High School Biology : Understanding the DNA Backbone

Study concepts, example questions & explanations for High School Biology

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Example Questions

Example Question #1 : Understanding The Dna Backbone

Which sugar is found in the back bone of DNA?

Possible Answers:

Deoxyribose

Dextran

Ribose

Dextrose

Correct answer:

Deoxyribose

Explanation:

DNA stands for "deoxyribonucleic acid." The backbone of DNA is comprised of alternating sugar and phosphate units, in which the sugar is deoxyribose. The backbone of RNA is also comprised of sugar and phosphate units, but uses the sugar ribose.

Example Question #2 : Understanding The Dna Backbone

Which of the following might you find in the backbone of DNA?

I. Phosphate group

II. Hexose sugar

III. Adenine

Possible Answers:

I only

I and II

II only

I, II, and III

Correct answer:

I only

Explanation:

A DNA molecule has two primary structural domains: the DNA backbone and the DNA bases. Recall that all DNA molecules are made from nucleotides. One nucleotide of a DNA molecule consists of a phosphate group, a pentose (five-carbon) sugar called deoxyribose, and a nitrogenous base (adenine, thymine, guanine, cytosine). Several of these nucleotide monomers are joined together by phosphodiester bonds to create a DNA molecule.

The backbone of a DNA molecule consists of the phosphate groups and the deoxyribose sugars, whereas the base region of the DNA molecule consists of the nitrogenous bases; therefore, the backbone of DNA is made up of phosphate groups and pentose sugars. Adenine is part of the base region of the molecule. DNA does not contain any hexose (six-carbon) sugars.

Example Question #3 : Understanding The Dna Backbone

DNA splicing is the process of removing DNA from one organism and inserting it into a new organism's genome. Which of the following is required to cut DNA molecules in this process?

Possible Answers:

Peptidase is needed to break the bonds between the phosphate group on the 3' carbon of one sugar and hydroxyl group on the 5' carbon of the adjacent sugar

Peptidase is needed to break the bonds between the phosphate group on the 5' carbon of one sugar and hydroxyl group on the 3' carbon of the adjacent sugar

Phosphodiesterase is needed to break the bonds between the phosphate group on the 3' carbon of one sugar and hydroxyl group on the 5' carbon of the adjacent sugar

Phosphodiesterase is needed to break the bonds between the phosphate group on the 5' carbon of one sugar and hydroxyl group on the 3' carbon of the adjacent sugar

Correct answer:

Phosphodiesterase is needed to break the bonds between the phosphate group on the 5' carbon of one sugar and hydroxyl group on the 3' carbon of the adjacent sugar

Explanation:

A DNA molecule is made up of multiple nucleotides that are connected by phosphodiester bonds. A nucleotide consists of a phosphate group, a pentose sugar (deoxyribose in DNA), and a nitrogenous base. The phosphodiester bond occurs between the phosphate group of one nucleotide and the hydroxyl group of the adjacent nucleotide. Recall that the phosphate group is always attached to the 5' carbon on the pentose sugar. There are multiple hydroxyl groups in a pentose sugar, but the hydroxyl group involved in the phosphodiester bond is attached to the 3' carbon; therefore, the phosphodiester bond occurs between a 5' phosphate group and a 3' hydroxyl group. To cut DNA molecules, you need to break these phosphodiester bonds, which is accomplished by the enzyme phosphodiesterase.

Peptide bonds are found in proteins. They are bonds that join adjacent amino acids together and are involved in the formation of a polypeptide (protein) chain. Peptidase proteins are used to break these bonds, effectively cutting proteins, not DNA.

Example Question #4 : Understanding The Dna Backbone

What can you conclude about the DNA backbones in a double-stranded DNA molecule?

Possible Answers:

The two strands are parallel; the 5' end on one strand contains a phosphate group, whereas the 5' end on the other contains a hydroxyl group

The two strands are antiparallel; the 5' end on one strand contains a phosphate group, whereas the 5' end on the other contains a hydroxyl group

The two strands are antiparallel and the 5' end on both strands contain a phosphate group

The two strands are parallel and the 5' ends on both strands contain a phosphate group

Correct answer:

The two strands are antiparallel and the 5' end on both strands contain a phosphate group

Explanation:

In the nucleus, DNA is always found as a double-stranded molecule. This means that one DNA molecule consists of two DNA strands. Each strand is made up of a DNA backbone (the phosphate groups and the pentose sugars) and the bases.

In a DNA molecule, the two strands are organized in such a way that the DNA backbone of one strand runs in the 5'-to-3' direction, whereas the DNA backbone of the other strand runs in the 3'-to-5' direction; therefore, the two strands are antiparallel to each other.

Recall that 5' and 3' refer to the carbons on the pentose sugar. A phosphate group is found on the 5' carbon of the sugar and a hydroxyl group is found on the 3'carbon of the sugar. This means that the 5' end of each strand is always characterized by the phosphate group, and the 3' end is always characterized by the hydroxyl group; therefore, both strands will have a phosphate group at their 5' end.

Example Question #5 : Understanding The Dna Backbone

What is the main difference between the backbone of an RNA molecule and the backbone of a DNA molecule?

Possible Answers:

The RNA backbone has uracil instead of thymine

The RNA backbone contains a pentose sugar, whereas the DNA backbone contains a hexose sugar

The RNA backbone contains no phosphate groups

The sugar in the RNA backbone has an extra hydroxyl group

Correct answer:

The sugar in the RNA backbone has an extra hydroxyl group

Explanation:

RNA and DNA are both types of nucleic acids; therefore, both molecules are made from nucleotide monomers. Recall that a nucleotide contains a phosphate group, a pentose sugar, and a nitrogenous base. The biggest difference between an RNA nucleotide and a DNA nucleotide is the type of pentose sugar. DNA, or deoxyribonucleic acid, contains deoxyribose sugar whereas RNA, or ribonucleic acid, contains ribose sugar. A ribose sugar contains a hydroxyl group on its 2’ carbon whereas the deoxyribose sugar contains a hydrogen; therefore, the RNA pentose sugar has an extra hydroxyl group.

RNA molecules do contain the nitrogenous base uracil in place of thymine; however, bases are not part of the RNA or DNA backbone structure. The backbone only consists of the phosphate groups and the pentose sugars.

Example Question #32 : Dna, Rna, And Proteins

Based on the structure of DNA, what is the charge of DNA?

Possible Answers:

Positive

Neutral 

Nonpolar

Depends on the surrounding conditions

Negative 

Correct answer:

Negative 

Explanation:

Given the backbone of DNA, with the phosphate group attached to the deoxyribose via a phosphodiester bond, DNA is negatively charged. For this reason, histones - the proteins around which DNA molecules are wrapped in eukaryotes - have lots of positively charged amino acids on their DNA-binding sites. This produces a strong attractive force between DNA and histones.

Example Question #6 : Understanding The Dna Backbone

Which of these can be found in the DNA backbone:

I. Phosphate group
II. Ribose
III. Deoxyribose
IV. Phosphodiester bond 

Possible Answers:

I, III, IV, and V

I, III, and IV

I, II, and III

I, II, and IV

II, IV, and IV 

Correct answer:

I, III, and IV

Explanation:

The backbone of DNA consists of a phosphate group and a deoxyribose. These two components are therefore connected by a phosphodiester bond. The nucleotides are not included in the backbone. Instead, they make up the "rungs" of the double helical structure of DNA, which are hydrogen bonded to the bases of the complementary antiparallel strand. 

Example Question #34 : Dna, Rna, And Proteins

Which of the following statements are incorrect?

Possible Answers:

The bond between the phosphate and 5-C deoxyribose sugar in DNA is called a phosphodiester bond

In DNA, adenine pairs with thymine, and cytosine pairs with guanine.

A phosphate group, 5-C sugar, and a nitrogenous base make up a nucleotide.

The two strands of DNA are connected by phosphodiester bonds

DNA is double stranded, while RNA is single stranded.

Correct answer:

The two strands of DNA are connected by phosphodiester bonds

Explanation:

The two strands of DNA are actually connected by hydrogen bonds (H-bonds) between the nitrogenous bases. There are 2 hydrogen bonds between adenine and thymine, and 3 hydrogen bonds between cytosine and guanine. All other statements are true.

Example Question #7 : Understanding The Dna Backbone

DNA’s backbone consists of which of the following?

Possible Answers:

Phosphate-sugar 

Phosphate only

Purine-pyrimidine H-bonds

Double helix

Correct answer:

Phosphate-sugar 

Explanation:

DNA is the hereditary material found in virtually all organisms; however, some viruses use RNA. DNA consists of several components. It has a phosphate-sugar (deoxyribose) backbone and is composed of two strands made from purine-pyrimidine hydrogen bonds in a double helix confirmation. The purines associated with DNA include adenine and guanine and the pyrimidines include cytosine and thymine. Adenine bonds with thymine and cytosine bonds with guanine. 

Example Question #8 : Understanding The Dna Backbone

Which of the following is not correct regarding eukaryotic DNA?

Possible Answers:

DNA is found in the nucleus, cytoplasm, and ribosomes.

DNA's bases include adenine, guanine, thymine, and cytosine. 

DNA is self-replicating. 

Deoxyribose is the sugar that composes DNA. 

DNA runs antiparallel in a 5' to 3' direction. 

Correct answer:

DNA is found in the nucleus, cytoplasm, and ribosomes.

Explanation:

All of the responses are correct except that eukaryotic DNA is found in the nucleus, cytoplasm, and ribosomes. In eukaryotes, DNA is only found in the nucleus, mitochondria, and only sometimes free floating in the cytoplasm. DNA is not found in ribosomes. RNA on the other hand, is found in the nucleus, cytoplasm, and ribosomes. Note that since prokaryotes lack membrane-bound organelles, their DNA is free-floating in the cytoplasm.

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