Practice question

Question

You have labelled DNA in a bacterium by flowing cells in medium containing either 14N nitrogen or the heavier isotope, 15N. Furthermore, you have isolated pure DNA from these organisms, and subjected it to CsCl density gradient centrifugation leading separation of light (14N) and heavy (15N) forms of DNA to different locations in the centrifuge tube. In the next experiment, bacteria were regrown first in medium containing 15N, so that all the DNA made by cells will be in heavy form. Then these cells were transferred to medium containing only 14N and allowed the cells to divide for one generation. DNAs were extracted and centrifuged as above in the CsCl gradient. A hybrid DNA band was observed at a positions located between and equidistant from the 15N and 14N DNA bands. Based on the above observation, which one of the following conclusions is correct