AP Biology : Understanding Nucleic Acids

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Understanding Nucleic Acids

Which of the following choices is a correct statement about DNA?

Possible Answers:

A segment of double-stranded DNA the a large composition of adenine will be more stable than a segment with a large composition of cytosine

In a segment of double-stranded DNA the number of thymine bases and the number of guanine bases should be equal

Two or more of these statements are correct

In a segment of double-stranded DNA the number of adenine bases and the number of cytosine bases should be equal

In a segment of double-stranded DNA the number of guanine bases and the number of cytosine bases should be equal

Correct answer:

In a segment of double-stranded DNA the number of guanine bases and the number of cytosine bases should be equal

Explanation:

Chargaff found that in double-stranded DNA, the number of guanine bases should be equal to the number of cytosine bases, and the number of adenine bases should equal the number of thymine bases. These rules proved to be important pieces of evidence for the idea of complementarity, the theory that each DNA base pairs only with a specific other base on its opposite strand.

According to Chargaff's rules, the statement regarding guanine and cytosine bases is correct. The two other statements that are similarly worded are not correct because they do not compare the frequencies of two bases that are complementary to each other (adenine will not bind cytosine and guanine will not bind thymine). Finally, guanine-cytosine bonds are more stable than adenine-thymine bonds.

Example Question #2 : Understanding Nucleic Acids

The DNA and amino acid sequences of two organisms belonging to the same species were analyzed. It was revealed that there was more variability between their DNA sequences, as compared to their amino acid sequences. What is one possible explanation for the reduced variability of the amino acid sequence?

Possible Answers:

DNA is more heat sensitive than amino acids, and is therefore prone to greater sequence variability

Amino acids are smaller, making them more difficult to detect

Electrophoresis of proteins is an unreliable method

The sequencing of DNA is more reliable than sequencing of amino acids

Different sequences of DNA may code for the same amino acid

Correct answer:

Different sequences of DNA may code for the same amino acid

Explanation:

An amino acid may be represented by multiple different codon sequences of base pairs of DNA. This codon degeneracy is what allows for the greater variability of DNA as compared to amino acid sequences.

For example, a DNA sequence may show variability without affecting the variability of the coded amino acids.

Organism 1 DNA: 5'-AAAGCAGGC-3' // Organism 2 DNA: 5'-AAGGCTGGT-3' // Differences: 3

Organism 1 RNA: 3'-UUU-CGU-CCG-5' // Organism 2 RNA: 3'-UUC-CGA-CCA-5' // Differences: 3

Organisms 1 Amino Acids: Phe-Arg-Pro // Organism 2 Amino Acids: Phe-Arg-Pro // Differences: 0

Example Question #123 : Macromolecules

What are the components of the DNA backbone?

Possible Answers:

Amino acids

Sugar and phosphate groups

Alpha-linked glucose residues

Glycerol and fatty acids

Correct answer:

Sugar and phosphate groups

Explanation:

The backbone of DNA is made up of deoxyribose sugars linked to phosphate groups. These units are joined by phosphodiester bonds into chains. Nitrogenous bases are bound to the sugars of these groups and join DNA strands together by hydrogen bonds with their complementary base pairs.

Amino acids are the building blocks of proteins, and are not found in DNA. Alpha-linked glucose residues describe a type of polysaccharide, namely glycogen. Glycerol and fatty acids describe a type of lipid known as a triglyceride. Triglycerides and glycogen are primarily used in energy storage.

Example Question #124 : Macromolecules

Phosphodiester bond forms between which two molecules?

Possible Answers:

A phosphate group and the 3’ carbon on pentose sugar

The 5’ carbon on pentose sugar and a nitrogenous base

A phosphate group and a nitrogenous base

A phosphate group and the 5’ carbon on pentose sugar

Correct answer:

A phosphate group and the 3’ carbon on pentose sugar

Explanation:

By looking at the name "phosphodiester bond" you should realize that a phosphate group is involved. A phosphodiester bond occurs between the phosphate group and a pentose sugar in the DNA backbone. The phosphate group from one nucleotide binds to the 3’ carbon on pentose sugar from the other nucleotide.

Nitrogenous bases are attached to the 1' carbon of the ring by a glycosidic linkage.

Example Question #3 : Understanding Nucleic Acids

Which of the following contains a five-carbon sugar?

I. DNA

II. RNA

III. Glycogen

Possible Answers:

I, II, and III

II only

I only

I and II

Correct answer:

I and II

Explanation:

Remember that all nucleic acids contain nucleotides, composed of a pentose (a five-carbon sugar), a nitrogenous base, and a phosphate group. DNA and RNA must both contain a five-carbon sugar. RNA contains the pentose ribose, while DNA contains the pentose deoxyribose.

Glycogen does not contain a five-carbon sugar because it is made up of glucose subunits. Glucose contains six carbons.

Example Question #4 : Understanding Nucleic Acids

ATP and GTP provide energy for several cellular processes, and are composed of all except which of the following components?

Possible Answers:

A pentose sugar

A pyrimidine

Three phosphate groups

A purine

Correct answer:

A pyrimidine

Explanation:

ATP stands for adenosine triphosphate and GTP stands for guanosine triphosphate. Both of them are nucleic acids, meaning that they must contain a pentose sugar, a nitrogenous base, and phosphate groups. Both ATP and GTP contain three phosphate groups. The only difference between ATP and GTP is their nitrogenous base. ATP contains adenine whereas GTP contains guanine. Recall that adenine and guanine are both purines.

ATP and GTP do not contain any pyrimidines (cytosine, thymine, and uracil).

Example Question #5 : Understanding Nucleic Acids

Which of the following describes polymerase chain reaction (PCR)? 

Possible Answers:

A technique that detects proteins in the cells of tissue

A technique that quickly amplifies a segment of DNA, resulting in a large number of copies of that segment

A technique that detects specific DNA sequences

A technique that detects RNA in a sample

A technique that separates DNA based on charge and size

Correct answer:

A technique that quickly amplifies a segment of DNA, resulting in a large number of copies of that segment

Explanation:

Polymerase chain reaction (PCR) is a technique used to generate thousands to millions of copies of a specific segment of DNA. There are three major steps to PCR. Denaturation occurs when heat separates the original DNA strand. Annealing follows, in which the DNA is cooled and primers bind to each separated strand. The final step is extension, in which DNA polymerase adds nucleotides to the 3' end of each primer. These steps are repeated for each PCR cycle.

Example Question #163 : Biochemical Concepts

Which of the following is not found in nucleotides?

Possible Answers:

A region of negative charge

A nitrogenous base

A five-carbon sugar

A carboxylic acid

A phosphate group

Correct answer:

A carboxylic acid

Explanation:

Nucleotides are the monomers that make up nucleic acids. They are composed of a five-carbon sugar, a nitrogenous base, and a phosphate group. In building the polymer nucleic acid chain, the sugar and phosphate of one nucleotide align with those of another to build the phosphate-sugar backbone, while the nitrogenous bases will form hydrogen bonds across the helix to link two chains of nucleotides together. Phosphate groups carry negative charge; this gives the cell nucleus an overall negative charge and can be used to generate electrochemical gradients across the nuclear membrane.

Carboxylic acids are found in amino acids, and are not present in nucleic acids.

Example Question #168 : Biochemical Concepts

Which of the following groups is contained in ATP?

Possible Answers:

Ribose

Glucose

Arginine

Alanine

Tyrosine

Correct answer:

Ribose

Explanation:

Adenosine triphosphate contains an adenine group, a ribose sugar, and three phosphates. ATP is known as the energy molecule since there is a lot of potential energy stored in the bonds between each of the three phosphate groups.

Example Question #169 : Biochemical Concepts

What type of bond holds together the nitrogenous bases between the two backbones of DNA?

Possible Answers:

Phosphodiester bonds

Polar covalent bonds

Covalent bonds

Ionic bonds

Hydrogen bonds

Correct answer:

Hydrogen bonds

Explanation:

Hydrogen bonds form between electronegative atoms such as nitrogen and hydrogen atoms on their complementary bases between the DNA backbones. Adenine and thymine make two hydrogen bonds, while cytosine and guanine made three hydrogen bonds. Phosphodiester bonds keep the DNA backbone bonded together. Ionic and covalent bonds are too strong to bond the two antiparallel strands together since the strands must be separated during DNA synthesis. Hydrogen bonds are the perfect bond since they are weak individually, but collectively very strong.

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