Question 1

Which of the following best explains why DNA contains thymine instead of uracil?

Accepted Answer

Repair enzymes cannot distinguish between natural uracil and cytosine deamination products, so thymine (5-methyl-U) is used in DNA.

Question 2

Which of the following is a correct representation of the Beer-Lambert Law used in nucleic acid quantification?

Accepted Answer

$$A = \epsilon l c$$

Question 3

A DNA sample is analyzed by UV spectroscopy. Which absorbance ratio would indicate a pure DNA sample with minimal protein contamination?

Accepted Answer

$$A_{260}/A_{280} = 1.8$$

Question 4

Which of the following statements about the chemical structure of DNA and RNA is correct?

Accepted Answer

RNA contains uracil, while DNA contains thymine.

Question 5

Which of the following best describes the forces that stabilize the DNA double helix?

Accepted Answer

Hydrogen bonds between paired bases and base stacking interactions.

Question 6

A circular DNA molecule has a linking number ($$L) of 200$$, a twist ($$T) of 190$$, and a writhe ($$W) of 10. If a $$topoisomerase introduces two negative supercoils, what will be the new linking number?

Accepted Answer

$$L = 198$$

Question 7

Which of the following is a palindromic DNA sequence?

Accepted Answer

5'-GAATTC-3'

Question 8

Which of the following best explains why RNA is less stable than DNA under alkaline conditions?

Accepted Answer

RNA has a 2'-OH group that can participate in intramolecular nucleophilic attack, leading to cleavage.

Question 9

Which of the following is NOT a function of ATP in the cell?

Accepted Answer

Serving as a coenzyme for hydride transfer reactions

Question 10

A DNA sample is heated and its absorbance at 260 nm increases sharply. What is the best explanation for this observation?

Accepted Answer

The DNA is denaturing, causing the bases to become unstacked and absorb more UV light (hyperchromic effect).

Question 11

Which of the following best describes the difference between a nucleoside and a nucleotide?

Accepted Answer

A nucleoside contains a base and a sugar; a nucleotide contains a base, a sugar, and a phosphate group.

Question 12

Which of the following is true regarding the A, B, and Z forms of DNA?

Accepted Answer

B-DNA is the most common form in cells, A-DNA is favored in low humidity, and Z-DNA is a left-handed helix.

Question 13

Which of the following best describes the role of restriction enzymes in molecular cloning?

Accepted Answer

They cleave DNA at specific palindromic sequences, allowing for the insertion of foreign DNA into vectors.

Question 14

A researcher wants to separate a mixture of nucleotides using electrophoresis. Which property of nucleotides is most important for their separation in this technique?

Accepted Answer

Their net negative charge at physiological pH

Question 15

Which of the following is a correct statement about the Meselson-Stahl experiment?

Accepted Answer

It demonstrated that DNA replication is semiconservative.

Question 16

Which of the following best describes the primary structure of a nucleic acid?

Accepted Answer

The sequence of nucleotide bases from the 5' to 3' end

Question 17

Which of the following is a function of cAMP in cells?

Accepted Answer

It activates protein kinases and regulates gene expression.

Question 18

Which of the following best explains why high ionic strength stabilizes the DNA duplex?

Accepted Answer

Cations shield the negative charges on the phosphate backbone, reducing electrostatic repulsion.

Question 19

Which of the following is true about the tautomeric forms of DNA bases?

Accepted Answer

Tautomeric shifts can lead to mutations during DNA replication.

Chapter 4: Nucleic Acids – Structure, Properties, and Biological Functions

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