AP Biology : Understanding Nucleotides and Base Pairs

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Understanding Nucleotides And Base Pairs

Which of the following nitrogenous bases is used to produce RNA, but not DNA?

Possible Answers:

Uracil

Guanine

Adenine

Thymine

Correct answer:

Uracil

Explanation:

Uracil is bound to adenine in the production of RNA, while thymine is used in its place in the production of DNA. Adenine, guanine, and cytosine are all used in the production of both RNA and DNA.

Example Question #71 : Dna, Rna, And Proteins

How many total hydrogen bonds would there be between base pairs of a piece of DNA if the sequence of one side was CGTTTGAC?

Possible Answers:

Correct answer:

Explanation:

Cytosine and guanine form three hydrogen bonds between each other, while tyrosine and adenine form two hydrogen bonds. We simply need to count how many of each base we have and multiple cytosine and guanine by three, and thymine and adenine by two.

CGTTTGAC has 2 cytosine, 2 guanine, 3 thymine, and 1 adenine.

Example Question #73 : Dna, Rna, And Proteins

What element would you NOT find in either DNA, or RNA?

Possible Answers:

Phosphorus

Carbon

Hydrogen

Potassium

Nitrogen

Correct answer:

Potassium

Explanation:

DNA and RNA are made of nucleotides, which contain oxygen, hydrogen, nitrogen, carbon, and phosphorus. The nucleic acid backbone is comprised of sugars, made of carbon, hydrogen, and oxygen, and phosphate groups, made of phosphorus, hydrogen, and oxygen. The backbone binds to bases, which contain a nitrogen element.

Potassium is not found in nucleic acid structure, and is used in other parts of the body like muscles and nerves for signal propagation.

Example Question #22 : Dna And Rna Structure

What are the components of a nucleoside?

Possible Answers:

A ribose or deoxyribose sugar and a phosphate

A nitrogenous base and a ribose or deoxyribose sugar

A nitrogenous base and a phosphate

A nitrogenous base, a ribose or deoxyribose sugar, and a phosphate group

Correct answer:

A nitrogenous base and a ribose or deoxyribose sugar

Explanation:

This question is mostly about the differentiations between a nucleoside and a nucleotide. A nucleoside is composed of a nitrogenous base and a ribose or deoxyribose sugar. A nitrogenous base, a ribose/deoxyribose sugar, and a phosphate describe a nucleotide. Remember that nucleosides are incomplete nucleotides, and lack a phosphate group.

Example Question #3 : Understanding Nucleotides And Base Pairs

Which of the following is a reason that cytosine pairs more favorably with guanine than thymine or adenine?

Possible Answers:

A covalent bond is formed

Ionic interactions between the bases

Bond energy is maximized

Maximized number of hydrogen bonds

Correct answer:

Maximized number of hydrogen bonds

Explanation:

Cytosine and guanine, when base paired, have three hydrogen bonds between them. Adenine and thymine only have two. This extra hydrogen bond helps make the cytosine-guanine pair favorable because it increases stability, and reduces bond energy.

Ionic and covalent bonds do not occur between nitrogenous bases in DNA. Covalent bonds are found in the DNA backbone (known as phosphodiester bonds).

 

Example Question #4 : Understanding Nucleotides And Base Pairs

Chargaff is credited with which of the following discoveries about DNA base pairs?

Possible Answers:

The ratio of adenine to guanine is close to  and the ratio of cytosine to thymine is close to 

The ratio of adenine to cytosine is close to  and the ratio of guanine to thymine is close to 

The ratio of adenine to thymine is close to  and the ratio of guanine to cytosine is close to 

Correct answer:

The ratio of adenine to thymine is close to  and the ratio of guanine to cytosine is close to 

Explanation:

Due to DNA's double-helical structure, the nucleotide bases are paired. Adenine is paired with thymine and guanine is paired with cytosine. Chargaff found that there is typically an equivalent number of adenine and thymine bases, and an equivalent number of guanine and cytosine bases. In a given sample of DNA, all adenine residues will have thymine counterparts on the complementary strand, and all cytosine residues will have complementary guanine counterparts. As a result, there will be equal numbers of each residue of the base pair in any sample of double-stranded DNA.

Example Question #5 : Understanding Nucleotides And Base Pairs

Which of the following choices displays complementary DNA strands?

Possible Answers:

3’-AUUGC-5’

5’-UAACG-3’

3’-ATTGC-5’

5’-ATTGC-3’

3'-ATTGC-5’

5’-TAACG-3’

3’-ATTGC-5’

3’-TAACG-5’

Correct answer:

3'-ATTGC-5’

5’-TAACG-3’

Explanation:

When determining complementary strands, there are a few important points to consider. First, remember that uracil is only found in RNA. We can eliminate any answer choices that contain uracil. Second, remember that DNA is anti-parallel. This means that the 3' end of the template strand must match up with the 5' end of the complementary strand. Flip any answers that have both 3' ends together. Finally, make sure that the bases align correctly. Adenine and thymine should always pair and cytosine and guanine should always pair.

Template strand: 3'-ATTGC-5’

Complementary strand is antiparallel: 5'-_ _ _ _ _-3'

Complementary strand must have right base pairs: 5'-TAACG-3'

An alternate way of writing this answer would be 3'-GCAAT-5'. This answer would still be correct! It would simply need to be flipped, as in the second step outlined above.

Example Question #1 : Understanding Nucleotides And Base Pairs

Which of the following could be a segment of DNA?

Possible Answers:

5'-AGATAG-3'

3'-GATCTC-5'

5'-GTCGAT-3'

3'-GATCTC-5'

5'-CTAGAG-3'

3'-GATCTC-5'

5'-CTCGTG-3'

3'-AATGTA-5'

5'-CTCGTG-3'

3'-GATCTC-5'

Correct answer:

5'-CTAGAG-3'

3'-GATCTC-5'

Explanation:

DNA is a double stranded molecule that is antiparallel, meaning the 5' end of one strand matches up with the 3' end of the other strand. Nitrogenous bases hold the strands together via hydrogen bonding. These bases are adenine (A), thymine (T), guanine (G), and cytosine (C). Adenine (A) can only pair with thymine (T), and guanine (G) can only pair with cytosine (C).

The correct answer presents two strands that are antiparallel and show proper binding patterns.

5'-CTAGAG-3'

3'-GATCTC-5'

All other answers have an error in base pairing or ordering.

Example Question #2 : Understanding Nucleotides And Base Pairs

Which of the following describes the components of a nucleotide? 

Possible Answers:

A phosphate group, a hexose sugar, and a nitrogenous base 

A phosphate group, a pentose sugar, and a nitrogenous base

A phosphate group, a pentose sugar, and an amino acid 

ATP, a pentose sugar, and a nitrogenous base 

ATP, a hexose sugar, and an amino acid 

Correct answer:

A phosphate group, a pentose sugar, and a nitrogenous base

Explanation:

Nucleic acids are one of the main biological macromolecules, and include DNA and RNA. A nucleotide is a DNA monomer, while a ribonucleotide is an RNA monomer. Numerous nucleotides are bonded together by phosphodiester bonds to form a single molecule of DNA. The pattern of nucleotides is used to store and transmit hereditary information. A nucleotide is composed of a phosphate group, a nitrogenous base, and a pentose sugar (deoxyribose).

Example Question #1 : Understanding Nucleotides And Base Pairs

Which of the following DNA primers has the lowest melting point?

Possible Answers:

GCGGATGTGTCC

CATGCGTAGATC

GCCACAGGTGGA

AGGTGGACACCG

TGACAAACTTGT

Correct answer:

TGACAAACTTGT

Explanation:

Cytosine and guanine form three hydrogen bonds with each other, while adenine and tyrosine only form two hydrogen bonds. This means that strands of DNA with a higher percentage of cytosine and guanine will have higher melting points.

Since we are looking for the sequence with the lowest melting point, we want the lowest percentage of cytosine and guanine, and the highest percentage of adenine and thymine.

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