Overview
This lecture explains Meselson and Stahl's experiment, which provided evidence for semiconservative DNA replication, distinguishing it from conservative replication models.
DNA Replication Hypotheses
- Watson and Crick proposed two DNA replication models: conservative and semiconservative.
- Semiconservative replication: new DNA consists of one original strand and one newly synthesized strand.
- Conservative replication: original DNA remains intact, and new DNA is entirely newly synthesized.
Background Facts for the Experiment
- DNA nucleotides contain nitrogenous bases (adenine, thymine, cytosine, guanine) that include nitrogen.
- The experiment uses two nitrogen isotopes: lighter 14N and heavier 15N.
- Isotopes are forms of an element with the same protons/electrons but different neutrons.
- Bacteria incorporate available nitrogen isotopes from growth media into their DNA.
Meselson and Stahl’s Experiment Steps
- Bacteria are initially grown in a 15N medium; DNA becomes heavy (all 15N).
- DNA from 15N bacteria settles at the bottom in a centrifuge due to density.
- Bacteria are transferred to 14N medium and allowed one round of replication.
- After one division, DNA has one 15N and one 14N strand (intermediate density, one band in the middle of the tube).
- After two divisions in 14N, half the DNA is hybrid (15N-14N), half is light (14N-14N), visible as two bands.
- After three divisions, 1/4 is hybrid, 3/4 is light, seen as two bands with different thicknesses in the centrifuge.
Evidence for Semiconservative Replication
- If replication were conservative, after the first division, two bands would appear (one heavy, one light).
- Instead, Meselson and Stahl observed only one intermediate-density band after the first division, disproving conservative replication.
- Subsequent divisions produced expected banding patterns for semiconservative replication.
Key Terms & Definitions
- Semiconservative replication — Each new DNA molecule contains one original and one newly synthesized strand.
- Conservative replication — Original DNA remains intact; all new DNA is fully synthesized.
- Isotope — Atoms of the same element with different neutron numbers.
- Centrifuge — A device to separate substances by density using rapid spinning.
- Nitrogenous base — Molecule in DNA containing nitrogen (adenine, thymine, guanine, cytosine).
Action Items / Next Steps
- Practice questions from the topic 1 assessment booklets on DNA replication and Meselson and Stahl's experiment.