Question 1

Which of the following best describes the relationship between linking number (L), twist (T), and writhe (W) in DNA topology?

Accepted Answer

$$L = T + W$$

Question 2

A researcher isolates a circular DNA molecule with a linking number ($$L) of 20. $$After treatment with a Type I topoisomerase, the linking number changes to 21. What does this indicate about the enzyme's action?

Accepted Answer

The enzyme introduced a single-strand break and changed the linking number by $$+1$$ without using ATP.

Question 3

Which of the following statements about DNA supercoiling is correct?

Accepted Answer

Negative supercoiling underwinds DNA, facilitating strand separation during replication and transcription.

Question 4

Which of the following is a unique feature of Type II topoisomerases compared to Type I?

Accepted Answer

They change the linking number ($$L) by$$ $$\pm 2$$ and require ATP.

Question 5

A mutation in a cell causes histone H1 to be nonfunctional. Which chromatin structure would be most directly affected?

Accepted Answer

The compaction of the 'beads-on-a-string' fiber into the 30 nm solenoid fiber.

Question 6

Which of the following best describes the 'beads-on-a-string' structure of chromatin?

Accepted Answer

It consists of nucleosomes connected by linker DNA, about 10 nm in diameter.

Question 7

Which of the following is NOT a function of nonhistone chromosomal proteins?

Accepted Answer

Forming the histone octamer core.

Question 8

A scientist observes that a DNA molecule is highly supercoiled and resists strand separation. Which type of supercoiling is most likely present?

Accepted Answer

Positive supercoiling

Question 9

Which of the following statements about nucleosomes is correct?

Accepted Answer

Each nucleosome contains two copies of H2A, H2B, H3, and H4 histones.

Question 10

Which of the following best explains why eukaryotic DNA must be highly compacted?

Accepted Answer

To fit the long DNA molecules into the small nuclear volume while maintaining accessibility for replication and gene expression.

Question 11

Which of the following is a correct sequence of DNA packaging in eukaryotic cells from least to most compact?

Accepted Answer

Nucleosome → 30 nm fiber → Looped domains → Miniband → Chromatid → Chromosome

Question 12

A patient is treated with a drug that inhibits Type II topoisomerases. Which cellular process would be most directly affected?

Accepted Answer

The ability to relax DNA knots and untangle chromosomes during cell division.

Question 13

Which of the following is true regarding the organization of DNA in bacteria, archaea, and eukaryotes?

Accepted Answer

Eukaryotes have multiple linear chromosomes organized by histones.

Question 14

If a circular DNA molecule has $$T = 10$$ and $$W = -2$$, what is the linking number ($$L$$)?

Accepted Answer

$$L = 8$$

Question 15

Which chromatin structure is most associated with regions of active gene expression?

Accepted Answer

Beads-on-a-string (10 nm fiber)

Question 16

A DNA molecule is nicked by an enzyme, allowing one strand to rotate freely and relieve supercoiling. Which enzyme is responsible for this action?

Accepted Answer

Type I topoisomerase

Question 17

Which of the following best describes the role of linker DNA in chromatin structure?

Accepted Answer

It connects adjacent nucleosomes and is stabilized by histone H1.

Question 18

During mitosis, which chromatin structure ensures accurate separation of genetic material?

Accepted Answer

Fully condensed chromosomes composed of stacked minibands

Question 19

Which of the following scenarios would most likely require the action of a Type II topoisomerase?

Accepted Answer

Untangling two interlinked circular DNA molecules after replication

DNA Dynamics and Chromatin Packaging: Structure, Topology, and Organization in Eukaryotes

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