The Logical Deduction Step-by-Step:

Step 1 — What were the three possible replication models?

• Conservative: Both original strands stay together; daughter molecule gets both new strands. After 1 generation: heavy (original) + light (new) = 2 bands.
• Semiconservative: Each daughter gets one original + one new strand. After 1 generation: all hybrid = 1 band.
• Dispersive: Original DNA fragmented and distributed randomly. After 1 generation: 1 hybrid band; after 2 generations: still 1 band (always mixed).

Step 2 — What was observed after 1 generation? ONE band at intermediate (hybrid) density. This rules out CONSERVATIVE replication (which would show 2 bands: heavy + light). Compatible with BOTH semiconservative and dispersive models.

Step 3 — What was observed after 2 generations? TWO bands: hybrid density AND light density (1:1 ratio). This rules out DISPERSIVE replication (which would give only 1 band). This is EXACTLY what semiconservative predicts: 2 hybrid molecules + 2 light molecules = 4 total.

Step 4 — Conclusion: Only semiconservative replication is consistent with BOTH observations (1 band after Gen1; 2 bands after Gen2). Therefore, DNA replication is semiconservative.

Step 5 — Why does this make biological sense? Semiconservative replication preserves the integrity of information: each daughter molecule gets a complete, accurate original strand as a proofreading reference. Dispersive replication would randomly distribute mutations, making quality control impossible.