5. A Holliday junction js a branched nucleic acid structure that contains four double stranded arms joined together. After strand invasion and branch migration the function must then be resolved into two separate duplexes, restoring either the parental configuration or a crossed-over configuration. According to double strand break model, crossover products are generated when
(1) There no resolution
(2) both the cut are in parental strand
(3) both the cut are in crossover strands
(4) One cut is in parental strand while another in crossover strand

Holliday junctions form during homologous recombination in the double-strand break repair (DSBR) model, where resolution determines crossover or non-crossover outcomes. Crossover products arise when the two Holliday junctions in the double Holliday junction (dHJ) intermediate are resolved by cutting crossing strands at one junction and non-crossing (parental) strands at the other. The correct answer is option (3): both the cuts are in crossover strands, as this configuration, combined with opposite cuts at the second junction, produces the exchanged duplexes characteristic of crossovers.

Question Overview

This CSIR NET-style question tests understanding of Holliday junction resolution in DSBR, focusing on how cuts dictate recombinant products after strand invasion and branch migration. The dHJ has two orientations: parental (non-crossing) strands maintain original configuration, while crossover (crossing) strands lead to exchange.

Option Analysis

• (1) There no resolution: Resolution is essential; without it, the branched structure persists, preventing separation into duplexes and recombinant products.

• (2) both the cut are in parental strand: Cutting both parental strands at each junction yields non-crossover products, restoring parental duplexes without exchange.

• (3) both the cut are in crossover strands: In dHJ, cutting crossover strands at both junctions (or equivalently, crossing at one and parental at the other in opposite sense) generates crossover products by swapping flanking regions.

• (4) One cut is in parental strand while another in crossover strand: This describes mixed cuts at a single junction or inconsistent resolution across dHJs, typically producing non-crossovers or requiring specific orientation for crossovers, not the defining mechanism.

DSBR Model Key Steps

The DSBR model initiates with a double-strand break, resection, strand invasion forming dHJs via second-end capture, branch migration, and biased resolution.

• Crossover requires opposite-sense cleavage: e.g., vertical (parental) at one junction, horizontal (crossover) at the other.

• Enzymes like RuvC or GEN1 cleave specifically, favoring crossovers in meiosis.
  This ensures genetic diversity while repairing breaks accurately.