Question
Describe the semi-conservative model of DNA replication. How did the Meselson-Stahl experiment prove this model? Explain the results expected for each generation.
Solution — Step by Step
Watson and Crick, after proposing the double helix structure in 1953, suggested that DNA replication is semi-conservative: during replication, the two strands of the parent double helix separate, and each serves as a template for a new complementary strand. The result is two daughter DNA molecules, each containing one original (parental) strand and one newly synthesised strand.
“Semi-conservative” = “half-conserved” — each daughter molecule conserves half of the original DNA.
Two other models were also proposed:
- Conservative model: Parental double helix remains intact; daughter molecule is entirely new
- Dispersive model: Both strands of both daughter molecules contain a mixture of old and new DNA
In 1958, Matthew Meselson and Franklin Stahl designed an elegant experiment using E. coli bacteria and two isotopes of nitrogen:
- ¹⁴N (light nitrogen): Normal nitrogen, found in ordinary growth medium
- ¹⁵N (heavy nitrogen): Heavier isotope, gives DNA a greater density
Setup:
- First, grow E. coli in ¹⁵N medium for several generations → all DNA becomes labelled with heavy nitrogen (¹⁵N–¹⁵N DNA)
- Then transfer bacteria to ¹⁴N medium — all newly synthesised DNA will use light nitrogen
- Sample at regular intervals (after 0, 1, 2 generations) and centrifuge DNA in CsCl density gradient centrifugation
- Heavier DNA settles lower in the tube; lighter DNA settles higher
Before transferring to ¹⁴N medium:
All DNA is ¹⁵N–¹⁵N (heavy–heavy). In the centrifuge tube, it forms a single band at the bottom (dense/heavy position).
This is the control — we see where pure heavy DNA sits.
After one replication in ¹⁴N medium:
Semi-conservative prediction: Each daughter molecule has one ¹⁵N strand (from parent) and one ¹⁴N strand (newly made) → hybrid/intermediate density DNA
Result observed: A single band at intermediate density (hybrid ¹⁵N–¹⁴N).
This rules out the conservative model (which would predict one heavy band + one light band). The dispersive model and semi-conservative model both predict intermediate DNA at this stage, so Generation 1 alone cannot distinguish them.
After two replications in ¹⁴N:
Semi-conservative prediction:
- The ¹⁵N–¹⁴N hybrid molecules from Generation 1 replicate → one daughter gets the ¹⁵N strand + new ¹⁴N strand = hybrid (intermediate density); the other gets the ¹⁴N strand + new ¹⁴N strand = light–light DNA
- So we expect two bands: one hybrid (intermediate) and one light
Result observed: Exactly two bands — one at intermediate density and one at light density, in equal proportions (1:1 ratio)
This result perfectly matches the semi-conservative model and distinguishes it from the dispersive model (which would predict a single band of intermediate density even at Generation 2).
Semi-conservative model proved.
Why This Works
The brilliance of the Meselson-Stahl experiment is that it used density as a proxy for whether DNA strands were old or new. Since ¹⁵N is physically heavier than ¹⁴N, DNA containing different proportions of old and new strands can be physically separated by centrifugation — no radioactivity needed, no complicated chemistry.
The key insight: if you can track “old” vs “new” material at the molecular level, you can directly test any model of replication. The unambiguous two-band result at Generation 2 is what made this experiment conclusive.
Alternative Method
The same principle was later confirmed using radioactive labelling with tritium (³H) and autoradiography, and also with bromodeoxyuridine (BrdU) substitution — both giving the same result.
In NEET and CBSE exams, a common question shows the three models (conservative, semi-conservative, dispersive) and asks which band pattern corresponds to which model after 1 and 2 generations. Know this grid: After Gen 1 — Conservative = two bands (heavy + light); Semi = one band (intermediate); Dispersive = one band (intermediate). After Gen 2 — Conservative = heavy + light + light; Semi = intermediate + light; Dispersive = single intermediate band.
Common Mistake
Students often say the Meselson-Stahl experiment was done with the isotope ³²P (phosphorus). The experiment used ¹⁵N (nitrogen), not phosphorus. ³²P is used in Hershey-Chase experiment (proving DNA is the genetic material). These are two completely different experiments from the same era — don’t confuse them.