The Meselson-Stahl experiment

• Why is DNA replication called semi-conservative?

  Because each new DNA molecule has one original strand and one new strand.
• What do the original DNA strands act as during replication?

  They act as templates for building new strands.
• What experiment provided evidence for semi-conservative replication?

  Meselson and Stahl's experiment.
• What isotopes of nitrogen were used in the Meselson and Stahl experiment?

  Nitrogen-14 (light) and Nitrogen-15 (heavy).
• How can 'heavy' and 'light' DNA be separated?

  By their weight in a centrifuge.
• What broth was used to grow bacteria for the experiment?

  Nutrient broth containing either light or heavy nitrogen.
• What happens to the original DNA strand when bacteria with heavy nitrogen are put into light nitrogen broth?

  The original strand contains heavy nitrogen, but the new strands contain light nitrogen.
• What result would indicate conservative replication in Meselson and Stahl's experiment?

  Two separate bands for heavy and light DNA.
• What result was observed in Meselson and Stahl's experiment after one generation?

  A single band halfway between the heavy and light samples.
• Why did the second generation show a light band in Meselson and Stahl's experiment?

  Because some DNA used light as the template, while others used a mix of light and heavy strands.