D1.1 Meselson and Stahl (1958)
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What question did Meselson and Stahl test?
How does DNA replicate?
What organism did they use?
E. coli bacteria.
How did they label DNA?
Grew bacteria in ¹⁵N (heavy nitrogen) medium → DNA became “heavy”.
What happened next?
Transferred bacteria to ¹⁴N (light nitrogen) medium.
How did they analyze DNA?
Density gradient centrifugation → separates heavy/light DNA bands.
What did they see after 1 generation?
One intermediate band → not conservative (which would show heavy + light separately).
What did they see after 2 generations?
One intermediate band + one light band → supports semi-conservative replication.
What does semi-conservative mean?
Each new DNA molecule = 1 old (parent) strand + 1 new (daughter) strand.