AP Biology : Understanding Nucleotides and Base Pairs

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #45 : Dna And Rna Structure

If a strand of DNA is ten percent adenine, then what percentage of guanine will it possess?

Possible Answers:

None of these

Correct answer:

Explanation:

If the imaginary DNA is ten percent adenine, then that means there must be ten percent thymine because they are complementary to one another. The rest of the eighty percent of the DNA strand must be made up of guanine and cytosine. Because they are complementary to each other, half will be guanine and half will be cytosine. This means that there will be forty percent guanine present in the DNA strand.

Example Question #46 : Dna And Rna Structure

A sample of double-stranded RNA is composed of thirty percent guanine. How much uracil is present in this sample?

Possible Answers:

None of these

Correct answer:

Explanation:

If the RNA is thirty percent guanine, then that means it must be composed of thirty percent cytosine because they are complementary to each other. The other forty percent of the RNA strand is made up of adenine and uracil. Because they are complementary to each other, half will be adenine and the other half will be uracil. This means that there will be twenty percent uracil present in the RNA strand.

Example Question #47 : Dna And Rna Structure

There are __________ hydrogen bonds between the nucleotides adenine and thymine and __________ hydrogen bonds between the nucleotides cytosine and guanine. 

Possible Answers:

three . . . two

one . . . three

two . . . two

two . . . three

three . . . three

Correct answer:

two . . . three

Explanation:

Hydrogen bonds between nucleotides of different strands are crucial for the stability of DNA. Adenine and thymine pair up with two hydrogen bonds between the nucleotides. On the other hand, cytosine and guanine pair up with three hydrogen bonds between the nucleotides.

Example Question #48 : Dna And Rna Structure

What would be the nucleotide sequence of the mRNA strand from the DNA strand below?

3'-ATCCCAGGTACC-5'

Possible Answers:

3'-UAGGGUCCAUGG-5'

3'-ATCCCAGGTACC-5'

3'-TAGGGTCCATGG-5'

5'-TAGGGTCCATGG-3'

5'-UAGGGUCCAUGG-3'

Correct answer:

5'-UAGGGUCCAUGG-3'

Explanation:

In order to solve this problem, it is imperative that you remember that RNA replaces thymine with uracil. This automatically eliminates three of the answer choices. Next, the RNA strand must have nucleotides that pair up with the DNA nucleotides (adenine pairs with thymine and cytosine pairs with guanine). For example, if DNA has a nucleotide of G, the RNA would have a nucleotide of C. Finally, since the strands must be antiparallel to one another, the mRNA must start with a 5' and end with a 3'. Thus, our answer is 5'-UAGGGUCCAUGG-3'. 

Below is a side by side comparison:

3'-ATCCCAGGTACC-5'  (DNA)

5'-UAGGGUCCAUGG-3 (mRNA)

Example Question #49 : Dna And Rna Structure

What are four possible chromosome structure abnormalities?

Possible Answers:

Polyploidy, deletion, inversion, translocation

Synapsis, duplication, inversion, translocation

Nondisjunction, deletion, duplication, inversion

Deletion, duplication, inversion, translocation

Deletion, independent assortment, inversion, duplication

Correct answer:

Deletion, duplication, inversion, translocation

Explanation:

Chromosomal related mutations include deletion, duplication, inversion, or translocation. Nondisjunction is a faulty separation event of homologous chromosomes but does not necessarily involve improper chromosome structures. Independent assortment is a mendelian inheritance principle which states chromosomes are divided randomly into two daughter cells. Synapsis is a normal pairing up event of homologous chromosomes in prophase I. Polyploidy is a condition of having more than two sets of chromosomes and is typically a characteristic of a species, not an abnormality.

Example Question #50 : Dna And Rna Structure

During DNA synthesis, DNA polymerase mistakenly substitutes a purine (adenine or guanine) for a pyrimidine (cytosine or thymine). This is known as what type of mutation?

Possible Answers:

Replacement substitution

Transition

Transversion

Silent mutation

Inversion

Correct answer:

Transversion

Explanation:

The substitution of a purine for a pyrimidine, or vice versa, is called a transversion. The substitution of one purine for another or one pyrimidine for another is called a transition. Either of these can be referred to as a point mutation, but transversion is the most specific answer to this question.

Example Question #51 : Dna, Rna, And Proteins

Which of the following is not a component of nucleic acids?

Possible Answers:

Sugar

Guanine

Phosphate group

Adenine

Tyrosine

Correct answer:

Tyrosine

Explanation:

Nucleic acids are made up of nitrogenous bases and a sugar-phosphate backbone. The sugar will vary depending on if it is an RNA or DNA molecule that's being discussed. RNA has ribose while DNA has deoxyribose. The nitrogenous bases are guanine, adenine, thymine, cytosine and uracil. Tyrosine is an amino acid, therefore not involved in the composition of nucleic acids.

Example Question #52 : Dna, Rna, And Proteins

What are the three major components that make up a nucleotide in DNA?

Possible Answers:

Glucose, ribose, and nitrogenous base

Phosphate, deoxyribose, and nitrogenous base

Phosphate, ribose, and nitrogenous base

Protein unit, deoxyribose, and nitrogenous base

Correct answer:

Phosphate, deoxyribose, and nitrogenous base

Explanation:

The three major components of a nucleotide in DNA are phosphate, deoxyribose, and one of the four nitrogenous bases. The phosphate and deoxyribose alternate along the backbone, and the nitrogenous base codes for the type of protein made in transcription and translation.

Example Question #53 : Dna, Rna, And Proteins

What is the nitrogenous base that adenine bonds to in the DNA strand?

Possible Answers:

Thymine

Guanine

Cytosine

Uracil

Correct answer:

Thymine

Explanation:

In the DNA strand, adenine bonds to thymine. It does not bond to the other bases in DNA. In RNA, adenine bonds to uracil.

Example Question #61 : Dna, Rna, And Proteins

In the DNA strand, with which nitrogenous base does guanine pair?

Possible Answers:

Uracil

Adenine

Thymine

Cytosine

Correct answer:

Cytosine

Explanation:

In the DNA strand, guanine bonds to cytosine. It does not bond to the other nitrogenous bases. Guanine and cytosine form three hydrogen bonds to keep complementary strands of DNA together.

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