Biochemistry : Nucleic Acid Structures and Functions

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #111 : Biochemistry

What type of bonds are found between the DNA sugar hydroxyl groups? What are their corresponding carbon numbers?

Possible Answers:

Phosphodiester; 2', 4'

Phosphoester; 1', 5'

Phosphodiester; 1', 3'

Phosphodiester; 3', 5'

Phosphoester; 3', 5'

Correct answer:

Phosphodiester; 3', 5'

Explanation:

The phosphodiester bonds in DNA occur between the 3' and 5' hydroxyl groups on deoxyribose. (This is related to DNA's 5' to 3' directionality as DNA polymerase can only synthesize DNA by adding nucleotides to the 3' hydroxyl group).

Example Question #1 : Sugar Phosphate Groups And Phosphodiester Bonds

In DNA, the 5-carbon sugar is attached to the nitrogenous base by a __________.

Possible Answers:

glycosidic bond

phosphodiester bond

deoxyribose

ester

phosphoester bond

Correct answer:

glycosidic bond

Explanation:

The beta-N-glycosidic bond attaches the nitrogen on the purine or pyrimidine base to the 1' anomeric carbon on the deoxyribose sugar. Phosphodiester linkages connect the 3' and 5' sugar hydroxyl groups on adjacent nucleotides.

Example Question #1 : Nucleic Acid Structures

Which of the following is not a reason that the anhydride linkage of pyrophosphate stores so much energy?

Possible Answers:

Orthophosphate by itself has more favorable interactions with water than two orthophosphates linked together (pyrophosphate).

The negative charges on the on the orthophosphates repel each other.

Phosphorus on pyrophosphate satisfies the octet rule, but does not on orthophosphate.

Pyrophosphate has less resonance forms than orthophosphate groups by themselves.

All of the choices are true

Correct answer:

Phosphorus on pyrophosphate satisfies the octet rule, but does not on orthophosphate.

Explanation:

Phosphorus on both orthophosphate and pyrophosphate is surrounded by 10 electrons. This does not satisfy the octet rule in either case, so that answer choice is false, and therefore the correct answer. With four negative charges close by, pyrophosphate is very unstable, and releases a lot of energy when cleaved. Finally, the two orthophosphates formed from a single pyrophosphate gain an additional resonance structure.

Example Question #1 : Nucleic Acid Structures

What type of bond holds the phosphate group to the sugar group in DNA's backbone?

Possible Answers:

Hydrogen bond

Glycosidic bond

Ionic bond

Phosphodiester bond

Covalent bond

Correct answer:

Phosphodiester bond

Explanation:

The type of bond that holds the phosphate group to the sugar in DNA's backbone is called a phosphodiester bond. Hydrogen bonds connect bases to one another and glycosidic bonds occur between deoxyribose groups and the base groups.

Example Question #1 : Nitrogenous Bases

Which statement about the anticodon loop is false?

Possible Answers:

Anticodons are antiparallel to their corresponding mRNA codon.

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

None of the other statements is false.

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

The anticodon sequence is located on a tRNA hairpin loop.

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 #1 : Nitrogenous Bases

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

Possible Answers:

adenine . . . guanine

cytosine . . . guanine

thymine . . . guanine

adenine . . . thymine

cytosine . . . thymine

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 #7 : Nucleic Acid Structures And Functions

Which of the following statements concerning DNA nucleotides is true?

Possible Answers:

DNA nucleotides have a maximum absorbance at 

Pyrimidines have a larger structure than purines

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

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 #1 : Nitrogenous Bases

What is the structural difference between thymine and uracil?

Possible Answers:

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

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

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

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 #2 : Nitrogenous Bases

Which of these is a purine?

Possible Answers:

Uracil

Adenine

Cytosine

Ribulose

Thymine

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 #10 : Nucleic Acid Structures And Functions

Which of the following nitrogenous bases is a purine?

Possible Answers:

Cytosine

Adenine

Uracil

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.

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