DNA replication — semiconservative model with Meselson-Stahl experiment

Question

Describe the semiconservative model of DNA replication. Explain how the Meselson-Stahl experiment provided evidence for this model.

(NCERT Class 12, very high-frequency NEET question)

Solution — Step by Step

In semiconservative replication, the two strands of the parent DNA molecule separate, and each strand serves as a template for a new complementary strand. The result: each daughter DNA molecule contains one old (parental) strand and one new strand.

This was proposed by Watson and Crick in 1953, but needed experimental proof.

Meselson and Stahl grew E. coli in a medium containing heavy nitrogen ( $^{15}\text{N}$ ) for many generations. This made all the DNA “heavy” (both strands contained $^{15}\text{N}$ ).

They then transferred the bacteria to a medium containing normal light nitrogen ( $^{14}\text{N}$ ) and allowed them to replicate. After each generation, they extracted DNA and separated it by density using CsCl density gradient centrifugation.

Generation 0 (before transfer): All DNA is heavy-heavy ( $^{15}\text{N}$ - $^{15}\text{N}$ ). One band at the bottom (high density).

Generation 1: All DNA shows a single band at intermediate density. This means every DNA molecule has one heavy strand ( $^{15}\text{N}$ ) and one light strand ( $^{14}\text{N}$ ). This rules out conservative replication (which would give one heavy and one light band, not an intermediate).

Generation 2: Two bands appear — one intermediate and one light. 50% intermediate ( $^{15}\text{N}$ - $^{14}\text{N}$ ) and 50% light ( $^{14}\text{N}$ - $^{14}\text{N}$ ). No heavy band at all.

This pattern perfectly matches the semiconservative model.

Conservative model (both old strands stay together): Would give heavy + light bands in Generation 1 — but Meselson-Stahl found only one intermediate band. Ruled out.
Dispersive model (old and new segments mixed randomly): Would give intermediate bands in all generations, gradually shifting lighter. But in Generation 2, Meselson-Stahl found two distinct bands (intermediate + light), not one gradually lighter band. Ruled out.

Only the semiconservative model correctly predicts: all intermediate in Gen 1, then 50:50 intermediate and light in Gen 2.

Why This Works

The elegance of this experiment lies in using isotope labelling to physically distinguish old and new DNA strands. The density difference between $^{15}\text{N}$ and $^{14}\text{N}$ is small but detectable by ultracentrifugation. By tracking how parental strands distribute across generations, the replication mechanism is directly revealed.

Taylor’s experiment later confirmed semiconservative replication in eukaryotes (using Vicia faba and radioactive thymidine with autoradiography).

For NEET, know the math: after $n$ generations of semiconservative replication, there are $2^n$ DNA molecules total. Of these, exactly 2 molecules will have one $^{15}\text{N}$ strand (hybrid), and the remaining $2^n - 2$ will be fully light. This counting pattern is a favourite NEET numerical.

Common Mistake

Students often confuse the dispersive and semiconservative models in Generation 1. Both models predict an intermediate band in Generation 1 — you cannot distinguish them after just one round of replication. The crucial difference appears in Generation 2: semiconservative gives two distinct bands, while dispersive gives only one intermediate band (slightly lighter than Gen 1). Always emphasise the Gen 2 results when writing your answer.

Another error: writing that DNA helicase is the main enzyme of replication. The main enzyme is DNA polymerase III (in prokaryotes), which synthesises the new strand. Helicase unwinds the DNA, but polymerase does the actual replication.

Question

Solution — Step by Step

Why This Works

Common Mistake

Try These Next