Question

Arrange DNA crossover between two identical circular molecules from recombination site to final stage.
A. DNA replication
B. Complete DNA replication
C. Site-specific recombination
D. Homologous recombination
Choose the correct answer from the options given below :

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

Hint:

Remember the "Chromosome Dimer Resolution" paradox. Homologous recombination (general) accidentally tangles circular chromosomes during replication, and Site-Specific recombination (precise) is strictly required at the very end to untangle them!

Answer 1

The Correct Option is D

Step 1: Concept:
The question asks to chronologically order the biological events surrounding genetic crossing over between two identical circular DNA molecules (such as bacterial chromosomes or replicating plasmids) from initiation to resolution.

Step 2: Step-by-step Explanation:

• In organisms possessing circular chromosomes (like bacteria), the physical topology of the DNA causes unique issues during reproduction that must be managed by sequential recombination mechanisms.

• Phase 1 (A - DNA Replication): The process naturally begins when the circular DNA molecule initiates duplication at the origin of replication to create two identical sister chromatids.

• Phase 2 (D - Homologous Recombination): As the replication forks progress, they can frequently stall or encounter breaks. To rescue the stalled fork and ensure genetic fidelity, the cell utilizes homologous recombination between the two identical newly replicating circular DNA strands. Because the DNA is circular, an odd number of homologous crossover events physically links the two sister chromatids together into a single, massive continuous loop called a "circular dimer."

• Phase 3 (B - Complete DNA Replication): The replication machinery eventually finishes copying the entire genome. However, the two identical circular chromosomes remain physically tethered together as a dimeric cointegrate.

• Phase 4 (C - Site-specific recombination): Before the cell can safely divide, this dimeric chromosome must be untangled (resolved) back into two independent circular monomers. The cell utilizes specialized enzymes (like the XerCD recombinase complex in E. coli) that perform site-specific recombination at specialized target sequences (like the dif site). This neatly separates the identical circular molecules so they can be segregated into daughter cells.

• Therefore, the biological sequence is: DNA Replication initiates (A) \(\rightarrow\) Homologous Recombination forms a dimer (D) \(\rightarrow\) Replication finishes (B) \(\rightarrow\) Site-specific Recombination resolves the dimer (C).

Step 3: Final Answer:
The sequence matching this logic is A, D, B, C, which is Option (D).