Question 1

Which of the following best explains why negative supercoiling is beneficial for DNA function in cells?

Accepted Answer

It promotes strand separation, which facilitates processes like replication and transcription.

Question 2

DNA gyrase (topoisomerase II) introduces negative supercoils into DNA. Which step in its mechanism directly requires ATP hydrolysis?

Accepted Answer

The passage of an unbroken DNA segment through a double-stranded break.

Question 3

A researcher isolates a mutant bacterial strain lacking functional DNA gyrase. Which of the following is the most likely consequence?

Accepted Answer

The DNA will accumulate positive supercoils, hindering replication and transcription.

Question 4

Topoisomerase I differs from DNA gyrase in its mechanism. Which statement accurately describes this difference?

Accepted Answer

Topoisomerase I relaxes negative supercoils by breaking one DNA strand and does not require ATP.

Question 5

If a circular bacterial chromosome is overwound during replication, which enzyme is primarily responsible for restoring the appropriate level of supercoiling?

Accepted Answer

DNA gyrase (topoisomerase II)

Question 6

A circular DNA molecule is subjected to the action of DNA gyrase, which introduces two negative supercoils per catalytic cycle. If the initial linking number ($$L_0) is 200$$, what will be the new linking number ($$L) $$after one complete catalytic cycle?

Accepted Answer

$$L = 198$$

Question 7

Which of the following best describes the role of the A and B subunits in DNA gyrase function?

Accepted Answer

A subunits bind and cleave DNA, while B subunits capture energy from ATP hydrolysis.

Question 8

During DNA replication in bacteria, two circular DNA molecules can become interlocked (catenated). Which enzyme is primarily responsible for resolving this issue?

Accepted Answer

DNA gyrase (topoisomerase II)

Question 9

A scientist treats eukaryotic cells with a drug that inhibits topoisomerase I. What is the most likely effect on the DNA within these cells?

Accepted Answer

DNA will accumulate negative supercoils, increasing tension and potentially blocking replication.

Question 10

Which of the following equations correctly represents the relationship between the linking number ($$L$$), twist ($$T$$), and writhe ($$W) in a $$closed circular DNA molecule?

Accepted Answer

$$L = T + W$$

Question 11

A mutation in a bacterial cell causes the lower jaws of DNA gyrase to lose their ability to bind DNA. Which step in the supercoiling process would be directly affected?

Accepted Answer

The initial capture and wrapping of DNA around the A subunits.

Question 12

Which of the following best describes the organization of eukaryotic chromosomes?

Accepted Answer

Each eukaryotic chromosome is linear and contains hundreds to thousands of genes.

Question 13

A patient is treated with an antibiotic that specifically inhibits bacterial DNA gyrase. What is the most likely effect on the bacteria?

Accepted Answer

The bacteria will accumulate positive supercoils, impairing DNA replication and transcription.

Question 14

Which process is directly facilitated by the negative supercoiling of DNA in prokaryotes?

Accepted Answer

Strand separation during replication and transcription

Question 15

If a circular DNA molecule has a linking number ($$L) of 100 $$and is relaxed, what will be the linking number after DNA gyrase introduces four negative supercoils?

Accepted Answer

$$L = 96$$

Question 16

Which of the following is a correct statement about the energy requirements of DNA gyrase and topoisomerase I?

Accepted Answer

DNA gyrase requires ATP, while topoisomerase I does not.

Question 17

A researcher observes that after treatment with a certain drug, bacterial DNA is no longer able to separate into single strands efficiently. Which enzyme is most likely inhibited by this drug?

Accepted Answer

DNA gyrase

Question 18

Which of the following best explains why eukaryotic chromosomes are typically much longer than prokaryotic chromosomes?

Accepted Answer

Eukaryotic chromosomes are linear and contain many more genes and regulatory sequences.

DNA Supercoiling and Chromosome Structure

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