DNA and RNA Structure

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AP Biology › DNA and RNA Structure

Questions 1 - 10
1

The most prevalent negative charge on DNA can be found on which of the following molecular components?

Phosphate backbone

Ribose sugar ring

Hydroxyl residues

Phosphodiester linkage

Hydrogen bonds between base pairs

Explanation

The phosphate backbone of DNA is negatively charged due to the bonds created between the phosphorous atoms and the oxygen atoms. Each phosphate group contains one negatively charged oxygen atom, therefore the entire strand of DNA is negatively charged due to repeated phosphate groups.

2

The most prevalent negative charge on DNA can be found on which of the following molecular components?

Phosphate backbone

Ribose sugar ring

Hydroxyl residues

Phosphodiester linkage

Hydrogen bonds between base pairs

Explanation

The phosphate backbone of DNA is negatively charged due to the bonds created between the phosphorous atoms and the oxygen atoms. Each phosphate group contains one negatively charged oxygen atom, therefore the entire strand of DNA is negatively charged due to repeated phosphate groups.

3

Please complete the analogy.

Nitrogen : Nucleic Acids :: Phosphorous : ______________.

All answer choices

Phospholipids

ATP

ADP

DNA

Explanation

Nitrogen is essential to create all the nucleic acids, and phosphorous is essential to create phospholipids (an obvious choice), ATP and ADP (they are the same class of molecule, and the P stands for phosphate), and DNA (for the phosphate-sugar backbone).

4

Please complete the analogy.

Nitrogen : Nucleic Acids :: Phosphorous : ______________.

All answer choices

Phospholipids

ATP

ADP

DNA

Explanation

Nitrogen is essential to create all the nucleic acids, and phosphorous is essential to create phospholipids (an obvious choice), ATP and ADP (they are the same class of molecule, and the P stands for phosphate), and DNA (for the phosphate-sugar backbone).

5

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

More information is needed in order to answer the question

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.

6

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

More information is needed in order to answer the question

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.

7

A __________ bond between the sugar of one nucleotide and the phosphate of an adjacent nucleotide stabilizes the backbone of the DNA.

covalent

ionic

hydrogen

metallic

weak

Explanation

The bond formed between the sugar of one nucleotide and the phosphate of an adjacent nucleotide is a covalent bond. A covalent bond is the sharing of electrons between atoms. A covalent bond is stronger than a hydrogen bond (hydrogen bonds hold pairs of nucleotides together on opposite strands in DNA). Thus, the covalent bond is crucial to the backbone of the DNA.

8

A __________ bond between the sugar of one nucleotide and the phosphate of an adjacent nucleotide stabilizes the backbone of the DNA.

covalent

ionic

hydrogen

metallic

weak

Explanation

The bond formed between the sugar of one nucleotide and the phosphate of an adjacent nucleotide is a covalent bond. A covalent bond is the sharing of electrons between atoms. A covalent bond is stronger than a hydrogen bond (hydrogen bonds hold pairs of nucleotides together on opposite strands in DNA). Thus, the covalent bond is crucial to the backbone of the DNA.

9

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% Guanine, 28% Cytosine, 28% Thymine

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

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

22% Adenine, 22% Uracil, 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.

10

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

Tyrosine

Phosphate group

Guanine

Sugar

Adenine

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.

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