Question

Choose the correct sequence for how a type I topoisomerase relaxes DNA.
A. DNA release
B. Cleavage and opening of gate
C. Rejoining of cleaved strand
D. Strand passage

• A. A, B, C, D
• B. B, D, C, A
• C. A, D, B, C
• D. C, A, D, B

Hint:

Type I topoisomerase cuts ONE strand (Type I = 1 strand), whereas Type II topoisomerase cuts TWO strands. The general mechanism is always: Cut (Cleavage) $\rightarrow$ Pass (Strand passage) $\rightarrow$ Seal (Rejoining) $\rightarrow$ Let go (Release).

Answer 1

The Correct Option is B

Step 1: Concept:
The question focuses on the mechanism of action of Type I topoisomerase, a crucial enzyme that regulates the topological state of DNA by removing supercoils during processes like DNA replication and transcription.

Step 2: Step-by-step Explanation:

• Topoisomerase I relaxes supercoiled DNA by creating transient single-strand breaks. This allows the DNA to swivel and relieve torsional strain through a very specific catalytic cycle.

• Step B (Cleavage and opening of gate): The enzyme first binds to the DNA duplex. A tyrosine residue in the active site attacks the phosphodiester backbone of one DNA strand, cleaving it and creating a covalent enzyme-DNA intermediate. This creates a transient "gate".

• Step D (Strand passage): The intact complementary DNA strand is then passed through this newly opened break (the gate) to relieve the torsional strain, effectively unwinding one supercoil.

• Step C (Rejoining of cleaved strand): Once the intact strand has successfully passed through, the enzyme reverses the cleavage reaction, ligating (rejoining) the broken phosphodiester bonds back together.

• Step A (DNA release): Finally, the enzyme undergoes a conformational change, releasing the newly relaxed DNA molecule and preparing for another catalytic cycle.

• Therefore, the chronologically accurate sequence is B $\rightarrow$ D $\rightarrow$ C $\rightarrow$ A.

Step 3: Final Answer:
The correct sequence is given by option (B).