Question 1

Which of the following best describes the effect of negative supercoiling on bacterial chromosomal DNA?

Accepted Answer

It makes the DNA more compact and promotes strand separation necessary for replication and transcription.

Question 2

A circular DNA molecule is anchored between two plates. If a left-handed twist of 360° is applied, which of the following is most likely to occur?

Accepted Answer

The DNA will form a negative supercoil.

Question 3

Which enzyme is primarily responsible for introducing negative supercoils into bacterial DNA?

Accepted Answer

DNA gyrase (topoisomerase II)

Question 4

In E. coli, approximately how many negative supercoils are present per 40 turns of the double helix?

Accepted Answer

1

Question 5

Which of the following best defines topoisomers?

Accepted Answer

DNA molecules that differ only in their degree of supercoiling.

Question 6

A researcher observes that after treating bacterial DNA with a certain enzyme, the DNA becomes more relaxed and less compact. Which enzyme was most likely used?

Accepted Answer

A topoisomerase that relaxes negative supercoils.

Question 7

If the diameter of a DNA microdomain is calculated as $$D = \frac{	ext{circumference}}{2}$$ and the circumference is $$3.4\ \mu m$$, what is the diameter?

Accepted Answer

$$1.1\ \mu m$$

Question 8

Which of the following statements about DNA supercoiling is FALSE?

Accepted Answer

Positive supercoiling is the predominant form found in bacterial chromosomes.

Question 9

A mutation in the gene encoding the B subunit of DNA gyrase would most likely result in which of the following?

Accepted Answer

Reduced ability to introduce negative supercoils into DNA.

Question 10

Which of the following best explains why negative supercoiling is advantageous for bacterial cells?

Accepted Answer

It both compacts the DNA and facilitates processes that require strand separation, such as replication and transcription.

Question 11

If a DNA molecule is overwound by a right-handed twist, what is the most likely structural outcome?

Accepted Answer

Formation of a positive supercoil.

Question 12

Which of the following scenarios would most likely require the action of DNA gyrase in a bacterial cell?

Accepted Answer

During rapid DNA replication, when positive supercoils accumulate ahead of the replication fork.

Question 13

A student claims that underwinding DNA always leads to fewer turns in the helix. Which of the following best evaluates this claim?

Accepted Answer

Underwinding can lead to either fewer turns or the formation of negative supercoils, depending on the structural stability.

Question 14

Which of the following is NOT a function of negative supercoiling in bacterial chromosomes?

Accepted Answer

Increasing the number of turns in the DNA helix.

Question 15

If a bacterial chromosome were unable to maintain negative supercoiling, which of the following would most likely occur?

Accepted Answer

The chromosome would become less compact and genetic processes like replication and transcription would be impaired.

Question 16

Which of the following best describes the relationship between DNA supercoiling and topoisomerases?

Accepted Answer

Topoisomerases regulate the degree of DNA supercoiling by introducing or removing supercoils.

Question 17

A scientist isolates a DNA molecule with 5000 base pairs and finds it has 480 helical turns. What is the linking number ($$L_k) of $$this DNA molecule? (Assume 10.4 base pairs per turn for relaxed B-DNA.)

Accepted Answer

$$L_k = 480$$

Question 18

Which of the following would be the most direct consequence of inhibiting DNA gyrase in a bacterial cell treated with the antibiotic ciprofloxacin?

Accepted Answer

Accumulation of positive supercoils ahead of the replication fork, stalling DNA replication.

Question 19

Which of the following best explains why the diameter of a DNA microdomain is 'a little too big to fit inside a bacterium such as E. coli' without further compaction?

Accepted Answer

NAPs and supercoiling are required to make the microdomains compact enough to fit within the nucleoid.

DNA Supercoiling and Chromosome Structure in Bacteria

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