DNA Replication Models
📌 The two competing models for DNA replication were semi-conservative replication and conservative replication.
🧬 Semi-conservative replication means each resulting DNA molecule has one original strand and one new complementary strand.
🧪 Conservative replication suggested that the original helix would separate, and both new helices would consist of two entirely new strands.

The Meselson-Stahl Experiment Fundamentals
🔬 The experiment utilized the two stable isotopes of nitrogen: Nitrogen 14 ($^{14}$N), the lighter, common form, and Nitrogen 15 ($^{15}$N), the heavier form.
⚖️ Heavy DNA (containing $^{15}$N) was separated from light DNA (containing $^{14}$N) using a centrifuge, as heavier molecules settle lower in the tube.
🦠 Bacteria were initially grown in a medium containing only $^{15}$N, resulting in heavy DNA banding near the bottom of the tube.

Experimental Results and Interpretation
🔄 After transferring $^{15}$N bacteria to a $^{14}$N medium and allowing one replication cycle, a single band appeared in the intermediate position.
✨ This intermediate band proved the DNA was hybrid (one $^{15}$N strand and one $^{14}$N strand), confirming semi-conservative replication.
🔬 After a second replication cycle in the $^{14}$N medium, two bands appeared: one intermediate band (hybrid DNA) and one band near the top (light DNA, two $^{14}$N strands).

Key Points & Insights
➡️ The centrifugation technique was crucial for separating DNA based on the mass difference introduced by the heavy nitrogen isotope ($^{15}$N).
➡️ The appearance of a single intermediate band after one generation definitively ruled out conservative replication, which would have produced two distinct bands ($^{15}$N/$^{15}$N and $^{14}$N/$^{14}$N).
➡️ If DNA replicated conservatively after two cycles, the results would show one molecule of $^{15}$N/$^{15}$N DNA and three molecules of $^{14}$N/$^{14}$N DNA, with no hybrid molecules present.

📸 Video summarized with SummaryTube.com on Oct 08, 2025, 19:09 UTC