Question

E.coli grown on ^{a 15}N medium is transferred to a ¹⁴N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment?

1. one high-density and one low-density band
2. one intermediate density band
3. one high density and one intermediate-density band
4. one low-density and one intermediate-density band

Short answer

1. The option “one high-density and one low-density band” is false.
2. The option “one intermediate density band” is false.
3. The option “one high density and one intermediate-density band” is false.
4. The option “one low-density and one intermediate-density band” is true.

Step-by-step solution

DNA

DNA is the genetic material that is found in most living organisms, including bacteria. It carries the genetic information which passes from one generation to the other generation.

Explanation of option (A)

The observation of one high density and low-density band is not seen since the experiment was conducted for two generations in the ¹⁴N medium.

After two rounds of DNA replication, no high-density band formation is observed because the E.coli was moved to the¹⁴N medium.

Therefore, the given option is false.

Explanation of option (B)

The observation of one intermediate band is not the only result of the experiment since the bacteria is grown in two different types of medium containing ¹⁵N and ¹⁴N.

Thus, there must be the formation of two different bands having varying densities.

Therefore, the given option is false.

Explanation of option (C)

The observation of one high density and one intermediate-density band is not the result of this experiment. Since the E.coli grown on the medium containing ¹⁵N is transferred to the ¹⁴N.

After this transfer, two round of replication is carried out only in the ¹⁴N medium.

Thus, the observation of a high-density band that corresponds to the ¹⁵N medium is not seen.

Therefore, the given option is false.

Explanation of option (D)

The transfer of E.coli from the ¹⁵N is transferred to the ¹⁴N, making the bacterium adapted to the ¹⁴N. Since the bacterium is grown initially in the¹⁵N heavier than the ¹⁴N, it produces the one intermediate band, not the high-density band.

The two rounds of replication are taking place in the¹⁴N that forms the low density and intermediate bands.

Therefore, the given option is true.