Question

E. coli cell grown on ¹⁵N medium are transferred to ¹⁴N medium and allowed to grow for two generations. DNA extracted from these cells is ultracentrifuged in a cesium chloride density gradient. What density distribution of DNA would you expect in this experiment?

Options

• One high and one low density band.

• One intermediate density band.

• One high and one intermediate density band.

• One low and one intermediate density band.

Answer 1

One low and one intermediate density band.

Explanation:

This experiment follows the Meselson-Stahl model of semi-conservative replication. When E. coli cells grown in heavy ¹⁵N are moved to light ¹⁴N, the first generation produces “hybrid” DNA molecules containing one heavy and one light strand, resulting in a single intermediate density band. In the second generation, these hybrid strands serve as templates; the heavy strands again produce hybrids, while the light strands pair with new ¹⁴N to form entirely “light” DNA. This results in two distinct bands: one intermediate (hybrid) and one low-density (light), proving that DNA replicates by keeping one old strand and synthesising one new one.