Biochemistry : Nitrogenous Bases

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Nitrogenous Bases

Which statement about the anticodon loop is false?

Possible Answers:

None of the other statements is false.

Anticodons are antiparallel to their corresponding mRNA codon.

One possible base in position one of the anticodon is hypoxanthine.

The anticodon sequence is located on a tRNA hairpin loop.

An anticodon with guanine in position one can appear with any other of the four codons to code for the same amino acid.

Correct answer:

An anticodon with guanine in position one can appear with any other of the four codons to code for the same amino acid.

Explanation:

The type of loop of tRNA at the anticodon sequence is indeed hairpin. Hypoxanthine is occasionally found in tRNA anticodons (as its nucleoside, inosine). tRNA anticodons and mRNA codons are indeed antiparallel (this is central to their functioning). However, just because an anticodon has guanine in position one, doesn’t mean in cannot code with any of the four codons to make the same amino acid; the amino acid produced will be subject to whatever combination of bases is present, as detailed in the genetic code.

Example Question #2 : Nitrogenous Bases

The two pyrimidine bases, which have a one-ring nitrogenous base are __________ and __________.

Possible Answers:

thymine . . . guanine

adenine . . . guanine

cytosine . . . thymine

adenine . . . thymine

cytosine . . . guanine

Correct answer:

cytosine . . . thymine

Explanation:

The two pyrimidine bases are cytosine and thymine. Purine bases are larger in structure than pyrimidines and possess a two-ring nitrogenous base. In RNA, uracil is also a pyrimidine. One way to help us remember which nitrogenous bases are which is to use the mnemonic CUT. Pyramids (like pyrimidines) are "sharp" and thus they CUT, Cytosine, Uracil and Thymine.

Example Question #3 : Nitrogenous Bases

Which of the following statements concerning DNA nucleotides is true?

Possible Answers:

The structure of adenine is identical to guanine with the exception of one additional amino group on carbon #2

DNA nucleotides have a maximum absorbance at 

Pyrimidines have a larger structure than purines

The structure of thymine is identical to cytosine with the exception of one additional methyl group on carbon #5

All DNA nucleotides have tautomeric forms, and the lactim or enol form is more common

Correct answer:

The structure of thymine is identical to cytosine with the exception of one additional methyl group on carbon #5

Explanation:

DNA nucleotides have a maximum absorbance at 280 260nm. Pyrimidines Purines have a larger structure than purines pyrimidines, since purines have two rings in their structures, and the pyrimidines have only one. All DNA nucleotides have tautomeric forms, and the lactim or enol lactam or keto form is more common. The structure of adenine is identical to guanine with the exception of one additional amino group on carbon #2 and a double bonded oxygen replacing the amino group on carbon #6.

Example Question #4 : Nitrogenous Bases

What is the structural difference between thymine and uracil?

Possible Answers:

Thymine has an amino group on the 5' carbon while uracil has a methyl group at this location

Uracil has a methyl group on the 5' carbon where thymine has no substitution on this carbon

Thymine has a methyl group on the 5' carbon where uracil has no substitution on this carbon

Uracil has a methyl group at the 5' carbon while thymine has a carbonyl group at this location

Uracil has an amino group at the 5' carbon while thymine has a methyl group at this location

Correct answer:

Thymine has a methyl group on the 5' carbon where uracil has no substitution on this carbon

Explanation:

Thymine and uracil a both pyrimidines with similar structure but a key difference at the 5' carbon. While this location is methylated in thymine, there is no functional group there in uracil.

Example Question #5 : Nitrogenous Bases

Which of these is a purine?

Possible Answers:

Uracil

Thymine

Cytosine

Ribulose

Adenine

Correct answer:

Adenine

Explanation:

Adenine and guanine are purines (contain two rings). Thymine, cytosine, and uracil are pyrimidines (contain one ring). In normal base pairing, a purine matches to a pyrimidine. Adenine matches to thymine in DNA (uracil in RNA), and cytosine matches to guanine. Ribulose is a carbohydrate.

Example Question #6 : Nitrogenous Bases

Which of the following nitrogenous bases is a purine?

Possible Answers:

Cytosine

Uracil

Adenine

Thymine

Correct answer:

Adenine

Explanation:

In DNA and RNA, there are two types of nitrogenous bases: pyrimidines and purines. A pyrimidine contains one carbon-nitrogen ring with two nitrogen atoms. A purine consists of a pyrimidine fused with an imidazole ring. Adenine and guanine are purines. Cytosine, thymine, and uracil are pyrimidines.

Example Question #7 : Nitrogenous Bases

Which of the following statements is true regarding the condition known as gout?

Possible Answers:

Gout is caused by a deficiency in the enzyme xanthine oxidase.

Gout is caused by excessive production of uric acid.

Fluorouracil is an effective treatment for gout.

Gout is caused by the excessive breakdown of pyrimidine bases.

Correct answer:

Gout is caused by excessive production of uric acid.

Explanation:

When purines are broken down, they go through the intermediates hypoxanthine and xanthine, then are converted to uric acid. Xanthine oxidase is the enzyme that converts xanthine to uric acid. Treatments for gout are suicide inhibitors of xanthine oxidase, lowering the production of uric acid and building up the intermediates. This is preferable, because unlike uric acid, the intermediates are highly soluble.

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