Q.87 In the Meselson and Stahl experiment, E. coli was grown in a medium containing ¹⁵NH₄Cl. After 24 hours, the E. coli cells were transferred to a medium containing ¹⁴NH₄Cl. After the fourth generation in the medium containing ¹⁴NH₄Cl, the ratio between hybrid (¹⁵N / ¹⁴N) DNA and light (¹⁴N / ¹⁴N) labeled DNA will be 1 : n. Find the value of n.

E. coli grown in 15NH4Cl medium produces fully heavy 15N/15N DNA. Transfer to 14NH4Cl reveals semiconservative replication, yielding a 1:7 hybrid-to-light DNA ratio after four generations.​

Experiment Overview

Matthew Meselson and Franklin Stahl used nitrogen isotopes to prove DNA replicates semiconservatively. Bacteria start with heavy 15N-labeled DNA, then switch to light 14N medium. Each replication pairs one parental strand with a new strand, tracked via density gradient centrifugation.​

Generation-by-Generation Breakdown

Initial (Gen 0): All DNA is heavy (15N/15N) – 1 molecule.

• Gen 1: All hybrid (15N/14N) – 2 molecules.

• Gen 2: 2 hybrid + 2 light (14N/14N) – total 4.

• Gen 3: 4 hybrid? No—2 hybrid replicate to 4 hybrid + 4 light; prior 2 light to 4 light → 4 hybrid + 8 light (total 12). Actually corrects to pattern below.​​

Precise counts per total 16 DNA molecules at Gen 4:

• Hybrid persists as 2 (original 15N strands pair with new 14N each time).

• Light dominates at 14. Ratio simplifies to 1:7.​​

Correct Answer and n Value

After fourth generation, hybrid:light DNA = 1:7, so n=7. Heavy DNA dilutes to zero by Gen 2. Formula: Hybrid remains 2/2^g; light = 2^g – 2 (g=generations).​​

Common Options Explained

Typical MCQ options test replication models:

• 1:1: Matches Gen 2 only, not Gen 4.

• 1:3: Gen 3 ratio (2 hybrid:6 light).

• 1:7: Correct for Gen 4 (2:14).

• 1:15: Gen 5 error (2:30). Conservative replication would show different bands.​​

This semiconservative proof remains foundational for molecular biology exams.