Biochemistry : Identifying Specific Nucleic Acid Structures

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #84 : Identification By Structure

Which of the following nucleotides are purines?

Possible Answers:

Cytosine and thymine

Adenine and cytosine

Thymine and guanine

Cytosine and adenine

Adenine and guanine

Correct answer:

Adenine and guanine

Explanation:

Adenine and guanine are purines, while cytosine and thymine are pyrimidines.  A helpful mnemonic for this is Pure (purine) As (adenine) Gold (guanine).  Note that uracil is a pyrimidine, but is only seen in RNA.

Example Question #85 : Identification By Structure

In the eukaryotic ribosome, the sedimentation coefficient for the small subunit is __________ and large subunit is __________.

Possible Answers:

35S . . . 70S

40S . . . 70S

20S . . . 60S

40S . . . 60S

60S . . . 80S

Correct answer:

40S . . . 60S

Explanation:

The 80S eukaryotic ribosome is made up of 40S and 60S subunits.

Example Question #91 : Identification By Structure

In the prokaryotic ribosome, the sedimentation coefficient for the small subunit is __________ and large subunit is __________.

Possible Answers:

40S . . . 60S

30S . . . 40S

20S . . . 60S

20S . . . 40S

30S . . . 50S

Correct answer:

30S . . . 50S

Explanation:

The 70S prokaryotic ribosome is made up of 30S and 50S subunits.

Example Question #351 : Biochemistry

Which prokaryotic translation initiation factor (IF) forms a ternary complex with mRNA, fMet-tRNA, and GTP?

Possible Answers:

IF-3

IF-5

IF-4

IF-2

IF-1

Correct answer:

IF-2

Explanation:

IF-3 binds to the 30S subunit first. Then IF-2 forms the ternary complex, and it binds with IF-1 to the 30S subunit. Upon release of IF-1 and IF-3, the 50S subunit will bind to prepare for translation.

Example Question #21 : Identifying Specific Nucleic Acid Structures

Which of the following is not considered to be a pyrimidine derivative?

Possible Answers:

Cytosine

Thymine and adenine

Guanine and adenine

Thymine

Uracil

Correct answer:

Guanine and adenine

Explanation:

The nitrogenous bases that are considered to be pyrimidine derivatives are thymine, cytosine, and uracil. Thus, the purines are adenine and guanine. The mnemonic Pure As Gold (Purines = Adenine and Guanine) can be used to remember which bases are purines.

Example Question #351 : Biochemistry

Which of the following components might be found in a nucleoside molecule?

I. Phosphate group

II. Pentose

III. Uracil

Possible Answers:

II only

II and III

I and II

I only

Correct answer:

II and III

Explanation:

Nucleotides are monomers of nucleic acids and are made up of three molecules: nitrogenous base (uracil, thymine, cytosine, adenine or guanine), pentose sugar (deoxyribose for DNA and ribose for RNA) and phosphate group. Nucleosides, on the other hand, are molecules that only contain nitrogenous base and pentose sugar; they do not contain phosphate groups.

Example Question #95 : Identification By Structure

A researcher is analyzing two nucleic acids. He observes that the molecular weight of molecule A is three times as much as molecule B. Based on the given information, which of the following can you conclude?

Possible Answers:

Molecule A has uracil while molecule B doesn’t

Molecule B has more adenine and guanine

Molecule A has more adenine and guanine

Cannot conclude any of these based on the given information

Correct answer:

Cannot conclude any of these based on the given information

Explanation:

The question states that molecule A is heavier than molecule B. This could result from several reasons. First, molecule A could be a double stranded DNA and molecule B could be a single stranded RNA. This will allow the DNA molecule to be a lot heavier than its RNA counterpart. Second, molecule A could contain more purines (adenine and guanine) than molecule B). Recall that purines are two membered rings whereas pyrimidines (uracil, cytosine and thymine) are one membered rings. This means purines have more atoms and, therefore, have a higher molecular weight.

We are not given enough information to determine the identity or the distribution of nitrogenous bases in molecule A and B; therefore, we cannot conclude any of the given statements.

Example Question #91 : Identification By Structure

Compared to an RNA molecule, a DNA molecule has __________ thymine molecules and __________ deoxygenated sugars.

Possible Answers:

fewer . . . fewer

more . . . more

more . . . fewer

fewer . . . more

Correct answer:

more . . . more

Explanation:

DNA and RNA molecules contain four types of nitrogenous bases. Guanine, cytosine and adenine are found in both whereas thymine is only found in DNA and uracil is only found in RNA. This means that DNA molecules will have more Thymine bases than RNA molecules. Another difference between DNA and RNA molecule is the type of pentose sugar. RNA contains ribose sugar whereas DNA contains deoxyribose. Deoxyribose contains one less hydroxyl group than ribose; therefore, DNA molecules have more deoxygenated sugars than RNA.

Example Question #92 : Identification By Structure

Which of the following are pyrimidines?

Possible Answers:

Adenine (A) and guanine (G)

Cytosine (C), uracil (U), and thymine (T) 

Cytosine (C) and uracil (U) 

Adenine (A), guanine (G), and uracil (U) 

Correct answer:

Cytosine (C), uracil (U), and thymine (T) 

Explanation:

Think of it as this. CUT the PY (pie) PYrimidines. PURe As Gold. PURines. A will pair with T (and U in RNA). G will pair with C. The deamination of cytosine makes uracil, which is in RNA. 

Example Question #91 : Identification By Structure

All of the following amino acids are chiral except __________.

Possible Answers:

alanine

threonine

cysteine

glycine

proline

Correct answer:

glycine

Explanation:

In a biochemistry course, we often rely on important information that was learned during general and organic chemistry. One important concept to remember is chirality. A molecule is chiral if it is not superimposable on its mirror image. These molecules lack a plane of symmetry.  

It is a common mistake for students to think that all amino acids are chiral. While most amino acids are chiral, glycine is in fact achiral. Remember, the side chain on glycine is simply a hydrogen. The central carbon therefore is connected to two hydrogens, and is not a stereocenter.

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