Biochemistry : Nucleic Acids: DNA and RNA

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Nucleic Acids: Dna And Rna

Which of the following statements is true?

Possible Answers:

DNA is usually double-stranded

RNA is usually single-stranded

DNA is stored in the nucleus of eukaryotic cells

DNA contains uracil; RNA contains thymine

RNA is less stable than DNA

Correct answer:

DNA contains uracil; RNA contains thymine

Explanation:

RNA contains uracil, while DNA contains thymine. All of the other statements are true. Note that DNA is usually double-stranded and RNA is usually single-stranded, but both can exist in the opposite configuration under certain conditions.

Example Question #2 : Nucleic Acids: Dna And Rna

Which of the following components must all free nucleotides contain?

Possible Answers:

5'  group

1'  group

2'  group

3'  group

2'  group

Correct answer:

3'  group

Explanation:

Nucleotides are the macromolecular building blocks of DNA and RNA. They contain a sugar that is attached to a nitrogenous base and a phosphate group. The nitrogenous base that makes up the nucleotide can vary between adenine, guanine, cytosine, thymine (in DNA) and uracil (in RNA). Furthermore, the sugar can also vary between deoxyribose in DNA and ribose in RNA. As nucleotides are added to a growing DNA strand, the enzyme DNA polymerase synthesizes the strand in the 5' to 3' direction. That is to say, the 3' end of the growing strand must have a  group in order for another nucleotide to be added onto it. Thus, the 3' carbon of a free nucleotide must contain a  group. The 1' carbon of the sugar moiety will always be bound to a nitrogenous base. Thus, this position cannot have a  group. The 5' carbon of the sugar moiety is occupied by a phosphate group. The 2' carbon of the sugar moiety can have a  group. In fact, in RNA the 2' position of the sugar has a  group. However, this position in DNA does not have a  group, but instead has a hydrogen bound in this position, hence the name deoxyribose. Thus, the 2' position does not have to have a hydroxyl group.

 

Example Question #3 : Nucleic Acids: Dna And Rna

In humans, DNA exists as double-stranded helices of complimentary strands with the __________ on the outside of the helix and the __________ on the inside of the helix. 

Possible Answers:

nitrogenous bases . . . sugar-phosphate chains

hydrogen bonds . . . nucleotides

sugar-phosphate chains . . . nitrogenous bases

purines . . . pyrimidines

nucleotides . . . hydrogen bonds

Correct answer:

sugar-phosphate chains . . . nitrogenous bases

Explanation:

The DNA structure consists of the nitrogenous bases on the inside of the helix, bound together by hydrogen bonds, and the sugar phosphate backbone outside of the helix, bound together by phosphodiester bonds. This arrangement allows the hydrogen bonding to stabilize the DNA molecule, while making it relatively easy to pull apart when it needs to be replicated/transcribed. 

Example Question #4 : Nucleic Acids: Dna And Rna

Which enzyme is responsible for the unwinding of the the DNA prior to DNA replication via breaking the hydrogen bonds between nitrogenous bases.

Possible Answers:

Replicase

Aldolase

Topoisomerase

Helicase

DNA polymerase

Correct answer:

Helicase

Explanation:

Helicase is used to separate annealed nitrogenous bases of double-stranded DNA in order to allow access by other fundamental enzymes such as DNA polymerase so that replication may proceed. Topoisomerase helps relieve tension of the double helix that arises as a result of separating the strands during replication and/or transcription. Aldolase catalyzes the conversion of fructose-1,6-bisphosphate into triose phosphates during glycolysis. DNA polymerase catalyzes the polymerization of deoxynucleoside triphosphates into deoxynucleoside monophosphates, known as DNA strands.

Example Question #5 : Nucleic Acids: Dna And Rna

Which of the following is true about mRNA?

Possible Answers:

mRNA makes up the majority of a cell’s RNA

mRNA is transcribed from functional genes, and is translated into proteins

All mRNA strands tend to stay intact for about the same length of time

mRNA strands are all about the same size

Correct answer:

mRNA is transcribed from functional genes, and is translated into proteins

Explanation:

Only about 5% of a cell’s RNA consists of mRNA. The highest percentage by type of RNA actually turns out to be rRNA. mRNA strands have great variation in size, from about 100 to more than 20,000 nucleotides. This variation is a reflection of the sizes of the genes which produce the mRNA. mRNA strands stay intact for very different lengths of time; gene expression is regulated by these varying lengths of time. mRNA is indeed transcribed from functional genes, and goes on further to be translated into proteins (DNA  RNA  protein).

Example Question #6 : Nucleic Acids: Dna And Rna

Which of the following statements is false about the double helix of B-DNA (the most commonly found form of DNA)?

Possible Answers:

The sugar-phosphate backbone is on the outside of the molecule.

The adenine and thymine bases are connected by two hydrogen bonds, and the cytosine and guanine are connected by three.

There is a complete turn of the helix at approximately every 10 base pairs.

The two strands are have polarities positioned opposite to each other.

The 5' end contains an  group, and the 3' end has a free phosphate group.

Correct answer:

The 5' end contains an  group, and the 3' end has a free phosphate group.

Explanation:

The 5' end has a phosphate group, while the 3' end has an . The two strands do, indeed, have opposing polarities; that is, the 5' end of one is positioned next to the 3' end of the other. The sugar-phosphate backbone is on the outside of the molecule, giving it a negative charge. DNA turns once typically every 10.4 pairs, and the distance between the center of nucleotide pairs is about 3.4nm. The adenine (A)-thymine (T) base pair is connected by two hydrogen bonds, and cytosine (C)-guanine (G), three.

Example Question #7 : Nucleic Acids: Dna And Rna

Which of the following is false concerning the nucleosome structure?

Possible Answers:

Histone folds have three alpha-helices and two loops.

DNA wraps around the histone core in a right-handed coil.

The DNA that links two nucleosome cores has a maximum length of about 80 base pairs.

Nucleosomes contain histone octamers, with eight histone proteins, as well as double-stranded DNA.

When a DNA molecule forms into a nucleosome, its length is decreased by about two-thirds.

Correct answer:

DNA wraps around the histone core in a right-handed coil.

Explanation:

Nucleosomes represent the first level of chromosome organization, and occur in the interphase of the cell cycle. The nucleosome core has an eight histone-DNA complex, called a histone octamer. The region between two cores includes DNA up to a length of 80 base pairs. This packs the DNA tightly, to about one-third its initial length (a decrease of two-thirds). Histones all have are folded in a manner containing three alpha-helices as two loops. The DNA wraps around the core in a left-handed coil.

Example Question #8 : Nucleic Acids: Dna And Rna

Which of the following shows a correct base pair that can occur in DNA?

Possible Answers:

Cytosine-Thymine

Adenine-Thymine

Guanine-Thymine

Adenine-Guanine

Adenine-Uracil

Correct answer:

Adenine-Thymine

Explanation:

Nucleic acids such as DNA and RNA contain a series of nucleotides, each of which contains a sugar, a nitrogenous base, and a phosphate group. The sugar and phosphate group form the backbone of the linear chain, while the nitrogenous bases are able to protrude out. When hydridizing with other nucleic acid strands, only certain nitrogenous bases can pair with others. This base pairing is due to hydrogen bonds that form between the two nitrogenous bases, and the number of hydrogen bonds differs depending on what bases are involved.

For both DNA and RNA strands, guanine and cytosine will pair with one another via three hydrogen bonds.

In RNA, the nucleotide thymine is absent, but in its place is the nucleotide uracil. Uracil is able to base pair with adenine via two hydrogen bonds, but this only happens in RNA, not DNA!

In DNA, thymine is present rather than uracil. The thymine found in DNA is also able to base pair with adenine via two hydrogen bonds. Thus, out of all the answer choices, this is the only correct one that can occur within DNA.

Example Question #9 : Nucleic Acids: Dna And Rna

Which of the following are true regarding nucleic acids?

I. All types of nucleic acids contain a pentose sugar

II. Increased degradation of DNA and RNA produces uric acid

III. A monomer of a nucleic acid is called a nucleoside

Possible Answers:

I only

III only

I and II

II and III

Correct answer:

I and II

Explanation:

Nucleic acids are made up of nucleotides that contain a pentose sugar, a phosphate group, and a nitrogenous base. There are two types of nucleic acids; DNA and RNA. The difference between these two nucleic acids is their pentose sugar. DNA has deoxyribose whereas RNA has ribose sugar residues. Note that both are types of pentose sugar; therefore, all types of nucleic acids have pentose sugars.

Nitrogenous bases found in nucleic acids are classified as either purines or pyrimidines. Free purines in the blood are broken down to uric acid in the liver. Recall that purines (guanine and adenine) are found in both RNA and DNA; therefore, breakdown of either type of nucleic acid will lead to release of purines and, subsequently, formation of uric acid in the liver.

As mentioned, nucleic acids are made up of monomers called nucleotides. Nucleosides, on the other hand, only have a pentose sugar and nitrogenous base (they lack the phosphate group); therefore, they are not the monomers of nucleic acids.

Example Question #10 : Nucleic Acids: Dna And Rna

A researcher is analyzing a molecule. He identifies it as a monomer of nucleic acids but can’t determine the type. Upon further analysis he observes that the molecule contains a phosphate group and guanine. What can the researcher conclude from the given information?

Possible Answers:

The molecule makes up RNA because it has a phosphate group

The molecule cannot be a monomer of nucleic acid because it has a phosphate group

The molecule makes up DNA because it has a phosphate group

The type of nucleic acid monomer cannot be determined from the given information

Correct answer:

The type of nucleic acid monomer cannot be determined from the given information

Explanation:

To answer this question we need to recall that the monomer of nucleic acids is a nucleotide. Nucleotides are made up of three molecules: pentose sugar, phosphate group and nitrogenous base. The question tells us that the monomer has a phosphate group and a guanine (nitrogenous base); however, we are not given any information regarding the pentose sugar. Recall that the two types of nucleic acids (DNA and RNA) are distinguished from each other by the pentose sugar. DNA has deoxyribose whereas RNA has ribose. Since we do not know the type of pentose sugar, we cannot determine the monomer’s identity.

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