An E coli bacterium divides in every 20 min so in 80 min it will be four generations.

¹⁵N isotope is present in the environment than shifted in ¹⁴N environment, so it will result in the presence of ¹⁵N and ¹⁴N strands in DNA.

Generation 0

DNA isolated from cells at the start of the experiment (“generation 0,” just before the switch to ¹⁴N medium) produced a single band after centrifugation. This result made sense because the DNA should have contained only heavy ¹⁵N at that time.

Generation 1

DNA isolated after one generation (one round of DNA replication) also produced a single band when centrifuged. However, this band was higher, intermediate in density between the heavy ¹⁵N DNA and the light ¹⁴N DNA.

The intermediate band told Meselson and Stahl that the DNA molecules made in the first round of replication was a hybrid of light and heavy DNA. This result fit with the dispersive and semi-conservative models, but not with the conservative model.

The conservative model would have predicted two distinct bands in this generation (a band for the heavy original molecule and a band for the light, newly made molecule).

Generation 2

Information from the second generation let Meselson and Stahl determine which of the remaining models (semi-conservative or dispersive) was actually correct.

When second-generation DNA was centrifuged, it produced two bands. One was in the same position as the intermediate band from the first generation, while the second was higher (appeared to be labeled only with ¹⁴N. This result told Meselson and Stahl that the DNA was being replicated semi-conservatively. The pattern of two distinct bands—one at the position of a hybrid molecule and one at the position of a light molecule—is just what we'd expect for semi-conservative replication In contrast, in dispersive replication, all the molecules should have bits of old and new DNA, making it impossible to get a "purely light" molecule.

Generations 3 and 4

In the semi-conservative model, each hybrid DNA molecule from the second generation would be expected to give rise to a hybrid molecule and a light molecule in the third generation, while each light DNA molecule would only yield more light molecules.

Thus, over the third and fourth generations, we'd expect the hybrid band to become progressively fainter (because it would represent a smaller fraction of the total DNA) and the light band to become progressively stronger (because it would represent a larger fraction).

Conclusions

• Results show that after one generation, the double stranded DNA is 1/2 heavy (from the parent) and 1/2 light (newly synthesized). This means that 100% of the strands are of intermediate density.

• After a second generation, one half of the new daughter strands are light (using 14N DNA as template and synthesizing 14N NA) and one half are intermediate density (using 15N DNA as a template and 14N DNA for synthesis). This result is predicted by semi-conservative replication.

• DNA strands serve as templates for their own replication.

Hence for E.Coli,

In the first generation two hybrid generations were produced. In the second generation, half will have the previous hybrid bacteria and half will have the light bacteria.

In the third generation, hybrid generation will again cut half, this time 25% hybrid and 75% light

In fourth generation, 2 copies will be hybrid and 14 will be light. This tells that light bands will be of higher densities than hybrid band.