# AP Biology : Understanding Nucleotides and Base Pairs

## Example Questions

### Example Question #35 : Dna And Rna Structure

A section of double-stranded DNA is composed of 35% adenine bases. What is the percentage of cytosine bases in the section of DNA?

Explanation:

Since we know that 35% of the bases in the section of DNA are adenine, we can conclude that 35% of the bases are thymine. This is because adenine will always pair with thymine, so there will be just as many thymine bases as adenine bases. Together, adenine and thymine compose 70% of the segment.

This means that 30% of the section is composed of guanine-cytosine pairs.

Since these two bases will be equal in quantity, 15% of the DNA section will be cytosine bases.

### Example Question #36 : Dna And Rna Structure

In a sample of DNA, 30% of the bases are found to be guanine. What percentage of the bases in the sample are thymine?

Explanation:

Guanine will pair with cytosine. From this knowledge, we can assume that there will be an equal number of guanine and cytosine residues in the sample. Each guanine must have a cytosine counterpart.

The total composition of the DNA sample must be accounted for by the sum of all the bases.

Use the known values for guanine and cytosine to find the sum of adenine and thymine.

Like cytosine and guanine, adenine and thymine must be present in equal amounts in order to form proper base pairs. We can reasonably assume that half of the remaining DNA will consist of each residue.

### Example Question #37 : Dna And Rna Structure

An analysis of a DNA sample indicates it contains 35% adenine. What percent of the sample would be cytosine?

Explanation:

There are four nitrogenous bases found in DNA: adenine, thymine, cytosine, and guanine. Adenine always binds with thymine, and cytosine always binds with guanine.

Since certain bases always appear in pairs, they will have equal percentages of the DNA composition. The percentage of adenine will equal the percentage of thymine, and the percentage of cytosine will equal the percentage of guanine. Together, these percentages will add to 100%.

We know that the sample is 35% adenine, which tells us that it is also 35% thymine.

We know that cytosine and guanine pair together and will be present in equal amounts, so we can divide this final total by 2 to find our answer.

The sample is 35% adenine, 35% thymine, 15% guanine, and 15% cytosine.

### Example Question #38 : Dna And Rna Structure

A strand of DNA was measured to contain 22% adenine. What is the DNA's composition of the other bases?

22% Adenine, 22% Thymine, 28% Cytosine, 28% Guanine

22% Adenine, 22% Cytosine, 28% Guanine, 28% Thymine

22% Adenine, 22% Guanine, 28% Cytosine, 28% Thymine

22% Adenine, 22% Guanine, 28% Cytosine, 28% Uracil

22% Adenine, 22% Uracil, 28% Cytosine, 28% Guanine

22% Adenine, 22% Thymine, 28% Cytosine, 28% Guanine

Explanation:

We can use Chargaff's rule to find the remaining compositional percentages. Adenine always pairs with thymine, so their percentages will be equal. Cytosine always pairs with guanine, so their percentages will also be equal. The sum of all four percentages must equal 100%.

We know that the sample is 22% adenine; this tells us it is also 22% thymine.

Since cytosine and guanine are present in equal amounts, we can simply divide their sum by 2.

The final composition is 22% adenine, 22% thymine, 28% cytosine, and 28% guanine.

Uracil is only found in RNA.

### Example Question #39 : Dna And Rna Structure

A strand of RNA is composed of 20% adenine and 30% guanine.

What is the strand's composition of the other bases?

20% Adenine, 30% Guanine, 30% Thymine, 20% Cytosine

20% Adenine, 30% Guanine, 20% Uracil, 30% Cytosine

Cannot determine from the given information

20% Adenine, 30% Guanine, 30% Uracil, 20% Cytosine

20% Adenine, 30% Guanine, 20% Thymine, 30% Cytosine

Cannot determine from the given information

Explanation:

Chargaff's rule only applies to DNA. RNA is single-stranded, and thus, no base pairing occurs.

Think of a strand of DNA. Each base pairs with a specific partner, allowing us to determine their percentages: adenine and thymine are always equal, and cytosine and guanine are always equal. In RNA, with this pairing absent, there is no correlation between the base percentages. A strand could be 20% adenine, 30% guanine, 5% cytosine, and 45% uracil; we simply cannot draw any conclusions.

### Example Question #40 : Dna And Rna Structure

What are the monomers of DNA?

Phosphate-sugar backbone

Deoxyribose sugars

Nucleotides

Nitrogen atoms

Nucleotides

Explanation:

A polymer is a macromolecule that is made up of subunits that are repeated or very similar. These subunits are called monomers. DNA is a polymer made up of monomer units called nucleotides. Nucleotides are made up of a phosphate group, a five-carbon sugar (deoxyribose, in the case of DNA), and a variable nitrogenous base. There are four different nucleotides that make up the polymer of DNA: thymine, cytosine, adenine, and guanine. These four nucleotides belong to two different classes based on structure. Adenine and guanine are purines that have two carbon-nitrogen rings. Thymine and cytosine are pyrimidines that have only one carbon-nitrogen ring.

### Example Question #41 : Dna And Rna Structure

Which base pair can be found in a DNA molecule?

Guanine-cytosine

Cytosine-thymine

Guanine-thymine

Guanine-cytosine

Explanation:

Within a DNA molecule, there are specific nucleotide binding patterns, a phenomenon called “complementary base pairing.” Specific pyrimidine nucleotides can only bind to specific purine bases: cytosine binds to guanine via three hydrogen bonds and adenine binds to thymine via hydrogen bonds. Normally, within a DNA molecule, no other base pair combinations exist. These specific complementary base pairs allow DNA to take the form of a double helix. The double helix can be most simply described as a twisted ladder; the base pairs and their hydrogen bonds represent the rungs, and the sugar-phosphate backbone represents the sides of the ladder.

### Example Question #42 : Dna And Rna Structure

The given diagrams depict which of the following?

Ribose and deoxyribose

Thymine and cytosine

Pyrimidines

Amino acids

Purines

Purines

Explanation:

The diagram depicts two purines (adenine and guanine), identifiable by their pyrimidine-imidazole double-ring structure. Pyrimidines (such at thymine and cytosine) have only one ring, amino acids have both amine and carboxylic acid groups, and ribose and deoxyribose are pentameric sugars (and contain no nitrogen).

### Example Question #43 : Dna And Rna Structure

What type of bonding occurs between the two parallel strands of nucleic acids in DNA?

Hydrogen bonding

Ionic bonding

Single bond

Double bond

Covalent bonding

Hydrogen bonding

Explanation:

The bonding that occurs between two parallel strands of nucleic acids in DNA is hydrogen bonding. As you know, hydrogen bonding occurs between molecules containing fluorine, nitrogen and oxygen with other fluorine, nitrogen and oxygen atoms. This is a fairly weak bond but there are so many hydrogen bonds along a strand of DNA making the attachment between the two quite strong, but the two strands can still be separated as needed (during replication and transcription). Adenine and thymine form two hydrogen bonds, while cytosine and guanine form three hydrogen bonds.

### Example Question #44 : Dna And Rna Structure

There are 4 types of bases in DNA: Guanine (G), Cytosine (C), Adenine (A), and Thymine (T). The complimentary pairing of these bases forms the double helix of DNA

Which bases are complimentary (bond together) to form the spiral staircase of the double helix present in DNA?

Thymine with uracil and cytosine with guanine

Adenine with uracil and cytosine with guanine

Adenine with thymine and guanine with cytosine

Adenine with cytosine and guanine with thymine

Adenine with guanine and cytosine with thymine