Question 1

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

Accepted Answer

Cytosine can spontaneously deaminate to form uracil, so using thymine allows repair enzymes to distinguish natural bases from deamination products.

Question 2

A sample of double-stranded DNA is analyzed and found to contain 30% adenine. What is the percentage of guanine in this DNA?

Accepted Answer

20%

Question 3

Which of the following statements about the difference between DNA and RNA is CORRECT?

Accepted Answer

DNA is more stable than RNA because it lacks the 2'-OH group, making it less susceptible to hydrolysis.

Question 4

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

Accepted Answer

Hydrogen bonds between base pairs and van der Waals stacking interactions between adjacent bases.

Question 5

A researcher is using agarose gel electrophoresis to separate DNA molecules. Which of the following statements is TRUE regarding the migration of DNA in the gel?

Accepted Answer

DNA migrates toward the anode because its phosphate backbone is negatively charged.

Question 6

Which of the following is a palindromic DNA sequence?

Accepted Answer

5'-GAATTC-3'

Question 7

A solution contains a mixture of DNA and protein. Which absorbance ratio at 260/280 nm would indicate a pure DNA sample?

Accepted Answer

$$1.8-2.0$$

Question 8

Which of the following best describes the effect of high ionic strength (e.g., high NaCl concentration) on DNA duplex stability?

Accepted Answer

It stabilizes the DNA duplex by shielding negative charges on the phosphate backbone.

Question 9

Which of the following is a correct statement about the topology of covalently closed circular DNA?

Accepted Answer

The linking number ($$L) $$can only be changed by breaking one or both DNA strands.

Question 10

A DNA sample is heated and its absorbance at 260 nm increases sharply. This phenomenon is known as:

Accepted Answer

The hyperchromic effect

Question 11

Which of the following best explains why RNA is not stable at high pH?

Accepted Answer

The 2'-OH group in ribose can attack the adjacent phosphate, leading to cleavage of the backbone.

Question 12

Which of the following is a function of cyclic AMP (cAMP) in cells?

Accepted Answer

It acts as a second messenger, regulating protein kinases and gene expression.

Question 13

Which of the following is TRUE regarding the A-form and B-form of DNA?

Accepted Answer

A-form DNA is favored in dehydrated conditions and in double-stranded RNA.

Question 14

A researcher wants to separate a mixture of nucleotides using ion-exchange chromatography. At pH 7.4, what is the approximate net charge of a nucleotide?

Accepted Answer

$$-1$$

Question 15

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

Accepted Answer

They cleave DNA at specific palindromic sequences, generating fragments for cloning.

Question 16

Which of the following is a correct representation of a nucleoside and its corresponding nucleotide?

Accepted Answer

Adenosine (nucleoside), Adenosine 5'-monophosphate (nucleotide)

Question 17

Which of the following statements about DNA denaturation is CORRECT?

Accepted Answer

Renaturation is promoted by slow cooling after denaturation.

Question 18

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

Accepted Answer

It used density gradient centrifugation to distinguish DNA of different isotopic composition.

Question 19

Which of the following is a correct description of the primary structure of nucleic acids?

Accepted Answer

The sequence of nucleotides from the 5' to 3' end.

Question 20

Which of the following is TRUE about the effect of ethidium bromide (EtBr) on covalently closed circular DNA?

Accepted Answer

EtBr intercalation increases the twist and induces positive supercoiling.

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

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