms come from aspartate (N1, C4, C5, C6), bicarbonate (C2) and glutamine (N3).

In de novo pyrimidine synthesis, carbamoyl phosphate synthetase II makes carbamoyl phosphate from bicarbonate and the amide nitrogen of glutamine; that molecule supplies C2 and N3. Aspartate transcarbamoylase then adds aspartate, which supplies N1, C4, C5 and C6, and the ring closes to form orotate.

The option swapping glutamine and aspartate has the sources reversed. Glutamate is what is left after glutamine gives up its amide nitrogen, so it is not the N3 donor, and asparagine is not used at all.

Answer: D

The end product of AMP and GMP catabolism in humans is uric acid.

Both purine nucleotides are broken down to free bases and then to xanthine: AMP via inosine and hypoxanthine, GMP via guanosine and guanine. Xanthine oxidase then oxidises xanthine to uric acid, which humans excrete because, like other higher primates, they lack urate oxidase (uricase).

Xanthine is an intermediate, not the end product. Urea is the end product of amino acid nitrogen through the urea cycle, and creatinine comes from creatine phosphate in muscle. Excess uric acid is what crystallises in gout.