Question 1

Which of the following best explains why DNA is considered a negatively charged molecule at physiological pH?

Accepted Answer

Because the phosphate groups in the backbone lose protons, resulting in negative charges.

Question 2

A scientist is analyzing a nucleic acid sample and finds it contains uracil, ribose, and a phosphate group. Which type of molecule is she most likely studying?

Accepted Answer

RNA

Question 3

Which of the following correctly describes the directionality of a DNA strand?

Accepted Answer

It is always written and synthesized from the 5' to 3' end.

Question 4

Which of the following is a pyrimidine found only in DNA and not in RNA?

Accepted Answer

Thymine

Question 5

During DNA replication, the double helix must be unwound. Which enzyme is primarily responsible for relieving the tension caused by this unwinding?

Accepted Answer

Topoisomerase

Question 6

Which of the following best describes the chemical bond that links nucleotides together in a DNA strand?

Accepted Answer

Phosphodiester bond

Question 7

A DNA molecule is found to have 20% adenine. According to Chargaff's rules, what percentage of guanine would you expect to find?

Accepted Answer

$$30\%$$

Question 8

Which of the following statements about DNA supercoiling is correct?

Accepted Answer

Negative supercoiling helps in DNA packaging and facilitates strand separation during replication.

Question 9

If a DNA molecule undergoes a decrease in its linking number ($$Lk) $$while the twist ($$Tw) $$remains constant, what must happen to the writhe ($$Wr) $$according to the equation $$Lk = Tw + Wr$$?

Accepted Answer

Writhe ($$Wr) $$must decrease.

Question 10

Which of the following best explains why the major and minor grooves of DNA are important?

Accepted Answer

They provide access for proteins to interact with specific base sequences.

Question 11

A mutation causes a cell to produce a defective topoisomerase that cannot reseal DNA breaks. Which of the following is a likely consequence?

Accepted Answer

DNA will become more supercoiled and may break.

Question 12

Which of the following is true regarding the hydrogen bonding in the DNA double helix?

Accepted Answer

Cytosine pairs with guanine via three hydrogen bonds.

Question 13

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

Accepted Answer

A nucleotide contains a nitrogenous base, a five-carbon sugar, and one or more phosphate groups.

Question 14

Which of the following is a real-world application of DNA denaturation and renaturation?

Accepted Answer

Polymerase Chain Reaction (PCR) in molecular diagnostics

Question 15

Which of the following statements about the B-form of DNA is correct?

Accepted Answer

It is a right-handed helix with 10 base pairs per turn and a diameter of 2 nm.

Question 16

Which of the following best explains the role of base stacking in DNA stability?

Accepted Answer

Base stacking involves hydrophobic interactions that minimize water contact and stabilize the helix.

Question 17

A researcher is working with a circular DNA molecule. Which of the following is a unique property of circular DNA compared to linear DNA?

Accepted Answer

It can be supercoiled without free ends, making it easier to compact.

Question 18

Which of the following is a correct statement about the function of topoisomerase enzymes?

Accepted Answer

They regulate the level of DNA supercoiling by making transient breaks in the DNA backbone.

Question 19

Which of the following is a correct description of the chemical structure of a nucleotide?

Accepted Answer

A nitrogenous base, a five-carbon sugar, and a phosphate group

Question 20

Which of the following best describes the importance of DNA supercoiling in the cell?

Accepted Answer

It allows the large DNA molecule to be compacted into the small space of the cell nucleus.

The Structure of DNA: Nucleic Acids and Nucleotides

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