Question
How did Meselson and Stahl experimentally prove that DNA replication is semiconservative? Explain.
✓
Answer
It was to show that DNA replication is semiconservative.
After completion of replication, each new/ daughter molecule of DNA has one parental strand and one newly synthesised strand; this is called semiconservative replication of DNA.
Experiment:
After completion of replication, each new/ daughter molecule of DNA has one parental strand and one newly synthesised strand; this is called semiconservative replication of DNA.
Experiment:
- Mathew Meselson and Franklin Stahl performed the experiment.
- They grew E.coli, in a medium containing $^{15} \mathrm{NH}_4 \mathrm{Cl}$, until $^{15} \mathrm{N}$ was incorporated in the two strands of newly formed E.coli cells; this 'heavy' DNA can be separated from the normal $^{14} \mathrm{N}$-DNA by centrifugation in cesium chloride (CsCl) density gradient.
- Then they transferred the cells into a medium with normal $^{14} \mathrm{NH}_4 \mathrm{Cl}$ and took out samples at various time intervals.
- They extracted the DNA and centrifuged it to measure the densities.
- The DNA extracted from cells after one generation after transfer from the $^{15} \mathrm{N}$ medium to $^{14} \mathrm{N}$ medium (i.e. after about 20 minutes), had an intermediate/ hybrid density.
- The DNA extracted after two generations (ie. after 40 minutes) consisted of equal amounts of 'light' DNA and 'hybrid' DNA.
- This proves that after replication, each DNA molecule has one parental strand and one newly-synthesised strand, i.e. replication is semiconservative.
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