GRE Subject Test: Biology : Understanding Nucleic Acids

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #34 : Cell Biology

Pick the reason that is least likely to explain why two purines will never be seen attached to each other in a DNA helix.

Possible Answers:

Two purines could cause a bump in the DNA, causing problems with transcription and replication.

The functional groups at the end of one purine would not correctly match with the other purine. 

Purine bases will never be found on opposite DNA strands, so they do not have the ability to pair with one another.

The bulky two-ring structure of purines would cause too much hindrance in the inside of the helix.

Correct answer:

Purine bases will never be found on opposite DNA strands, so they do not have the ability to pair with one another.

Explanation:

DNA strands are composed of millions of nucleotides. As a result, it would be virtually impossible to find a single strand that did not have all four nucleotides.

Nucleotides combine in purine-pyrimidine pairs due to the sterically appropriate fit of the bases, as well as the preferred combination of hydrogen bonds between the two nucleotides. As a result, two purines would not be seen combined. This is due to both being too large when together, and the incorrect hydrigen bonding between their functional groups.

Example Question #1 : Understanding Nucleic Acids

Which of the following is not true of nucleic acids?

Possible Answers:

Both DNA and RNA have nucleotides held together by phosphodiester bonds

Only RNA has a hydroxide group attached to the 2' carbon

Only DNA is read in the 5'-to-3' direction

ATP and GTP are nucleic acid derivatives

Correct answer:

Only DNA is read in the 5'-to-3' direction

Explanation:

DNA and RNA share very similar structures, with two primary differences: DNA lacks a hydroxide group on the 2' carbon of the ribose sugar and RNA uses uracil in place of thymine.

Both DNA and RNA have phosphate groups attached to the 5' carbon of the sugar, which can be joined to the 3' carbon of an adjacent nucleotide by a phosphodiester bond. As a result, both RNA and DNA are read in the 5'-to-3' direction.

Adenosine triphosphate (ATP) and guanosine triphosphate (GTP) are derived from adenine and guanine, two of the fundamental nitrogenous bases in nucleic acids, making them nucleic acid derivatives.

Example Question #2 : Understanding Nucleic Acids

What is the main difference between DNA nucleotides and RNA nucleotides?

Possible Answers:

DNA nucleotides are bound by phosphodiester bonds, but RNA nucleotides are bound by glycosidic bonds

RNA nucleotides have five-carbon sugars, while DNA nucleotides have six-carbon sugars

DNA has uracil, while RNA has thymine 

RNA nucleotide bases pair via hydrogen bonds, but DNA nucleotide bases do not 

RNA nucleotides have two hydroxide groups on the sugar, but DNA nucleotides have only one

Correct answer:

RNA nucleotides have two hydroxide groups on the sugar, but DNA nucleotides have only one

Explanation:

The correct answer is that RNA nucleotides have two hydroxide groups on the sugar, whereas DNA nucleotides have only one hydroxide group. RNA uses uracil in place of thymine; not DNA. Both DNA and RNA have five-carbon sugars and are bound together along the backbone by phosphodiester bonds. Though base pairing is more common in DNA (double-stranded RNA is less common), both utilize hydrogen bonding.

Example Question #3 : Understanding Nucleic Acids

Which of the following are components of a nucleotide?

I. Nitrogenous base

II. Glucose sugar

III. Ribose sugar

IV. Three sulfates

V. Three phosphates

Possible Answers:

I, II, III, IV, and V

I, II, and V

I, III, and V

I, III, and IV

I, II, and IV

Correct answer:

I, III, and V

Explanation:

A nucleotide consists of a nitrogenous base (adenine, guanine, cytosine, thymine, or uracil), a pentose sugar (either ribose or deoxyribose), and three phosphates. These nucleotide monomers can be strung together via phosphodiester linkages to form a polynucleotide. This polynucleotide can base pair with another polynucleotide through hydrogen bonding to form double-stranded DNA.

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