A NASA probe returned samples of rocks from the surface of Mars.From these rocks,a prokaryotic organism was isolated and remained viable under conditions similar to that of Mars.The organisms are capable of dividing to replicate themselves.In order to determine whether the DNA replication of these organisms was semiconservative,an experiment using N15/N14 (similar to that of Mendelson and Stahl)was performed.If the method of DNA replication were conservative,after one generation one would expect to see
A)all of the DNA in the heavy (N¹⁵)band.
B)all of the DNA in the light (N¹⁴)band.
C)all of the DNA at a point midway between the heavy and light bands.
D)half of the DNA in the light band, the other half in the heavy band.
E)none of the above

Question

Quizplus · Accepted Answer

If the DNA replication was conservative, all the parental DNA strands would remain intact and hybridize with each other to form a heavy (N¹⁵) band, and all newly synthesized strands would hybridize to form a light (N¹⁴) band. Therefore, after one generation, half of the DNA would be present in the heavy band (original parental strands) and the other half in the light band (newly synthesized strands). This is consistent with the predictions of the semiconservative model of DNA replication proposed by Watson and Crick. Hence, the correct answer is D.

A NASA Probe Returned Samples of Rocks from the Surface